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llvm/docs/AMDGPUDwarfExtensionAllowLocationDescriptionOnTheDwarfExpressionStack/AMDGPUDwarfExtensionAllowLocationDescriptionOnTheDwarfExpressionStack.md

Show All 19 Lines	- [Examples](#examples-1)
- [Source Language Variable Spread Across Multiple Kinds of Locations](#source-language-variable-spread-across-multiple-kinds-of-locations)		- [Source Language Variable Spread Across Multiple Kinds of Locations](#source-language-variable-spread-across-multiple-kinds-of-locations)
- [Address Spaces](#address-spaces)		- [Address Spaces](#address-spaces)
- [Bit Offsets](#bit-offsets)		- [Bit Offsets](#bit-offsets)
- [Call Frame Information (CFI)](#call-frame-information-cfi)		- [Call Frame Information (CFI)](#call-frame-information-cfi)
- [Objects Not In Byte Aligned Global Memory](#objects-not-in-byte-aligned-global-memory)		- [Objects Not In Byte Aligned Global Memory](#objects-not-in-byte-aligned-global-memory)
- [Higher Order Operations](#higher-order-operations)		- [Higher Order Operations](#higher-order-operations)
- [Objects In Multiple Places](#objects-in-multiple-places)		- [Objects In Multiple Places](#objects-in-multiple-places)
- [Conclusion](#conclusion)		- [Conclusion](#conclusion)
		- [Changes to DWARF Debugging Information Format Version 5](#changes-to-dwarf-debugging-information-format-version-5)
		- [2 General Description](#general-description)
		- [2.5 DWARF Expressions](#dwarf-expressions)
		- [2.5.1 DWARF Expression Evaluation Context](#dwarf-expression-evaluation-context)
		- [2.5.2 DWARF Expression Value](#dwarf-expression-value)
		- [2.5.3 DWARF Location Description](#dwarf-location-description)
		- [2.5.4 DWARF Operation Expressions](#dwarf-operation-expressions)
		- [2.5.4.1 Stack Operations](#stack-operations)
		- [2.5.4.2 Control Flow Operations](#control-flow-operations)
		- [2.5.4.3 Value Operations](#value-operations)
		- [2.5.4.3.1 Literal Operations](#literal-operations)
		- [2.5.4.3.2 Arithmetic and Logical Operations](#arithmetic-and-logical-operations)
		- [2.5.4.3.3 Type Conversion Operations](#type-conversion-operations)
		- [2.5.4.3.4 Special Value Operations](#special-value-operations)
		- [2.5.4.4 Location Description Operations](#location-description-operations)
		- [2.5.4.4.1 General Location Description Operations](#general-location-description-operations)
		- [2.5.4.4.2 Undefined Location Description Operations](#undefined-location-description-operations)
		- [2.5.4.4.3 Memory Location Description Operations](#memory-location-description-operations)
		- [2.5.4.4.4 Register Location Description Operations](#register-location-description-operations)
		- [2.5.4.4.5 Implicit Location Description Operations](#implicit-location-description-operations)
		- [2.5.4.4.6 Composite Location Description Operations](#composite-location-description-operations)
		- [2.5.5 DWARF Location List Expressions](#dwarf-location-list-expressions)
		- [3 Program Scope Entries](#program-scope-entries)
		- [3.3 Subroutine and Entry Point Entries](#subroutine-and-entry-point-entries)
		- [3.3.5 Low-Level Information](#low-level-information)
		- [3.4 Call Site Entries and Parameters](#call-site-entries-and-parameters)
		- [3.4.2 Call Site Parameters](#call-site-parameters)
		- [3.5 Lexical Block Entries](#lexical-block-entries)
		- [4 Data Object and Object List Entries](#data-object-and-object-list-entries)
		- [4.1 Data Object Entries](#data-object-entries)
		- [5 Type Entries](#type-entries)
		- [5.7 Structure, Union, Class and Interface Type Entries](#structure-union-class-and-interface-type-entries)
		- [5.7.3 Derived or Extended Structures, Classes and Interfaces](#derived-or-extended-structures-classes-and-interfaces)
		- [5.7.8 Member Function Entries](#member-function-entries)
		- [5.14 Pointer to Member Type Entries](#pointer-to-member-type-entries)
		- [5.16 Dynamic Type Entries](#dynamic-type-entries)
		- [6 Other Debugging Information](#other-debugging-information)
		- [6.2 Line Number Information](#line-number-information)
		- [6.4 Call Frame Information](#call-frame-information)
		- [6.4.1 Structure of Call Frame Information](#structure-of-call-frame-information)
		- [6.4.2 Call Frame Instructions](#call-frame-instructions)
		- [6.4.2.1 Row Creation Instructions](#row-creation-instructions)
		- [6.4.2.2 CFA Definition Instructions](#cfa-definition-instructions)
		- [6.4.2.3 Register Rule Instructions](#register-rule-instructions)
		- [6.4.2.4 Row State Instructions](#row-state-instructions)
		- [6.4.2.5 Padding Instruction](#padding-instruction)
		- [6.4.3 Call Frame Instruction Usage](#call-frame-instruction-usage)
		- [6.4.4 Call Frame Calling Address](#call-frame-calling-address)
		- [7 Data Representation](#data-representation)
		- [7.4 32-Bit and 64-Bit DWARF Formats](#32-bit-and-64-bit-dwarf-formats)
		- [7.5 Format of Debugging Information](#format-of-debugging-information)
		- [7.5.5 Classes and Forms](#classes-and-forms)
		- [7.7 DWARF Expressions](#dwarf-expressions)
		- [7.7.1 Operation Expressions](#operation-expressions)
		- [7.7.3 Location List Expressions](#location-list-expressions)
- [Further Information](#further-information)		- [Further Information](#further-information)

# Extension		# Extension

In DWARF 5, expressions are evaluated using a typed value stack, a separate		In DWARF 5, expressions are evaluated using a typed value stack, a separate
location area, and an independent loclist mechanism. This extension unifies all		location area, and an independent loclist mechanism. This extension unifies all
three mechanisms into a single generalized DWARF expression evaluation model		three mechanisms into a single generalized DWARF expression evaluation model
that allows both typed values and location descriptions to be manipulated on the		that allows both typed values and location descriptions to be manipulated on the
Show All 16 Lines
corner cases that had to be defined. The observation was that numerous		corner cases that had to be defined. The observation was that numerous
specialized context sensitive operations are harder for both produces and		specialized context sensitive operations are harder for both produces and
consumers than a smaller number of general composable operations that have		consumers than a smaller number of general composable operations that have
consistent semantics regardless of context.		consistent semantics regardless of context.

The following sections first describe heterogeneous devices and the features		The following sections first describe heterogeneous devices and the features
they have that are not addressed by DWARF 5. Then a brief simplified overview of		they have that are not addressed by DWARF 5. Then a brief simplified overview of
the DWARF 5 expression evaluation model is presented that highlights the		the DWARF 5 expression evaluation model is presented that highlights the
difficulties for supporting the heterogeneous features. Finally, an overview of		difficulties for supporting the heterogeneous features. Next, an overview of the
the extension is presented, using simplified examples to illustrate how it can		extension is presented, using simplified examples to illustrate how it can
address the issues of heterogeneous devices and also benefit non-heterogeneous		address the issues of heterogeneous devices and also benefit non-heterogeneous
devices. References to further information are provided.		devices. Finally, changes relative to the DWARF Version 5 standard are
		presented. References to further information are provided.

# Heterogeneous Computing Devices		# Heterogeneous Computing Devices

GPUs and other heterogeneous computing devices have features not common to CPU		GPUs and other heterogeneous computing devices have features not common to CPU
computing devices.		computing devices.

These devices often have many more registers than a CPU. This helps reduce		These devices often have many more registers than a CPU. This helps reduce
memory accesses which tend to be more expensive than on a CPU due to the much		memory accesses which tend to be more expensive than on a CPU due to the much
▲ Show 20 Lines • Show All 275 Lines • ▼ Show 20 Lines
is implicitly converted to a complete composite location description. The result		is implicitly converted to a complete composite location description. The result
of the expression is the location description in the location area.		of the expression is the location description in the location area.

![Variable Spread Across Different Locations Example: Step 7](images/04-composite.example.frame.7.png)		![Variable Spread Across Different Locations Example: Step 7](images/04-composite.example.frame.7.png)

### Offsetting a Composite Location		### Offsetting a Composite Location

This example attempts to extend the previous example to offset the composite		This example attempts to extend the previous example to offset the composite
location description it created. The Variable Location in Memory example		location description it created. The [Variable Location in
conveniently used the DW_OP_plus operation to offset a memory address.		Memory](#variable-location-in-memory) example conveniently used the DW_OP_plus
		operation to offset a memory address.

DW_OP_regx SGPR3		DW_OP_regx SGPR3
DW_OP_piece 4		DW_OP_piece 4
DW_OP_piece 2		DW_OP_piece 2
DW_OP_bregx SGPR0 0x10		DW_OP_bregx SGPR0 0x10
DW_OP_piece 2		DW_OP_piece 2
DW_OP_plus_uconst 5		DW_OP_plus_uconst 5

▲ Show 20 Lines • Show All 562 Lines • ▼ Show 20 Lines
A strength of DWARF is that it has generally sought to provide generalized		A strength of DWARF is that it has generally sought to provide generalized
composable solutions that address many problems, rather than solutions that only		composable solutions that address many problems, rather than solutions that only
address one-off issues. This extension attempts to follow that tradition by		address one-off issues. This extension attempts to follow that tradition by
defining a backwards compatible composable generalization that can address a		defining a backwards compatible composable generalization that can address a
significant family of issues. It addresses the specific issues present for		significant family of issues. It addresses the specific issues present for
heterogeneous computing devices, provides benefits for non-heterogeneous		heterogeneous computing devices, provides benefits for non-heterogeneous
devices, and can help address a number of other previously reported issues.		devices, and can help address a number of other previously reported issues.

		# Changes to DWARF Debugging Information Format Version 5

		> NOTE: This section provides changes relative to DWARF Version 5. The section
		> numbers generally correspond to those in the DWARF Version 5 standard.
		>
		> > NOTE: Notes are included to describe how the changes are to be applied to
		> > the DWARF Version 5 standard. They also describe rational and issues that
		> > may need further consideration.

		## 2 General Description

		### 2.5 DWARF Expressions

		> NOTE: This section, and its nested sections, replaces DWARF Version 5 section
		> 2.5 and section 2.6. It is based on the text of the existing DWARF Version 5
		> standard.

		DWARF expressions describe how to compute a value or specify a location.

		<i>The evaluation of a DWARF expression can provide the location of an object,
		the value of an array bound, the length of a dynamic string, the desired value
		itself, and so on.</i>

		If the evaluation of a DWARF expression does not encounter an error, then it can
		either result in a value (see [2.5.2 DWARF Expression
		Value](#dwarf-expression-value)) or a location description (see [2.5.3 DWARF
		Location Description](#dwarf-location-description)). When a DWARF expression
		is evaluated, it may be specified whether a value or location description is
		required as the result kind.

		If a result kind is specified, and the result of the evaluation does not match
		the specified result kind, then the implicit conversions described in [2.5.4.4.3
		Memory Location Description
		Operations](#memory-location-description-operations) are performed if
		valid. Otherwise, the DWARF expression is ill-formed.

		If the evaluation of a DWARF expression encounters an evaluation error, then the
		result is an evaluation error.

		> NOTE: Decided to define the concept of an evaluation error. An alternative is
		> to introduce an undefined value base type in a similar way to location
		> descriptions having an undefined location description. Then operations that
		> encounter an evaluation error can return the undefined location description or
		> value with an undefined base type.
		>
		> All operations that act on values would return an undefined entity if given an
		> undefined value. The expression would then always evaluate to completion, and
		> can be tested to determine if it is an undefined entity.
		>
		> However, this would add considerable additional complexity and does not match
		> that GDB throws an exception when these evaluation errors occur.

		If a DWARF expression is ill-formed, then the result is undefined.

		The following sections detail the rules for when a DWARF expression is
		ill-formed or results in an evaluation error.

		A DWARF expression can either be encoded as an operation expression (see [2.5.4
		DWARF Operation Expressions](#dwarf-operation-expressions)), or as a
		location list expression (see [2.5.5 DWARF Location List
		Expressions](#dwarf-location-list-expressions)).

		#### 2.5.1 DWARF Expression Evaluation Context

		A DWARF expression is evaluated in a context that can include a number of
		context elements. If multiple context elements are specified then they must be
		self consistent or the result of the evaluation is undefined. The context
		elements that can be specified are:

		1. <i>A current result kind</i>

		The kind of result required by the DWARF expression evaluation. If specified
		it can be a location description or a value.

		2. <i>A current thread</i>

		The target architecture thread identifier of the source program thread of
		execution for which a user presented expression is currently being
		evaluated.

		It is required for operations that are related to target architecture
		threads.

		<i>For example, the `DW_OP_regval_type` operation.</i>

		3. <i>A current call frame</i>

		The target architecture call frame identifier. It identifies a call frame
		that corresponds to an active invocation of a subprogram in the current
		thread. It is identified by its address on the call stack. The address is
		referred to as the Canonical Frame Address (CFA). The call frame information
		is used to determine the CFA for the call frames of the current thread's
		call stack (see [6.4 Call Frame Information](#call-frame-information)).

		It is required for operations that specify target architecture registers to
		support virtual unwinding of the call stack.

		<i>For example, the `DW_OP_reg` operations.</i>

		If specified, it must be an active call frame in the current thread.
		Otherwise the result is undefined.

		If it is the currently executing call frame, then it is termed the top call
		frame.

		4. <i>A current program location</i>

		The target architecture program location corresponding to the current call
		frame of the current thread.

		The program location of the top call frame is the target architecture
		program counter for the current thread. The call frame information is used
		to obtain the value of the return address register to determine the program
		location of the other call frames (see [6.4 Call Frame
		Information](#call-frame-information)).

		It is required for the evaluation of location list expressions to select
		amongst multiple program location ranges. It is required for operations that
		specify target architecture registers to support virtual unwinding of the
		call stack (see [6.4 Call Frame Information](#call-frame-information)).

		If specified:

		- If the current call frame is the top call frame, it must be the current
		target architecture program location.
		- If the current call frame F is not the top call frame, it must be the
		program location associated with the call site in the current caller frame
		F that invoked the callee frame.
		- Otherwise the result is undefined.

		5. <i>A current compilation unit</i>

		The compilation unit debug information entry that contains the DWARF
		expression being evaluated.

		It is required for operations that reference debug information associated
		with the same compilation unit, including indicating if such references use
		the 32-bit or 64-bit DWARF format. It can also provide the default address
		space address size if no current target architecture is specified.

		<i>For example, the `DW_OP_constx` and `DW_OP_addrx` operations.</i>

		<i>Note that this compilation unit may not be the same as the compilation
		unit determined from the loaded code object corresponding to the current
		program location. For example, the evaluation of the expression E associated
		with a `DW_AT_location` attribute of the debug information entry operand of
		the `DW_OP_call*` operations is evaluated with the compilation unit that
		contains E and not the one that contains the `DW_OP_call*` operation
		expression.</i>

		6. <i>A current target architecture</i>

		The target architecture.

		It is required for operations that specify target architecture specific
		entities.

		<i>For example, target architecture specific entities include DWARF register
		identifiers, DWARF address space identifiers, the default address space, and
		the address space address sizes.</i>

		If specified:

		- If the current thread is specified, then the current target architecture
		must be the same as the target architecture of the current thread.
		- If the current compilation unit is specified, then the current target
		architecture default address space address size must be the same as the
		`address_size` field in the header of the current compilation unit and any
		associated entry in the `.debug_aranges` section.
		- If the current program location is specified, then the current target
		architecture must be the same as the target architecture of any line
		number information entry (see [6.2 Line Number
		Information](#line-number-information)) corresponding to the current
		program location.
		- If the current program location is specified, then the current target
		architecture default address space address size must be the same as the
		`address_size` field in the header of any entry corresponding to the
		current program location in the `.debug_addr`, `.debug_line`,
		`.debug_rnglists`, `.debug_rnglists.dwo`, `.debug_loclists`, and
		`.debug_loclists.dwo` sections.
		- Otherwise the result is undefined.

		7. <i>A current object</i>

		The location description of a program object.

		It is required for the `DW_OP_push_object_address` operation.

		<i>For example, the `DW_AT_data_location` attribute on type debug
		information entries specifies the program object corresponding to a runtime
		descriptor as the current object when it evaluates its associated
		expression.</i>

		The result is undefined if the location descriptor is invalid (see [3.5.3
		DWARF Location Description](#dwarf-location-description)).

		8. <i>An initial stack</i>

		This is a list of values or location descriptions that will be pushed on the
		operation expression evaluation stack in the order provided before
		evaluation of an operation expression starts.

		Some debugger information entries have attributes that evaluate their DWARF
		expression value with initial stack entries. In all other cases the initial
		stack is empty.

		The result is undefined if any location descriptors are invalid (see [3.5.3
		DWARF Location Description](#dwarf-location-description)).

		If the evaluation requires a context element that is not specified, then the
		result of the evaluation is an error.

		<i>A DWARF expression for a location description may be able to be evaluated
		without a thread, call frame, program location, or architecture context. For
		example, the location of a global variable may be able to be evaluated without
		such context. If the expression evaluates with an error then it may indicate the
		variable has been optimized and so requires more context.</i>

		<i>The DWARF expression for call frame information (see [6.4 Call Frame
		Information](#call-frame-information)) operations are restricted to those
		that do not require the compilation unit context to be specified.</i>

		The DWARF is ill-formed if all the `address_size` fields in the headers of all
		the entries in the `.debug_info`, `.debug_addr`, `.debug_line`,
		`.debug_rnglists`, `.debug_rnglists.dwo`, `.debug_loclists`, and
		`.debug_loclists.dwo` sections corresponding to any given program location do
		not match.

		#### 2.5.2 DWARF Expression Value

		A value has a type and a literal value. It can represent a literal value of any
		supported base type of the target architecture. The base type specifies the
		size, encoding, and endianity of the literal value.

		> NOTE: It may be desirable to add an implicit pointer base type encoding. It
		> would be used for the type of the value that is produced when the
		> `DW_OP_deref*` operation retrieves the full contents of an implicit pointer
		> location storage created by the `DW_OP_implicit_pointer` operation. The
		> literal value would record the debugging information entry and byte
		> displacement specified by the associated `DW_OP_implicit_pointer` operation.

		There is a distinguished base type termed the generic type, which is an integral
		type that has the size of an address in the target architecture default address
		space, a target architecture defined endianity, and unspecified signedness.

		<i>The generic type is the same as the unspecified type used for stack
		operations defined in DWARF Version 4 and before.</i>

		An integral type is a base type that has an encoding of `DW_ATE_signed`,
		`DW_ATE_signed_char`, `DW_ATE_unsigned`, `DW_ATE_unsigned_char`,
		`DW_ATE_boolean`, or any target architecture defined integral encoding in the
		inclusive range `DW_ATE_lo_user` to `DW_ATE_hi_user`.

		> NOTE: It is unclear if `DW_ATE_address` is an integral type. GDB does not seem
		> to consider it as integral.

		#### 2.5.3 DWARF Location Description

		<i>Debugging information must provide consumers a way to find the location of
		program variables, determine the bounds of dynamic arrays and strings, and
		possibly to find the base address of a subprogram's call frame or the return
		address of a subprogram. Furthermore, to meet the needs of recent computer
		architectures and optimization techniques, debugging information must be able to
		describe the location of an object whose location changes over the object's
		lifetime, and may reside at multiple locations simultaneously during parts of an
		object's lifetime.</i>

		Information about the location of program objects is provided by location
		descriptions.

		Location descriptions can consist of one or more single location descriptions.

		A single location description specifies the location storage that holds a
		program object and a position within the location storage where the program
		object starts. The position within the location storage is expressed as a bit
		offset relative to the start of the location storage.

		A location storage is a linear stream of bits that can hold values. Each
		location storage has a size in bits and can be accessed using a zero-based bit
		offset. The ordering of bits within a location storage uses the bit numbering
		and direction conventions that are appropriate to the current language on the
		target architecture.

		There are five kinds of location storage:

		1. <i>memory location storage</i>

		Corresponds to the target architecture memory address spaces.

		2. <i>register location storage</i>

		Corresponds to the target architecture registers.

		3. <i>implicit location storage</i>

		Corresponds to fixed values that can only be read.

		4. <i>undefined location storage</i>

		Indicates no value is available and therefore cannot be read or written.

		5. <i>composite location storage</i>

		Allows a mixture of these where some bits come from one location storage and
		some from another location storage, or from disjoint parts of the same
		location storage.

		> NOTE: It may be better to add an implicit pointer location storage kind used
		> by the `DW_OP_implicit_pointer` operation. It would specify the debugger
		> information entry and byte offset provided by the operations.

		<i>Location descriptions are a language independent representation of addressing
		rules.</i>

		- <i>They can be the result of evaluating a debugger information entry attribute
		that specifies an operation expression of arbitrary complexity. In this usage
		they can describe the location of an object as long as its lifetime is either
		static or the same as the lexical block (see [3.5 Lexical Block
		Entries](#lexical-block-entries)]) that owns it, and it does not move during
		its lifetime.</i>

		- <i>They can be the result of evaluating a debugger information entry attribute
		that specifies a location list expression. In this usage they can describe the
		location of an object that has a limited lifetime, changes its location during
		its lifetime, or has multiple locations over part or all of its lifetime.</i>

		If a location description has more than one single location description, the
		DWARF expression is ill-formed if the object value held in each single location
		description's position within the associated location storage is not the same
		value, except for the parts of the value that are uninitialized.

		<i>A location description that has more than one single location description can
		only be created by a location list expression that has overlapping program
		location ranges, or certain expression operations that act on a location
		description that has more than one single location description. There are no
		operation expression operations that can directly create a location description
		with more than one single location description.</i>

		<i>A location description with more than one single location description can be
		used to describe objects that reside in more than one piece of storage at the
		same time. An object may have more than one location as a result of
		optimization. For example, a value that is only read may be promoted from memory
		to a register for some region of code, but later code may revert to reading the
		value from memory as the register may be used for other purposes. For the code
		region where the value is in a register, any change to the object value must be
		made in both the register and the memory so both regions of code will read the
		updated value.</i>

		<i>A consumer of a location description with more than one single location
		description can read the object's value from any of the single location
		descriptions (since they all refer to location storage that has the same value),
		but must write any changed value to all the single location descriptions.</i>

		Updating a location description L by a bit offset B is defined as adding the
		value of B to the bit offset of each single location description SL of L. It is
		an evaluation error if the updated bit offset of any SL is less than 0 or
		greater than or equal to the size of the location storage specified by SL.

		The evaluation of an expression may require context elements to create a
		location description. If such a location description is accessed, the storage it
		denotes is that associated with the context element values specified when the
		location description was created, which may differ from the context at the time
		it is accessed.

		<i>For example, creating a register location description requires the thread
		context: the location storage is for the specified register of that thread.
		Creating a memory location description for an address space may required a
		thread context: the location storage is the memory associated with that
		thread.</i>

		If any of the context elements required to create a location description change,
		the location description becomes invalid and accessing it is undefined.

		<i>Examples of context that can invalidate a location description are:</i>

		- <i>The thread context is required and execution causes the thread to
		terminate.</i>
		- <i>The call frame context is required and further execution causes the call
		frame to return to the calling frame.</i>
		- <i>The program location is required and further execution of the thread
		occurs. That could change the location list entry or call frame information
		entry that applies.</i>
		- <i>An operation uses call frame information:</i>
		- <i>Any of the frames used in the virtual call frame unwinding return.</i>
		- <i>The top call frame is used, the program location is used to select the
		call frame information entry, and further execution of the thread
		occurs.</i>

		<i>A DWARF expression can be used to compute a location description for an
		object. A subsequent DWARF expression evaluation can be given the object
		location description as the object context or initial stack context to compute a
		component of the object. The final result is undefined if the object location
		description becomes invalid between the two expression evaluations.</i>

		A change of a thread's program location may not make a location description
		invalid, yet may still render it as no longer meaningful. Accessing such a
		location description, or using it as the object context or initial stack context
		of an expression evaluation, may produce an undefined result.

		<i>For example, a location description may specify a register that no longer
		holds the intended program object after a program location change. One way to
		avoid such problems is to recompute location descriptions associated with
		threads when their program locations change.</i>

		#### 2.5.4 DWARF Operation Expressions

		An operation expression is comprised of a stream of operations, each consisting
		of an opcode followed by zero or more operands. The number of operands is
		implied by the opcode.

		Operations represent a postfix operation on a simple stack machine. Each stack
		entry can hold either a value or a location description. Operations can act on
		entries on the stack, including adding entries and removing entries. If the kind
		of a stack entry does not match the kind required by the operation and is not
		implicitly convertible to the required kind (see [2.5.4.4.3 Memory Location
		Description Operations](#memory-location-description-operations)), then
		the DWARF operation expression is ill-formed.

		Evaluation of an operation expression starts with an empty stack on which the
		entries from the initial stack provided by the context are pushed in the order
		provided. Then the operations are evaluated, starting with the first operation
		of the stream. Evaluation continues until either an operation has an evaluation
		error, or until one past the last operation of the stream is reached.

		The result of the evaluation is:

		- If an operation has an evaluation error, or an operation evaluates an
		expression that has an evaluation error, then the result is an evaluation
		error.
		- If the current result kind specifies a location description, then:
		- If the stack is empty, the result is a location description with one
		undefined location description.

		<i>This rule is for backwards compatibility with DWARF Version 5 which uses
		an empty operation expression for this purpose.</i>

		- If the top stack entry is a location description, or can be converted to one
		(see [2.5.4.4.3 Memory Location Description
		Operations](#memory-location-description-operations)), then the result
		is that, possibly converted, location description. Any other entries on the
		stack are discarded.
		- Otherwise the DWARF expression is ill-formed.

		> NOTE: Could define this case as returning an implicit location description
		> as if the `DW_OP_implicit` operation is performed.

		- If the current result kind specifies a value, then:
		- If the top stack entry is a value, or can be converted to one (see
		[2.5.4.4.3 Memory Location Description
		Operations](#memory-location-description-operations)), then the
		result is that, possibly converted, value. Any other entries on the stack
		are discarded.
		- Otherwise the DWARF expression is ill-formed.
		- If the current result kind is not specified, then:
		- If the stack is empty, the result is a location description with one
		undefined location description.

		<i>This rule is for backwards compatibility with DWARF Version 5 which uses
		an empty operation expression for this purpose.</i>

		> NOTE: This rule is consistent with the rule above for when a location
		> description is requested. However, GDB appears to report this as an error
		> and no GDB tests appear to cause an empty stack for this case.

		- Otherwise, the top stack entry is returned. Any other entries on the stack
		are discarded.

		An operation expression is encoded as a byte block with some form of prefix that
		specifies the byte count. It can be used:

		- as the value of a debugging information entry attribute that is encoded using
		class `exprloc` (see [7.5.5 Classes and Forms](#classes-and-forms)),
		- as the operand to certain operation expression operations,
		- as the operand to certain call frame information operations (see [6.4 Call
		Frame Information](#call-frame-information)),
		- and in location list entries (see [2.5.5 DWARF Location List
		Expressions](#dwarf-location-list-expressions)).

		##### 2.5.4.1 Stack Operations

		> NOTE: This section replaces DWARF Version 5 section 2.5.1.3.

		The following operations manipulate the DWARF stack. Operations that index the
		stack assume that the top of the stack (most recently added entry) has index 0.
		They allow the stack entries to be either a value or location description.

		If any stack entry accessed by a stack operation is an incomplete composite
		location description (see [2.5.4.4.6 Composite Location Description
		Operations](#composite-location-description-operations)), then the DWARF
		expression is ill-formed.

		> NOTE: These operations now support stack entries that are values and location
		> descriptions.

		> NOTE: If it is desired to also make them work with incomplete composite
		> location descriptions, then would need to define that the composite location
		> storage specified by the incomplete composite location description is also
		> replicated when a copy is pushed. This ensures that each copy of the
		> incomplete composite location description can update the composite location
		> storage they specify independently.

		1. `DW_OP_dup`

		`DW_OP_dup` duplicates the stack entry at the top of the stack.

		2. `DW_OP_drop`

		`DW_OP_drop` pops the stack entry at the top of the stack and discards it.

		3. `DW_OP_pick`

		`DW_OP_pick` has a single unsigned 1-byte operand that represents an index
		I. A copy of the stack entry with index I is pushed onto the stack.

		4. `DW_OP_over`

		`DW_OP_over` pushes a copy of the entry with index 1.

		<i>This is equivalent to a `DW_OP_pick 1` operation.</i>

		5. `DW_OP_swap`

		`DW_OP_swap` swaps the top two stack entries. The entry at the top of the
		stack becomes the second stack entry, and the second stack entry becomes the
		top of the stack.

		6. `DW_OP_rot`

		`DW_OP_rot` rotates the first three stack entries. The entry at the top of
		the stack becomes the third stack entry, the second entry becomes the top of
		the stack, and the third entry becomes the second entry.

		##### 2.5.4.2 Control Flow Operations

		> NOTE: This section replaces DWARF Version 5 section 2.5.1.5.

		The following operations provide simple control of the flow of a DWARF operation
		expression.

		1. `DW_OP_nop`

		`DW_OP_nop` is a place holder. It has no effect on the DWARF stack entries.

		2. `DW_OP_le`, `DW_OP_ge`, `DW_OP_eq`, `DW_OP_lt`, `DW_OP_gt`,
		`DW_OP_ne`

		> NOTE: The same as in DWARF Version 5 section 2.5.1.5.

		3. `DW_OP_skip`

		`DW_OP_skip` is an unconditional branch. Its single operand is a 2-byte
		signed integer constant. The 2-byte constant is the number of bytes of the
		DWARF expression to skip forward or backward from the current operation,
		beginning after the 2-byte constant.

		If the updated position is at one past the end of the last operation, then
		the operation expression evaluation is complete.

		Otherwise, the DWARF expression is ill-formed if the updated operation
		position is not in the range of the first to last operation inclusive, or
		not at the start of an operation.

		4. `DW_OP_bra`

		`DW_OP_bra` is a conditional branch. Its single operand is a 2-byte signed
		integer constant. This operation pops the top of stack. If the value popped
		is not the constant 0, the 2-byte constant operand is the number of bytes of
		the DWARF operation expression to skip forward or backward from the current
		operation, beginning after the 2-byte constant.

		If the updated position is at one past the end of the last operation, then
		the operation expression evaluation is complete.

		Otherwise, the DWARF expression is ill-formed if the updated operation
		position is not in the range of the first to last operation inclusive, or
		not at the start of an operation.

		5. `DW_OP_call2, DW_OP_call4, DW_OP_call_ref`

		`DW_OP_call2`, `DW_OP_call4`, and `DW_OP_call_ref` perform DWARF procedure
		calls during evaluation of a DWARF expression.

		`DW_OP_call2` and `DW_OP_call4`, have one operand that is, respectively, a
		2-byte or 4-byte unsigned offset DR that represents the byte offset of a
		debugging information entry D relative to the beginning of the current
		compilation unit.

		`DW_OP_call_ref` has one operand that is a 4-byte unsigned value in the
		32-bit DWARF format, or an 8-byte unsigned value in the 64-bit DWARF format,
		that represents the byte offset DR of a debugging information entry D
		relative to the beginning of the `.debug_info` section that contains the
		current compilation unit. D may not be in the current compilation unit.

		> NOTE: DWARF Version 5 states that DR can be an offset in a `.debug_info`
		> section other than the one that contains the current compilation unit. It
		> states that relocation of references from one executable or shared object
		> file to another must be performed by the consumer. But given that DR is
		> defined as an offset in a `.debug_info` section this seems impossible. If
		> DR was defined as an implementation defined value, then the consumer could
		> choose to interpret the value in an implementation defined manner to
		> reference a debug information in another executable or shared object.
		>
		> In ELF the `.debug_info` section is in a non-`PT_LOAD` segment so standard
		> dynamic relocations cannot be used. But even if they were loaded segments
		> and dynamic relocations were used, DR would need to be the address of D,
		> not an offset in a `.debug_info` section. That would also need DR to be
		> the size of a global address. So it would not be possible to use the
		> 32-bit DWARF format in a 64-bit global address space. In addition, the
		> consumer would need to determine what executable or shared object the
		> relocated address was in so it could determine the containing compilation
		> unit.
		>
		> GDB only interprets DR as an offset in the `.debug_info` section that
		> contains the current compilation unit.
		>
		> This comment also applies to `DW_OP_implicit_pointer`.

		<i>Operand interpretation of `DW_OP_call2`, `DW_OP_call4`, and
		`DW_OP_call_ref` is exactly like that for `DW_FORM_ref2`, `DW_FORM_ref4`,
		and `DW_FORM_ref_addr`, respectively.</i>

		The call operation is evaluated by:

		- If D has a `DW_AT_location` attribute that is encoded as a `exprloc` that
		specifies an operation expression E, then execution of the current
		operation expression continues from the first operation of E. Execution
		continues until one past the last operation of E is reached, at which
		point execution continues with the operation following the call operation.
		The operations of E are evaluated with the same current context, except
		current compilation unit is the one that contains D and the stack is the
		same as that being used by the call operation. After the call operation
		has been evaluated, the stack is therefore as it is left by the evaluation
		of the operations of E. Since E is evaluated on the same stack as the call
		operation, E can use, and/or remove entries already on the stack, and can
		add new entries to the stack.

		<i>Values on the stack at the time of the call may be used as parameters
		by the called expression and values left on the stack by the called
		expression may be used as return values by prior agreement between the
		calling and called expressions.</i>

		- If D has a `DW_AT_location` attribute that is encoded as a `loclist` or
		`loclistsptr`, then the specified location list expression E is evaluated.
		The evaluation of E uses the current context, except the result kind is a
		location description, the compilation unit is the one that contains D, and
		the initial stack is empty. The location description result is pushed on
		the stack.

		> NOTE: This rule avoids having to define how to execute a matched
		> location list entry operation expression on the same stack as the call
		> when there are multiple matches. But it allows the call to obtain the
		> location description for a variable or formal parameter which may use a
		> location list expression.
		>
		> An alternative is to treat the case when D has a `DW_AT_location`
		> attribute that is encoded as a `loclist` or `loclistsptr`, and the
		> specified location list expression E' matches a single location list
		> entry with operation expression E, the same as the `exprloc` case and
		> evaluate on the same stack.
		>
		> But this is not attractive as if the attribute is for a variable that
		> happens to end with a non-singleton stack, it will not simply put a
		> location description on the stack. Presumably the intent of using
		> `DW_OP_call*` on a variable or formal parameter debugger information
		> entry is to push just one location description on the stack. That
		> location description may have more than one single location description.
		>
		> The previous rule for `exprloc` also has the same problem, as normally a
		> variable or formal parameter location expression may leave multiple
		> entries on the stack and only return the top entry.
		>
		> GDB implements `DW_OP_call*` by always executing E on the same stack. If
		> the location list has multiple matching entries, it simply picks the
		> first one and ignores the rest. This seems fundamentally at odds with
		> the desire to support multiple places for variables.
		>
		> So, it feels like `DW_OP_call*` should both support pushing a location
		> description on the stack for a variable or formal parameter, and also
		> support being able to execute an operation expression on the same stack.
		> Being able to specify a different operation expression for different
		> program locations seems a desirable feature to retain.
		>
		> A solution to that is to have a distinct `DW_AT_proc` attribute for the
		> `DW_TAG_dwarf_procedure` debugging information entry. Then the
		> `DW_AT_location` attribute expression is always executed separately and
		> pushes a location description (that may have multiple single location
		> descriptions), and the `DW_AT_proc` attribute expression is always
		> executed on the same stack and can leave anything on the stack.
		>
		> The `DW_AT_proc` attribute could have the new classes `exprproc`,
		> `loclistproc`, and `loclistsptrproc` to indicate that the expression is
		> executed on the same stack. `exprproc` is the same encoding as
		> `exprloc`. `loclistproc` and `loclistsptrproc` are the same encoding as
		> their non-`proc` counterparts, except the DWARF is ill-formed if the
		> location list does not match exactly one location list entry and a
		> default entry is required. These forms indicate explicitly that the
		> matched single operation expression must be executed on the same stack.
		> This is better than ad hoc special rules for `loclistproc` and
		> `loclistsptrproc` which are currently clearly defined to always return a
		> location description. The producer then explicitly indicates the intent
		> through the attribute classes.
		>
		> Such a change would be a breaking change for how GDB implements
		> `DW_OP_call*`. However, are the breaking cases actually occurring in
		> practice? GDB could implement the current approach for DWARF Version 5,
		> and the new semantics for DWARF Version 6 which has been done for some
		> other features.
		>
		> Another option is to limit the execution to be on the same stack only to
		> the evaluation of an expression E that is the value of a
		> `DW_AT_location` attribute of a `DW_TAG_dwarf_procedure` debugging
		> information entry. The DWARF would be ill-formed if E is a location list
		> expression that does not match exactly one location list entry. In all
		> other cases the evaluation of an expression E that is the value of a
		> `DW_AT_location` attribute would evaluate E with the current context,
		> except the result kind is a location description, the compilation unit
		> is the one that contains D, and the initial stack is empty. The location
		> description result is pushed on the stack.

		- If D has a `DW_AT_const_value` attribute with a value V, then it is as if
		a `DW_OP_implicit_value V` operation was executed.

		<i>This allows a call operation to be used to compute the location
		description for any variable or formal parameter regardless of whether the
		producer has optimized it to a constant. This is consistent with the
		`DW_OP_implicit_pointer` operation.</i>

		> NOTE: Alternatively, could deprecate using `DW_AT_const_value` for
		> `DW_TAG_variable` and `DW_TAG_formal_parameter` debugger information
		> entries that are constants and instead use `DW_AT_location` with an
		> operation expression that results in a location description with one
		> implicit location description. Then this rule would not be required.

		- Otherwise, there is no effect and no changes are made to the stack.

		> NOTE: In DWARF Version 5, if D does not have a `DW_AT_location` then
		> `DW_OP_call*` is defined to have no effect. It is unclear that this is
		> the right definition as a producer should be able to rely on using
		> `DW_OP_call*` to get a location description for any
		> non-`DW_TAG_dwarf_procedure` debugging information entries. Also, the
		> producer should not be creating DWARF with `DW_OP_call*` to a
		> `DW_TAG_dwarf_procedure` that does not have a `DW_AT_location`
		> attribute. So, should this case be defined as an ill-formed DWARF
		> expression?

		<i>The `DW_TAG_dwarf_procedure` debugging information entry can be used to
		define DWARF procedures that can be called.</i>

		##### 2.5.4.3 Value Operations

		This section describes the operations that push values on the stack.

		Each value stack entry has a type and a literal value. It can represent a
		literal value of any supported base type of the target architecture. The base
		type specifies the size, encoding, and endianity of the literal value.

		The base type of value stack entries can be the distinguished generic type.

		###### 2.5.4.3.1 Literal Operations

		> NOTE: This section replaces DWARF Version 5 section 2.5.1.1.

		The following operations all push a literal value onto the DWARF stack.

		Operations other than `DW_OP_const_type` push a value V with the generic type.
		If V is larger than the generic type, then V is truncated to the generic type
		size and the low-order bits used.

		1. `DW_OP_lit0`, `DW_OP_lit1`, ..., `DW_OP_lit31`

		`DW_OP_lit<N>` operations encode an unsigned literal value N from 0 through
		31, inclusive. They push the value N with the generic type.

		2. `DW_OP_const1u`, `DW_OP_const2u`, `DW_OP_const4u`, `DW_OP_const8u`

		`DW_OP_const<N>u` operations have a single operand that is a 1, 2, 4, or
		8-byte unsigned integer constant U, respectively. They push the value U with
		the generic type.

		3. `DW_OP_const1s`, `DW_OP_const2s`, `DW_OP_const4s`, `DW_OP_const8s`

		`DW_OP_const<N>s` operations have a single operand that is a 1, 2, 4, or
		8-byte signed integer constant S, respectively. They push the value S with
		the generic type.

		4. `DW_OP_constu`

		`DW_OP_constu` has a single unsigned LEB128 integer operand N. It pushes the
		value N with the generic type.

		5. `DW_OP_consts`

		`DW_OP_consts` has a single signed LEB128 integer operand N. It pushes the
		value N with the generic type.

		6. `DW_OP_constx`

		`DW_OP_constx` has a single unsigned LEB128 integer operand that represents
		a zero-based index into the `.debug_addr` section relative to the value of
		the `DW_AT_addr_base` attribute of the associated compilation unit. The
		value N in the `.debug_addr` section has the size of the generic type. It
		pushes the value N with the generic type.

		<i>The `DW_OP_constx` operation is provided for constants that require
		link-time relocation but should not be interpreted by the consumer as a
		relocatable address (for example, offsets to thread-local storage).</i>

		7. `DW_OP_const_type`

		`DW_OP_const_type` has three operands. The first is an unsigned LEB128
		integer DR that represents the byte offset of a debugging information entry
		D relative to the beginning of the current compilation unit, that provides
		the type T of the constant value. The second is a 1-byte unsigned integral
		constant S. The third is a block of bytes B, with a length equal to S.

		TS is the bit size of the type T. The least significant TS bits of B are
		interpreted as a value V of the type D. It pushes the value V with the type
		D.

		The DWARF is ill-formed if D is not a `DW_TAG_base_type` debugging
		information entry in the current compilation unit, or if TS divided by 8
		(the byte size) and rounded up to a whole number is not equal to S.

		<i>While the size of the byte block B can be inferred from the type D
		definition, it is encoded explicitly into the operation so that the
		operation can be parsed easily without reference to the `.debug_info`
		section.</i>

		###### 2.5.4.3.2 Arithmetic and Logical Operations

		> NOTE: This section is the same as DWARF Version 5 section 2.5.1.4.

		###### 2.5.4.3.3 Type Conversion Operations

		> NOTE: This section is the same as DWARF Version 5 section 2.5.1.6.

		###### 2.5.4.3.4 Special Value Operations

		> NOTE: This section replaces parts of DWARF Version 5 sections 2.5.1.2,
		2.5.1.3, and 2.5.1.7.

		There are these special value operations currently defined:

		1. `DW_OP_regval_type`

		`DW_OP_regval_type` has two operands. The first is an unsigned LEB128
		integer that represents a register number R. The second is an unsigned
		LEB128 integer DR that represents the byte offset of a debugging information
		entry D relative to the beginning of the current compilation unit, that
		provides the type T of the register value.

		The operation is equivalent to performing `DW_OP_regx R; DW_OP_deref_type
		DR`.

		> NOTE: Should DWARF allow the type T to be a larger size than the size of
		> the register R? Restricting a larger bit size avoids any issue of
		> conversion as the, possibly truncated, bit contents of the register is
		> simply interpreted as a value of T. If a conversion is wanted it can be
		> done explicitly using a `DW_OP_convert` operation.
		>
		> GDB has a per register hook that allows a target specific conversion on a
		> register by register basis. It defaults to truncation of bigger registers.
		> Removing use of the target hook does not cause any test failures in common
		> architectures. If the compiler for a target architecture did want some
		> form of conversion, including a larger result type, it could always
		> explicitly used the `DW_OP_convert` operation.
		>
		> If T is a larger type than the register size, then the default GDB
		> register hook reads bytes from the next register (or reads out of bounds
		> for the last register!). Removing use of the target hook does not cause
		> any test failures in common architectures (except an illegal hand written
		> assembly test). If a target architecture requires this behavior, these
		> extensions allow a composite location description to be used to combine
		> multiple registers.

		2. `DW_OP_deref`

		S is the bit size of the generic type divided by 8 (the byte size) and
		rounded up to a whole number. DR is the offset of a hypothetical debug
		information entry D in the current compilation unit for a base type of the
		generic type.

		The operation is equivalent to performing `DW_OP_deref_type S, DR`.

		3. `DW_OP_deref_size`

		`DW_OP_deref_size` has a single 1-byte unsigned integral constant that
		represents a byte result size S.

		TS is the smaller of the generic type bit size and S scaled by 8 (the byte
		size). If TS is smaller than the generic type bit size then T is an unsigned
		integral type of bit size TS, otherwise T is the generic type. DR is the
		offset of a hypothetical debug information entry D in the current
		compilation unit for a base type T.

		> NOTE: Truncating the value when S is larger than the generic type matches
		> what GDB does. This allows the generic type size to not be an integral
		> byte size. It does allow S to be arbitrarily large. Should S be restricted
		> to the size of the generic type rounded up to a multiple of 8?

		The operation is equivalent to performing `DW_OP_deref_type S, DR`, except
		if T is not the generic type, the value V pushed is zero-extended to the
		generic type bit size and its type changed to the generic type.

		4. `DW_OP_deref_type`

		`DW_OP_deref_type` has two operands. The first is a 1-byte unsigned integral
		constant S. The second is an unsigned LEB128 integer DR that represents the
		byte offset of a debugging information entry D relative to the beginning of
		the current compilation unit, that provides the type T of the result value.

		TS is the bit size of the type T.

		<i>While the size of the pushed value V can be inferred from the type T, it
		is encoded explicitly as the operand S so that the operation can be parsed
		easily without reference to the `.debug_info` section.</i>

		> NOTE: It is unclear why the operand S is needed. Unlike
		> `DW_OP_const_type`, the size is not needed for parsing. Any evaluation
		> needs to get the base type T to push with the value to know its encoding
		> and bit size.

		It pops one stack entry that must be a location description L.

		A value V of TS bits is retrieved from the location storage LS specified by
		one of the single location descriptions SL of L.

		<i>If L, or the location description of any composite location description
		part that is a subcomponent of L, has more than one single location
		description, then any one of them can be selected as they are required to
		all have the same value. For any single location description SL, bits are
		retrieved from the associated storage location starting at the bit offset
		specified by SL. For a composite location description, the retrieved bits
		are the concatenation of the N bits from each composite location part PL,
		where N is limited to the size of PL.</i>

		V is pushed on the stack with the type T.

		> NOTE: This definition makes it an evaluation error if L is a register
		> location description that has less than TS bits remaining in the register
		> storage. Particularly since these extensions extend location descriptions
		> to have a bit offset, it would be odd to define this as performing sign
		> extension based on the type, or be target architecture dependent, as the
		> number of remaining bits could be any number. This matches the GDB
		> implementation for `DW_OP_deref_type`.
		>
		> These extensions define `DW_OP_breg` in terms of `DW_OP_regval_type`.
		> `DW_OP_regval_type` is defined in terms of `DW_OP_regx`, which uses a 0
		> bit offset, and `DW_OP_deref_type`. Therefore, it requires the register
		> size to be greater or equal to the address size of the address space. This
		> matches the GDB implementation for `DW_OP_breg`.

		The DWARF is ill-formed if D is not in the current compilation unit, D is
		not a `DW_TAG_base_type` debugging information entry, or if TS divided by 8
		(the byte size) and rounded up to a whole number is not equal to S.

		> NOTE: This definition allows the base type to be a bit size since there
		> seems no reason to restrict it.

		It is an evaluation error if any bit of the value is retrieved from the
		undefined location storage or the offset of any bit exceeds the size of the
		location storage LS specified by any single location description SL of L.

		See [2.5.4.4.5 Implicit Location Description
		Operations](#implicit-location-description-operations) for special
		rules concerning implicit location descriptions created by the
		`DW_OP_implicit_pointer` operation.

		5. `DW_OP_xderef`

		`DW_OP_xderef` pops two stack entries. The first must be an integral type
		value that represents an address A. The second must be an integral type
		value that represents a target architecture specific address space
		identifier AS.

		The address size S is defined as the address bit size of the target
		architecture specific address space that corresponds to AS.

		A is adjusted to S bits by zero extending if necessary, and then treating
		the least significant S bits as a twos-complement unsigned value A'.

		It creates a location description L with one memory location description SL.
		SL specifies the memory location storage LS that corresponds to AS with a
		bit offset equal to A' scaled by 8 (the byte size).

		If AS is an address space that is specific to context elements, then LS
		corresponds to the location storage associated with the current context.

		<i>For example, if AS is for per thread storage then LS is the location
		storage for the current thread. Therefore, if L is accessed by an operation,
		the location storage selected when the location description was created is
		accessed, and not the location storage associated with the current context
		of the access operation.</i>

		The DWARF expression is ill-formed if AS is not one of the values defined by
		the target architecture specific `DW_ASPACE_*` values.

		The operation is equivalent to popping A and AS, pushing L, and then
		performing `DW_OP_deref`. The value V retrieved is left on the stack with
		the generic type.

		6. `DW_OP_xderef_size`

		`DW_OP_xderef_size` has a single 1-byte unsigned integral constant that
		represents a byte result size S.

		It pops two stack entries. The first must be an integral type value
		that represents an address A. The second must be an integral type
		value that represents a target architecture specific address space
		identifier AS.

		It creates a location description L as described for `DW_OP_xderef`.

		The operation is equivalent to popping A and AS, pushing L, and then
		performing `DW_OP_deref_size S` . The zero-extended value V retrieved is
		left on the stack with the generic type.

		7. `DW_OP_xderef_type`

		`DW_OP_xderef_type` has two operands. The first is a 1-byte unsigned
		integral constant S. The second operand is an unsigned LEB128 integer DR
		that represents the byte offset of a debugging information entry D relative
		to the beginning of the current compilation unit, that provides the type T
		of the result value.

		It pops two stack entries. The first must be an integral type value that
		represents an address A. The second must be an integral type value that
		represents a target architecture specific address space identifier AS.

		It creates a location description L as described for `DW_OP_xderef`.

		The operation is equivalent to popping A and AS, pushing L, and then
		performing `DW_OP_deref_type DR` . The value V retrieved is left on the
		stack with the type T.

		8. `DW_OP_entry_value` <i>Deprecated</i>

		`DW_OP_entry_value` pushes the value of an expression that is evaluated in
		the context of the calling frame.

		<i>It may be used to determine the value of arguments on entry to the
		current call frame provided they are not clobbered.</i>

		It has two operands. The first is an unsigned LEB128 integer S. The second
		is a block of bytes, with a length equal S, interpreted as a DWARF operation
		expression E.

		E is evaluated with the current context, except the result kind is
		unspecified, the call frame is the one that called the current frame, the
		program location is the call site in the calling frame, the object is
		unspecified, and the initial stack is empty. The calling frame information
		is obtained by virtually unwinding the current call frame using the call
		frame information (see [6.4 Call Frame
		Information](#call-frame-information)).

		If the result of E is a location description L (see [2.5.4.4.4 Register
		Location Description
		Operations](#register-location-description-operations)), and the last
		operation executed by E is a `DW_OP_reg*` for register R with a target
		architecture specific base type of T, then the contents of the register are
		retrieved as if a `DW_OP_deref_type DR` operation was performed where DR is
		the offset of a hypothetical debug information entry in the current
		compilation unit for T. The resulting value V s pushed on the stack.

		<i>Using `DW_OP_reg*` provides a more compact form for the case where the
		value was in a register on entry to the subprogram.</i>

		> NOTE: It is unclear how this provides a more compact expression, as
		> `DW_OP_regval_type` could be used which is marginally larger.

		If the result of E is a value V, then V is pushed on the stack.

		Otherwise, the DWARF expression is ill-formed.

		<i>The `DW_OP_entry_value` operation is deprecated as its main usage is
		provided by other means. DWARF Version 5 added the
		`DW_TAG_call_site_parameter` debugger information entry for call sites that
		has `DW_AT_call_value`, `DW_AT_call_data_location`, and
		`DW_AT_call_data_value` attributes that provide DWARF expressions to compute
		actual parameter values at the time of the call, and requires the producer
		to ensure the expressions are valid to evaluate even when virtually
		unwound.</i>

		> NOTE: GDB only implements `DW_OP_entry_value` when E is exactly
		> `DW_OP_reg` or `DW_OP_breg; DW_OP_deref*`.

		##### 2.5.4.4 Location Description Operations

		This section describes the operations that push location descriptions on the
		stack.

		###### 2.5.4.4.1 General Location Description Operations

		> NOTE: This section replaces part of DWARF Version 5 section 2.5.1.3.

		1. `DW_OP_push_object_address`

		`DW_OP_push_object_address` pushes the location description L of the current
		object.

		<i>This object may correspond to an independent variable that is part of a
		user presented expression that is being evaluated. The object location
		description may be determined from the variable's own debugging information
		entry or it may be a component of an array, structure, or class whose
		address has been dynamically determined by an earlier step during user
		expression evaluation.</i>

		<i>This operation provides explicit functionality (especially for arrays
		involving descriptors) that is analogous to the implicit push of the base
		location description of a structure prior to evaluation of a
		`DW_AT_data_member_location` to access a data member of a structure.</i>

		> NOTE: This operation could be removed and the object location description
		> specified as the initial stack as for `DW_AT_data_member_location`. Or
		> this operation could be used instead of needing to specify an initial
		> stack. The latter approach is more composable as access to the object may
		> be needed at any point of the expression, and passing it as the initial
		> stack requires the entire expression to be aware where on the stack it is.
		> If this were done, `DW_AT_use_location` would require a
		> `DW_OP_push_object2_address` operation for the second object.
		>
		> The only attribute that specifies a current object is
		> `DW_AT_data_location` so the non-normative text seems to overstate how
		> this is being used. Or are there other attributes that need to state they
		> pass an object?

		###### 2.5.4.4.2 Undefined Location Description Operations

		> NOTE: This section replaces DWARF Version 5 section 2.6.1.1.1.

		<i>The undefined location storage represents a piece or all of an object that is
		present in the source but not in the object code (perhaps due to optimization).
		Neither reading nor writing to the undefined location storage is meaningful.</i>

		An undefined location description specifies the undefined location storage.
		There is no concept of the size of the undefined location storage, nor of a bit
		offset for an undefined location description. The `DW_OP_*piece` operations can
		implicitly specify an undefined location description, allowing any size and
		offset to be specified, and results in a part with all undefined bits.

		###### 2.5.4.4.3 Memory Location Description Operations

		> NOTE: This section replaces parts of DWARF Version 5 section 2.5.1.1, 2.5.1.2,
		2.5.1.3, and 2.6.1.1.2.

		Each of the target architecture specific address spaces has a corresponding
		memory location storage that denotes the linear addressable memory of that
		address space. The size of each memory location storage corresponds to the range
		of the addresses in the corresponding address space.

		<i>It is target architecture defined how address space location storage maps to
		target architecture physical memory. For example, they may be independent
		memory, or more than one location storage may alias the same physical memory
		possibly at different offsets and with different interleaving. The mapping may
		also be dictated by the source language address classes.</i>

		A memory location description specifies a memory location storage. The bit
		offset corresponds to a bit position within a byte of the memory. Bits accessed
		using a memory location description, access the corresponding target
		architecture memory starting at the bit position within the byte specified by
		the bit offset.

		A memory location description that has a bit offset that is a multiple of 8 (the
		byte size) is defined to be a byte address memory location description. It has a
		memory byte address A that is equal to the bit offset divided by 8.

		A memory location description that does not have a bit offset that is a multiple
		of 8 (the byte size) is defined to be a bit field memory location description.
		It has a bit position B equal to the bit offset modulo 8, and a memory byte
		address A equal to the bit offset minus B that is then divided by 8.

		The address space AS of a memory location description is defined to be the
		address space that corresponds to the memory location storage associated with
		the memory location description.

		A location description that is comprised of one byte address memory location
		description SL is defined to be a memory byte address location description. It
		has a byte address equal to A and an address space equal to AS of the
		corresponding SL.

		`DW_ASPACE_none` is defined as the target architecture default address space.

		If a stack entry is required to be a location description, but it is a value V
		with the generic type, then it is implicitly converted to a location description
		L with one memory location description SL. SL specifies the memory location
		storage that corresponds to the target architecture default address space with a
		bit offset equal to V scaled by 8 (the byte size).

		> NOTE: If it is wanted to allow any integral type value to be implicitly
		> converted to a memory location description in the target architecture default
		> address space:
		>
		> > If a stack entry is required to be a location description, but is a value V
		> > with an integral type, then it is implicitly converted to a location
		> > description L with a one memory location description SL. If the type size of
		> > V is less than the generic type size, then the value V is zero extended to
		> > the size of the generic type. The least significant generic type size bits
		> > are treated as an unsigned value to be used as an address A. SL specifies
		> > memory location storage corresponding to the target architecture default
		> > address space with a bit offset equal to A scaled by 8 (the byte size).
		>
		> The implicit conversion could also be defined as target architecture specific.
		> For example, GDB checks if V is an integral type. If it is not it gives an
		> error. Otherwise, GDB zero-extends V to 64 bits. If the GDB target defines a
		> hook function, then it is called. The target specific hook function can modify
		> the 64-bit value, possibly sign extending based on the original value type.
		> Finally, GDB treats the 64-bit value V as a memory location address.

		If a stack entry is required to be a location description, but it is an implicit
		pointer value IPV with the target architecture default address space, then it is
		implicitly converted to a location description with one single location
		description specified by IPV. See [2.5.4.4.5 Implicit Location Description
		Operations](#implicit-location-description-operations).

		If a stack entry is required to be a value, but it is a location description L
		with one memory location description SL in the target architecture default
		address space with a bit offset B that is a multiple of 8, then it is implicitly
		converted to a value equal to B divided by 8 (the byte size) with the generic
		type.

		1. `DW_OP_addr`

		`DW_OP_addr` has a single byte constant value operand, which has the size of
		the generic type, that represents an address A.

		It pushes a location description L with one memory location description SL
		on the stack. SL specifies the memory location storage corresponding to the
		target architecture default address space with a bit offset equal to A
		scaled by 8 (the byte size).

		<i>If the DWARF is part of a code object, then A may need to be relocated.
		For example, in the ELF code object format, A must be adjusted by the
		difference between the ELF segment virtual address and the virtual address
		at which the segment is loaded.</i>

		2. `DW_OP_addrx`

		`DW_OP_addrx` has a single unsigned LEB128 integer operand that represents a
		zero-based index into the `.debug_addr` section relative to the value of the
		`DW_AT_addr_base` attribute of the associated compilation unit. The address
		value A in the `.debug_addr` section has the size of the generic type.

		It pushes a location description L with one memory location description SL
		on the stack. SL specifies the memory location storage corresponding to the
		target architecture default address space with a bit offset equal to A
		scaled by 8 (the byte size).

		<i>If the DWARF is part of a code object, then A may need to be relocated.
		For example, in the ELF code object format, A must be adjusted by the
		difference between the ELF segment virtual address and the virtual address
		at which the segment is loaded.</i>

		3. `DW_OP_form_tls_address`

		`DW_OP_form_tls_address` pops one stack entry that must be an integral type
		value and treats it as a thread-local storage address TA.

		It pushes a location description L with one memory location description SL
		on the stack. SL is the target architecture specific memory location
		description that corresponds to the thread-local storage address TA.

		The meaning of the thread-local storage address TA is defined by the
		run-time environment. If the run-time environment supports multiple
		thread-local storage blocks for a single thread, then the block
		corresponding to the executable or shared library containing this DWARF
		expression is used.

		<i>Some implementations of C, C++, Fortran, and other languages support a
		thread-local storage class. Variables with this storage class have distinct
		values and addresses in distinct threads, much as automatic variables have
		distinct values and addresses in each subprogram invocation. Typically,
		there is a single block of storage containing all thread-local variables
		declared in the main executable, and a separate block for the variables
		declared in each shared library. Each thread-local variable can then be
		accessed in its block using an identifier. This identifier is typically a
		byte offset into the block and pushed onto the DWARF stack by one of the
		`DW_OP_const*` operations prior to the `DW_OP_form_tls_address` operation.
		Computing the address of the appropriate block can be complex (in some
		cases, the compiler emits a function call to do it), and difficult to
		describe using ordinary DWARF location descriptions. Instead of forcing
		complex thread-local storage calculations into the DWARF expressions, the
		`DW_OP_form_tls_address` allows the consumer to perform the computation
		based on the target architecture specific run-time environment.</i>

		4. `DW_OP_call_frame_cfa`

		`DW_OP_call_frame_cfa` pushes the location description L of the Canonical
		Frame Address (CFA) of the current subprogram, obtained from the call frame
		information on the stack. See [6.4 Call Frame
		Information](#call-frame-information).

		<i>Although the value of the `DW_AT_frame_base` attribute of the debugger
		information entry corresponding to the current subprogram can be computed
		using a location list expression, in some cases this would require an
		extensive location list because the values of the registers used in
		computing the CFA change during a subprogram execution. If the call frame
		information is present, then it already encodes such changes, and it is
		space efficient to reference that using the `DW_OP_call_frame_cfa`
		operation.</i>

		5. `DW_OP_fbreg`

		`DW_OP_fbreg` has a single signed LEB128 integer operand that represents a
		byte displacement B.

		The location description L for the <i>frame base</i> of the current
		subprogram is obtained from the `DW_AT_frame_base` attribute of the debugger
		information entry corresponding to the current subprogram as described in
		[3.3.5 Low-Level Information](#low-level-information).

		The location description L is updated by bit offset B scaled by 8 (the byte
		size) and pushed on the stack.

		6. `DW_OP_breg0`, `DW_OP_breg1`, ..., `DW_OP_breg31`

		The `DW_OP_breg<N>` operations encode the numbers of up to 32 registers,
		numbered from 0 through 31, inclusive. The register number R corresponds to
		the N in the operation name.

		They have a single signed LEB128 integer operand that represents a byte
		displacement B.

		The address space identifier AS is defined as the one corresponding to the
		target architecture specific default address space.

		The address size S is defined as the address bit size of the target
		architecture specific address space corresponding to AS.

		The contents of the register specified by R are retrieved as if a
		`DW_OP_regval_type R, DR` operation was performed where DR is the offset of
		a hypothetical debug information entry in the current compilation unit for
		an unsigned integral base type of size S bits. B is added and the least
		significant S bits are treated as an unsigned value to be used as an address
		A.

		They push a location description L comprising one memory location
		description LS on the stack. LS specifies the memory location storage that
		corresponds to AS with a bit offset equal to A scaled by 8 (the byte size).

		7. `DW_OP_bregx`

		`DW_OP_bregx` has two operands. The first is an unsigned LEB128 integer that
		represents a register number R. The second is a signed LEB128 integer that
		represents a byte displacement B.

		The action is the same as for `DW_OP_breg<N>`, except that R is used as the
		register number and B is used as the byte displacement.

		###### 2.5.4.4.4 Register Location Description Operations

		> NOTE: This section replaces DWARF Version 5 section 2.6.1.1.3.

		There is a register location storage that corresponds to each of the target
		architecture registers. The size of each register location storage corresponds
		to the size of the corresponding target architecture register.

		A register location description specifies a register location storage. The bit
		offset corresponds to a bit position within the register. Bits accessed using a
		register location description access the corresponding target architecture
		register starting at the specified bit offset.

		1. `DW_OP_reg0`, `DW_OP_reg1`, ..., `DW_OP_reg31`

		`DW_OP_reg<N>` operations encode the numbers of up to 32 registers, numbered
		from 0 through 31, inclusive. The target architecture register number R
		corresponds to the N in the operation name.

		The operation is equivalent to performing `DW_OP_regx R`.

		2. `DW_OP_regx`

		`DW_OP_regx` has a single unsigned LEB128 integer operand that represents a
		target architecture register number R.

		If the current call frame is the top call frame, it pushes a location
		description L that specifies one register location description SL on the
		stack. SL specifies the register location storage that corresponds to R with
		a bit offset of 0 for the current thread.

		If the current call frame is not the top call frame, call frame information
		(see [6.4 Call Frame Information](#call-frame-information)) is used to
		determine the location description that holds the register for the current
		call frame and current program location of the current thread. The resulting
		location description L is pushed.

		<i>Note that if call frame information is used, the resulting location
		description may be register, memory, or undefined.</i>

		<i>An implementation may evaluate the call frame information immediately, or
		may defer evaluation until L is accessed by an operation. If evaluation is
		deferred, R and the current context can be recorded in L. When accessed, the
		recorded context is used to evaluate the call frame information, not the
		current context of the access operation.</i>

		<i>These operations obtain a register location. To fetch the contents of a
		register, it is necessary to use `DW_OP_regval_type`, use one of the
		`DW_OP_breg` register-based addressing operations, or use `DW_OP_deref` on a
		register location description.</i>

		###### 2.5.4.4.5 Implicit Location Description Operations

		> NOTE: This section replaces DWARF Version 5 section 2.6.1.1.4.

		Implicit location storage represents a piece or all of an object which has no
		actual location in the program but whose contents are nonetheless known, either
		as a constant or can be computed from other locations and values in the program.

		An implicit location description specifies an implicit location storage. The bit
		offset corresponds to a bit position within the implicit location storage. Bits
		accessed using an implicit location description, access the corresponding
		implicit storage value starting at the bit offset.

		1. `DW_OP_implicit_value`

		`DW_OP_implicit_value` has two operands. The first is an unsigned LEB128
		integer that represents a byte size S. The second is a block of bytes with a
		length equal to S treated as a literal value V.

		An implicit location storage LS is created with the literal value V and a
		size of S.

		It pushes location description L with one implicit location description SL
		on the stack. SL specifies LS with a bit offset of 0.

		2. `DW_OP_stack_value`

		`DW_OP_stack_value` pops one stack entry that must be a value V.

		An implicit location storage LS is created with the literal value V using
		the size, encoding, and enianity specified by V's base type.

		It pushes a location description L with one implicit location description SL
		on the stack. SL specifies LS with a bit offset of 0.

		<i>The `DW_OP_stack_value` operation specifies that the object does not
		exist in memory, but its value is nonetheless known. In this form, the
		location description specifies the actual value of the object, rather than
		specifying the memory or register storage that holds the value.</i>

		See [2.5.4.4.5 Implicit Location Description
		Operations](#implicit-location-description-operations) for special
		rules concerning implicit pointer values produced by dereferencing implicit
		location descriptions created by the `DW_OP_implicit_pointer` operation.

		> NOTE: Since location descriptions are allowed on the stack, the
		> `DW_OP_stack_value` operation no longer terminates the DWARF operation
		> expression execution as in DWARF Version 5.

		3. `DW_OP_implicit_pointer`

		<i>An optimizing compiler may eliminate a pointer, while still retaining the
		value that the pointer addressed. `DW_OP_implicit_pointer` allows a producer
		to describe this value.</i>

		<i>`DW_OP_implicit_pointer` specifies an object is a pointer to the target
		architecture default address space that cannot be represented as a real
		pointer, even though the value it would point to can be described. In this
		form, the location description specifies a debugging information entry that
		represents the actual location description of the object to which the
		pointer would point. Thus, a consumer of the debug information would be able
		to access the dereferenced pointer, even when it cannot access the pointer
		itself.</i>

		`DW_OP_implicit_pointer` has two operands. The first operand is a 4-byte
		unsigned value in the 32-bit DWARF format, or an 8-byte unsigned value in
		the 64-bit DWARF format, that represents the byte offset DR of a debugging
		information entry D relative to the beginning of the `.debug_info` section
		that contains the current compilation unit. The second operand is a signed
		LEB128 integer that represents a byte displacement B.

		<i>Note that D may not be in the current compilation unit.</i>

		<i>The first operand interpretation is exactly like that for
		`DW_FORM_ref_addr`.</i>

		The address space identifier AS is defined as the one corresponding to the
		target architecture specific default address space.

		The address size S is defined as the address bit size of the target
		architecture specific address space corresponding to AS.

		An implicit location storage LS is created with the debugging information
		entry D, address space AS, and size of S.

		It pushes a location description L that comprises one implicit location
		description SL on the stack. SL specifies LS with a bit offset of 0.

		It is an evaluation error if a `DW_OP_deref*` operation pops a location
		description L', and retrieves S bits, such that any retrieved bits come from
		an implicit location storage that is the same as LS, unless both the
		following conditions are met:

		1. All retrieved bits come from an implicit location description that
		refers to an implicit location storage that is the same as LS.

		<i>Note that all bits do not have to come from the same implicit
		location description, as L' may involve composite location
		descriptors.</i>

		2. The bits come from consecutive ascending offsets within their respective
		implicit location storage.

		<i>These rules are equivalent to retrieving the complete contents of LS.</i>

		If both the above conditions are met, then the value V pushed by the
		`DW_OP_deref*` operation is an implicit pointer value IPV with a target
		architecture specific address space of AS, a debugging information entry of
		D, and a base type of T. If AS is the target architecture default address
		space, then T is the generic type. Otherwise, T is a target architecture
		specific integral type with a bit size equal to S.

		If IPV is either implicitly converted to a location description (only done
		if AS is the target architecture default address space), then the resulting
		location description RL is:

		- If D has a `DW_AT_location` attribute, the DWARF expression E from the
		`DW_AT_location` attribute is evaluated with the current context, except
		that the result kind is a location description, the compilation unit is
		the one that contains D, the object is unspecified, and the initial stack
		is empty. RL is the expression result.

		<i>Note that E is evaluated with the context of the expression accessing
		IPV, and not the context of the expression that contained the
		`DW_OP_implicit_pointer` operation that created L.</i>

		- If D has a `DW_AT_const_value` attribute, then an implicit location
		storage RLS is created from the `DW_AT_const_value` attribute's value with
		a size matching the size of the `DW_AT_const_value` attribute's value. RL
		comprises one implicit location description SRL. SRL specifies RLS with a
		bit offset of 0.

		> NOTE: If using `DW_AT_const_value` for variables and formal parameters
		> is deprecated and instead `DW_AT_location` is used with an implicit
		> location description, then this rule would not be required.

		- Otherwise, it is an evaluation error.

		The location description RL is updated by bit offset B scaled by 8 (the byte
		size).

		If a `DW_OP_stack_value` operation pops a value that is the same as IPV,
		then it pushes a location description that is the same as L.

		It is an evaluation error if LS or IPV is accessed in any other manner.

		<i>The restrictions on how an implicit pointer location description created
		by `DW_OP_implicit_pointer` can be used are to simplify the DWARF consumer.
		Similarly, for an implicit pointer value created by `DW_OP_deref*` and
		`DW_OP_stack_value`.</i>

		<i>Typically a `DW_OP_implicit_pointer` operation is used in a DWARF expression
		E<sub>1</sub> of a `DW_TAG_variable` or `DW_TAG_formal_parameter` debugging
		information entry D<sub>1</sub>'s `DW_AT_location` attribute. The debugging
		information entry referenced by the `DW_OP_implicit_pointer` operation is
		typically itself a `DW_TAG_variable` or `DW_TAG_formal_parameter` debugging
		information entry D<sub>2</sub> whose `DW_AT_location` attribute gives a second
		DWARF expression E<sub>2</sub>.</i>

		<i>D<sub>1</sub> and E<sub>1</sub> are describing the location of a pointer type
		object. D<sub>2</sub> and E<sub>2</sub> are describing the location of the
		object pointed to by that pointer object.</i>

		<i>However, D<sub>2</sub> may be any debugging information entry that contains a
		`DW_AT_location` or `DW_AT_const_value` attribute (for example,
		`DW_TAG_dwarf_procedure`). By using E<sub>2</sub>, a consumer can reconstruct
		the value of the object when asked to dereference the pointer described by
		E<sub>1</sub> which contains the `DW_OP_implicit_pointer` operation.</i>

		###### 2.5.4.4.6 Composite Location Description Operations

		> NOTE: This section replaces DWARF Version 5 section 2.6.1.2.

		A composite location storage represents an object or value which may be
		contained in part of another location storage or contained in parts of more than
		one location storage.

		Each part has a part location description L and a part bit size S. L can have
		one or more single location descriptions SL. If there are more than one SL then
		that indicates that part is located in more than one place. The bits of each
		place of the part comprise S contiguous bits from the location storage LS
		specified by SL starting at the bit offset specified by SL. All the bits must be
		within the size of LS or the DWARF expression is ill-formed.

		A composite location storage can have zero or more parts. The parts are
		contiguous such that the zero-based location storage bit index will range over
		each part with no gaps between them. Therefore, the size of a composite location
		storage is the sum of the size of its parts. The DWARF expression is ill-formed
		if the size of the contiguous location storage is larger than the size of the
		memory location storage corresponding to the largest target architecture
		specific address space.

		A composite location description specifies a composite location storage. The bit
		offset corresponds to a bit position within the composite location storage.

		There are operations that create a composite location storage.

		There are other operations that allow a composite location storage to be
		incrementally created. Each part is created by a separate operation. There may
		be one or more operations to create the final composite location storage. A
		series of such operations describes the parts of the composite location storage
		that are in the order that the associated part operations are executed.

		To support incremental creation, a composite location storage can be in an
		incomplete state. When an incremental operation operates on an incomplete
		composite location storage, it adds a new part.

		A composite location description that specifies a composite location storage
		that is incomplete is termed an incomplete composite location description. A
		composite location description that specifies a composite location storage that
		is complete is termed a complete composite location description.

		If the top stack entry is a location description that has one incomplete
		composite location description SL after the execution of an operation expression
		has completed, SL is converted to a complete composite location description.

		<i>Note that this conversion does not happen after the completion of an
		operation expression that is evaluated on the same stack by the `DW_OP_call*`
		operations. Such executions are not a separate evaluation of an operation
		expression, but rather the continued evaluation of the same operation expression
		that contains the `DW_OP_call*` operation.</i>

		If a stack entry is required to be a location description L, but L has an
		incomplete composite location description, then the DWARF expression is
		ill-formed. The exception is for the operations involved in incrementally
		creating a composite location description as described below.

		<i>Note that a DWARF operation expression may arbitrarily compose composite
		location descriptions from any other location description, including those that
		have multiple single location descriptions, and those that have composite
		location descriptions.</i>

		<i>The incremental composite location description operations are defined to be
		compatible with the definitions in DWARF Version 5.</i>

		1. `DW_OP_piece`

		`DW_OP_piece` has a single unsigned LEB128 integer that represents a byte
		size S.

		The action is based on the context:

		- If the stack is empty, then a location description L comprised of one
		incomplete composite location description SL is pushed on the stack.

		An incomplete composite location storage LS is created with a single part
		P. P specifies a location description PL and has a bit size of S scaled by
		8 (the byte size). PL is comprised of one undefined location description
		PSL.

		SL specifies LS with a bit offset of 0.

		- Otherwise, if the top stack entry is a location description L comprised of
		one incomplete composite location description SL, then the incomplete
		composite location storage LS that SL specifies is updated to append a new
		part P. P specifies a location description PL and has a bit size of S
		scaled by 8 (the byte size). PL is comprised of one undefined location
		description PSL. L is left on the stack.
		- Otherwise, if the top stack entry is a location description or can be
		converted to one, then it is popped and treated as a part location
		description PL. Then:

		- If the top stack entry (after popping PL) is a location description L
		comprised of one incomplete composite location description SL, then the
		incomplete composite location storage LS that SL specifies is updated to
		append a new part P. P specifies the location description PL and has a
		bit size of S scaled by 8 (the byte size). L is left on the stack.
		- Otherwise, a location description L comprised of one
		incomplete composite location description SL is pushed on
		the stack.

		An incomplete composite location storage LS is created with a single
		part P. P specifies the location description PL and has a bit size of S
		scaled by 8 (the byte size).

		SL specifies LS with a bit offset of 0.

		- Otherwise, the DWARF expression is ill-formed

		<i>Many compilers store a single variable in sets of registers or store a
		variable partially in memory and partially in registers. `DW_OP_piece`
		provides a way of describing where a part of a variable is located.</i>

		<i>The evaluation rules for the `DW_OP_piece` operation allow it to be
		compatible with the DWARF Version 5 definition.</i>

		> NOTE: Since these extensions allow location descriptions to be entries on
		> the stack, a simpler operation to create composite location descriptions
		> could be defined. For example, just one operation that specifies how many
		> parts, and pops pairs of stack entries for the part size and location
		> description. Not only would this be a simpler operation and avoid the
		> complexities of incomplete composite location descriptions, but it may
		> also have a smaller encoding in practice. However, the desire for
		> compatibility with DWARF Version 5 is likely a stronger consideration.

		2. `DW_OP_bit_piece`

		`DW_OP_bit_piece` has two operands. The first is an unsigned LEB128 integer
		that represents the part bit size S. The second is an unsigned LEB128
		integer that represents a bit displacement B.

		The action is the same as for `DW_OP_piece`, except that any part created
		has the bit size S, and the location description PL of any created part is
		updated by a bit offset B.

		<i>`DW_OP_bit_piece` is used instead of `DW_OP_piece` when the piece to be
		assembled is not byte-sized or is not at the start of the part location
		description.</i>

		#### 2.5.5 DWARF Location List Expressions

		> NOTE: This section replaces DWARF Version 5 section 2.6.2.

		<i>To meet the needs of recent computer architectures and optimization
		techniques, debugging information must be able to describe the location of an
		object whose location changes over the object's lifetime, and may reside at
		multiple locations during parts of an object's lifetime. Location list
		expressions are used in place of operation expressions whenever the object whose
		location is being described has these requirements.</i>

		A location list expression consists of a series of location list entries. Each
		location list entry is one of the following kinds:

		1. <i>Bounded location description</i>

		This kind of location list entry provides an operation expression that
		evaluates to the location description of an object that is valid over a
		lifetime bounded by a starting and ending address. The starting address is
		the lowest address of the address range over which the location is valid.
		The ending address is the address of the first location past the highest
		address of the address range.

		The location list entry matches when the current program location is within
		the given range.

		There are several kinds of bounded location description entries which differ
		in the way that they specify the starting and ending addresses.

		2. <i>Default location description</i>

		This kind of location list entry provides an operation expression that
		evaluates to the location description of an object that is valid when no
		bounded location description entry applies.

		The location list entry matches when the current program location is not
		within the range of any bounded location description entry.

		3. <i>Base address</i>

		This kind of location list entry provides an address to be used as the base
		address for beginning and ending address offsets given in certain kinds of
		bounded location description entries. The applicable base address of a
		bounded location description entry is the address specified by the closest
		preceding base address entry in the same location list. If there is no
		preceding base address entry, then the applicable base address defaults to
		the base address of the compilation unit (see DWARF Version 5 section
		3.1.1).

		In the case of a compilation unit where all of the machine code is contained
		in a single contiguous section, no base address entry is needed.

		4. <i>End-of-list</i>

		This kind of location list entry marks the end of the location list
		expression.

		The address ranges defined by the bounded location description entries of a
		location list expression may overlap. When they do, they describe a situation in
		which an object exists simultaneously in more than one place.

		If all of the address ranges in a given location list expression do not
		collectively cover the entire range over which the object in question is
		defined, and there is no following default location description entry, it is
		assumed that the object is not available for the portion of the range that is
		not covered.

		The result of the evaluation of a DWARF location list expression is:

		- If the current program location is not specified, then it is an evaluation
		error.

		> NOTE: If the location list only has a single default entry, should that be
		> considered a match if there is no program location? If there are non-default
		> entries then it seems it has to be an evaluation error when there is no
		> program location as that indicates the location depends on the program
		> location which is not known.

		- If there are no matching location list entries, then the result is a location
		description that comprises one undefined location description.
		- Otherwise, the operation expression E of each matching location list entry is
		evaluated with the current context, except that the result kind is a location
		description, the object is unspecified, and the initial stack is empty. The
		location list entry result is the location description returned by the
		evaluation of E.

		The result is a location description that is comprised of the union of the
		single location descriptions of the location description result of each
		matching location list entry.

		A location list expression can only be used as the value of a debugger
		information entry attribute that is encoded using class `loclist` or
		`loclistsptr` (see [7.5.5 Classes and Forms](#classes-and-forms)). The value of
		the attribute provides an index into a separate object file section called
		`.debug_loclists` or `.debug_loclists.dwo` (for split DWARF object files) that
		contains the location list entries.

		A `DW_OP_call*` and `DW_OP_implicit_pointer` operation can be used to specify a
		debugger information entry attribute that has a location list expression.
		Several debugger information entry attributes allow DWARF expressions that are
		evaluated with an initial stack that includes a location description that may
		originate from the evaluation of a location list expression.

		<i>This location list representation, the `loclist` and `loclistsptr` class, and
		the related `DW_AT_loclists_base` attribute are new in DWARF Version 5. Together
		they eliminate most, or all of the code object relocations previously needed for
		location list expressions.</i>

		> NOTE: The rest of this section is the same as DWARF Version 5 section 2.6.2.

		## 3 Program Scope Entries

		> NOTE: This section provides changes to existing debugger information entry
		> attributes. These would be incorporated into the corresponding DWARF Version 5
		> chapter 3 sections.

		### 3.3 Subroutine and Entry Point Entries

		#### 3.3.5 Low-Level Information

		1. A `DW_TAG_subprogram`, `DW_TAG_inlined_subroutine`, or `DW_TAG_entry_point`
		debugger information entry may have a `DW_AT_return_addr` attribute, whose
		value is a DWARF expression E.

		The result of the attribute is obtained by evaluating E with a context that
		has a result kind of a location description, an unspecified object, the
		compilation unit that contains E, an empty initial stack, and other context
		elements corresponding to the source language thread of execution upon which
		the user is focused, if any. The result of the evaluation is the location
		description L of the place where the return address for the current call
		frame's subprogram or entry point is stored.

		The DWARF is ill-formed if L is not comprised of one memory location
		description for one of the target architecture specific address spaces.

		> NOTE: It is unclear why `DW_TAG_inlined_subroutine` has a
		> `DW_AT_return_addr` attribute but not a `DW_AT_frame_base` or
		> `DW_AT_static_link` attribute. Seems it would either have all of them or
		> none. Since inlined subprograms do not have a call frame it seems they
		> would have none of these attributes.

		2. A `DW_TAG_subprogram` or `DW_TAG_entry_point` debugger information entry may
		have a `DW_AT_frame_base` attribute, whose value is a DWARF expression E.

		The result of the attribute is obtained by evaluating E with a context that
		has a result kind of a location description, an unspecified object, the
		compilation unit that contains E, an empty initial stack, and other context
		elements corresponding to the source language thread of execution upon which
		the user is focused, if any.

		The DWARF is ill-formed if E contains an `DW_OP_fbreg` operation, or the
		resulting location description L is not comprised of one single location
		description SL.

		If SL is a register location description for register R, then L is replaced
		with the result of evaluating a `DW_OP_bregx R, 0` operation. This computes
		the frame base memory location description in the target architecture
		default address space.

		<i>This allows the more compact `DW_OP_reg*` to be used instead of
		`DW_OP_breg* 0`.</i>

		> NOTE: This rule could be removed and require the producer to create the
		> required location description directly using `DW_OP_call_frame_cfa` or
		> `DW_OP_breg*`. This would also then allow a target to implement the call
		> frames within a large register.

		Otherwise, the DWARF is ill-formed if SL is not a memory location
		description in any of the target architecture specific address spaces.

		The resulting L is the <i>frame base</i> for the subprogram or entry point.

		<i>Typically, E will use the `DW_OP_call_frame_cfa` operation or be a stack
		pointer register plus or minus some offset.</i>

		3. If a `DW_TAG_subprogram` or `DW_TAG_entry_point` debugger information entry
		is lexically nested, it may have a `DW_AT_static_link` attribute, whose
		value is a DWARF expression E.

		The result of the attribute is obtained by evaluating E with a context that
		has a result kind of a location description, an unspecified object, the
		compilation unit that contains E, an empty initial stack, and other context
		elements corresponding to the source language thread of execution upon which
		the user is focused, if any. The result of the evaluation is the location
		description L of the <i>canonical frame address</i> (see [6.4 Call Frame
		Information](#call-frame-information)) of the relevant call frame of the
		subprogram instance that immediately lexically encloses the current call
		frame's subprogram or entry point.

		The DWARF is ill-formed if L is is not comprised of one memory location
		description for one of the target architecture specific address spaces.

		### 3.4 Call Site Entries and Parameters

		#### 3.4.2 Call Site Parameters

		1. A `DW_TAG_call_site_parameter` debugger information entry may have a
		`DW_AT_call_value` attribute, whose value is a DWARF operation expression
		E<sub>1</sub>.

		The result of the `DW_AT_call_value` attribute is obtained by evaluating
		E<sub>1</sub> with a context that has a result kind of a value, an unspecified
		object, the compilation unit that contains E, an empty initial stack, and other
		context elements corresponding to the source language thread of execution upon
		which the user is focused, if any. The resulting value V<sub>1</sub> is the
		value of the parameter at the time of the call made by the call site.

		For parameters passed by reference, where the code passes a pointer to a
		location which contains the parameter, or for reference type parameters, the
		`DW_TAG_call_site_parameter` debugger information entry may also have a
		`DW_AT_call_data_location` attribute whose value is a DWARF operation expression
		E<sub>2</sub>, and a `DW_AT_call_data_value` attribute whose value is a DWARF
		operation expression E<sub>3</sub>.

		The value of the `DW_AT_call_data_location` attribute is obtained by evaluating
		E<sub>2</sub> with a context that has a result kind of a location description,
		an unspecified object, the compilation unit that contains E, an empty initial
		stack, and other context elements corresponding to the source language thread of
		execution upon which the user is focused, if any. The resulting location
		description L<sub>2</sub> is the location where the referenced parameter lives
		during the call made by the call site. If E<sub>2</sub> would just be a
		`DW_OP_push_object_address`, then the `DW_AT_call_data_location` attribute may
		be omitted.

		> NOTE: The DWARF Version 5 implies that `DW_OP_push_object_address` may be
		> used but does not state what object must be specified in the context.
		> Either `DW_OP_push_object_address` cannot be used, or the object to be
		> passed in the context must be defined.

		The value of the `DW_AT_call_data_value` attribute is obtained by evaluating
		E<sub>3</sub> with a context that has a result kind of a value, an unspecified
		object, the compilation unit that contains E, an empty initial stack, and other
		context elements corresponding to the source language thread of execution upon
		which the user is focused, if any. The resulting value V<sub>3</sub> is the
		value in L<sub>2</sub> at the time of the call made by the call site.

		The result of these attributes is undefined if the current call frame is not for
		the subprogram containing the `DW_TAG_call_site_parameter` debugger information
		entry or the current program location is not for the call site containing the
		`DW_TAG_call_site_parameter` debugger information entry in the current call
		frame.

		<i>The consumer may have to virtually unwind to the call site (see [6.4 Call
		Frame Information](#call-frame-information)) in order to evaluate these
		attributes. This will ensure the source language thread of execution upon which
		the user is focused corresponds to the call site needed to evaluate the
		expression.</i>

		If it is not possible to avoid the expressions of these attributes from
		accessing registers or memory locations that might be clobbered by the
		subprogram being called by the call site, then the associated attribute should
		not be provided.

		<i>The reason for the restriction is that the parameter may need to be accessed
		during the execution of the callee. The consumer may virtually unwind from the
		called subprogram back to the caller and then evaluate the attribute
		expressions. The call frame information (see [6.4 Call Frame
		Information](#call-frame-information)) will not be able to restore registers
		that have been clobbered, and clobbered memory will no longer have the value at
		the time of the call.</i>

		### 3.5 Lexical Block Entries

		> NOTE: This section is the same as DWARF Version 5 section 3.5.

		## 4 Data Object and Object List Entries

		> NOTE: This section provides changes to existing debugger information entry
		> attributes. These would be incorporated into the corresponding DWARF Version 5
		> chapter 4 sections.

		### 4.1 Data Object Entries

		1. Any debugging information entry describing a data object (which includes
		variables and parameters) or common blocks may have a `DW_AT_location`
		attribute, whose value is a DWARF expression E.

		The result of the attribute is obtained by evaluating E with a context that
		has a result kind of a location description, an unspecified object, the
		compilation unit that contains E, an empty initial stack, and other context
		elements corresponding to the source language thread of execution upon which
		the user is focused, if any. The result of the evaluation is the location
		description of the base of the data object.

		See [2.5.4.2 Control Flow Operations](#control-flow-operations) for special
		evaluation rules used by the `DW_OP_call*` operations.

		> NOTE: Delete the description of how the `DW_OP_call*` operations evaluate
		> a `DW_AT_location` attribute as that is now described in the operations.

		> NOTE: See the discussion about the `DW_AT_location` attribute in the
		> `DW_OP_call*` operation. Having each attribute only have a single purpose
		> and single execution semantics seems desirable. It makes it easier for the
		> consumer that no longer have to track the context. It makes it easier for
		> the producer as it can rely on a single semantics for each attribute.
		>
		> For that reason, limiting the `DW_AT_location` attribute to only
		> supporting evaluating the location description of an object, and using a
		> different attribute and encoding class for the evaluation of DWARF
		> expression <i>procedures</i> on the same operation expression stack seems
		> desirable.

		2. `DW_AT_const_value`

		> NOTE: Could deprecate using the `DW_AT_const_value` attribute for
		> `DW_TAG_variable` or `DW_TAG_formal_parameter` debugger information
		> entries that have been optimized to a constant. Instead, `DW_AT_location`
		> could be used with a DWARF expression that produces an implicit location
		> description now that any location description can be used within a DWARF
		> expression. This allows the `DW_OP_call*` operations to be used to push
		> the location description of any variable regardless of how it is
		> optimized.

		## 5 Type Entries

		> NOTE: This section provides changes to existing debugger information entry
		> attributes. These would be incorporated into the corresponding DWARF Version 5
		> chapter 5 sections.

		### 5.7 Structure, Union, Class and Interface Type Entries

		#### 5.7.3 Derived or Extended Structures, Classes and Interfaces

		1. For a `DW_AT_data_member_location` attribute there are two cases:

		1. If the attribute is an integer constant B, it provides the offset in
		bytes from the beginning of the containing entity.

		The result of the attribute is obtained by updating the bit offset of
		the location description of the beginning of the containing entity by B
		scaled by 8 (the byte size). The result is the location description of
		the base of the member entry.

		<i>If the beginning of the containing entity is not byte aligned, then
		the beginning of the member entry has the same bit displacement within a
		byte.</i>

		2. Otherwise, the attribute must be a DWARF expression E which is evaluated
		with a context that has a result kind of a location description, an
		unspecified object, the compilation unit that contains E, an initial
		stack comprising the location description of the beginning of the
		containing entity, and other context elements corresponding to the
		source language thread of execution upon which the user is focused, if
		any. The result of the evaluation is the location description of the
		base of the member entry.

		> NOTE: The beginning of the containing entity can now be any location
		> description, including those with more than one single location
		> description, and those with single location descriptions that are of any
		> kind and have any bit offset.

		#### 5.7.8 Member Function Entries

		1. An entry for a virtual function also has a `DW_AT_vtable_elem_location`
		attribute whose value is a DWARF expression E.

		The result of the attribute is obtained by evaluating E with a context that
		has a result kind of a location description, an unspecified object, the
		compilation unit that contains E, an initial stack comprising the location
		description of the object of the enclosing type, and other context elements
		corresponding to the source language thread of execution upon which the user
		is focused, if any. The result of the evaluation is the location description
		of the slot for the function within the virtual function table for the
		enclosing class.

		### 5.14 Pointer to Member Type Entries

		1. The `DW_TAG_ptr_to_member_type` debugging information entry has a
		`DW_AT_use_location` attribute whose value is a DWARF expression E. It is used
		to compute the location description of the member of the class to which the
		pointer to member entry points.

		<i>The method used to find the location description of a given member of a
		class, structure, or union is common to any instance of that class, structure,
		or union and to any instance of the pointer to member type. The method is thus
		associated with the pointer to member type, rather than with each object that
		has a pointer to member type.</i>

		The `DW_AT_use_location` DWARF expression is used in conjunction with the
		location description for a particular object of the given pointer to member type
		and for a particular structure or class instance.

		The result of the attribute is obtained by evaluating E with a context that has
		a result kind of a location description, an unspecified object, the compilation
		unit that contains E, an initial stack comprising two entries, and other context
		elements corresponding to the source language thread of execution upon which the
		user is focused, if any. The first stack entry is the value of the pointer to
		member object itself. The second stack entry is the location description of the
		base of the entire class, structure, or union instance containing the member
		whose location is being calculated. The result of the evaluation is the location
		description of the member of the class to which the pointer to member entry
		points.

		### 5.16 Dynamic Type Entries

		1. The `DW_AT_data_location` attribute may be used with any type that provides one
		or more levels of hidden indirection and/or run-time parameters in its
		representation. Its value is a DWARF operation expression E which computes the
		location description of the data for an object. When this attribute is omitted,
		the location description of the data is the same as the location description of
		the object.

		The result of the attribute is obtained by evaluating E with a context that has
		a result kind of a location description, an object that is the location
		description of the data descriptor, the compilation unit that contains E, an
		empty initial stack, and other context elements corresponding to the source
		language thread of execution upon which the user is focused, if any. The result
		of the evaluation is the location description of the base of the member entry.

		<i>E will typically involve an operation expression that begins with a
		`DW_OP_push_object_address` operation which loads the location description
		of the object which can then serve as a descriptor in subsequent
		calculation.</i>

		> NOTE: Since `DW_AT_data_member_location`, `DW_AT_use_location`, and
		> `DW_AT_vtable_elem_location` allow both operation expressions and location
		> list expressions, why does `DW_AT_data_location` not allow both? In all cases
		> they apply to data objects so less likely that optimization would cause
		> different operation expressions for different program location ranges. But if
		> supporting for some then should be for all.
		>
		> It seems odd this attribute is not the same as `DW_AT_data_member_location` in
		> having an initial stack with the location description of the object since the
		> expression has to need it.

		## 6 Other Debugging Information

		> NOTE: This section provides changes to existing debugger information entry
		> attributes. These would be incorporated into the corresponding DWARF Version 5
		> chapter 6 sections.

		### 6.2 Line Number Information

		> NOTE: This section is the same as DWARF Version 5 section 6.2.

		### 6.4 Call Frame Information

		> NOTE: This section provides changes to DWARF Version 5 section 6.4. Register
		> unwind DWARF expressions are generalized to allow any location description,
		> including those with composite and implicit location descriptions.

		#### 6.4.1 Structure of Call Frame Information

		The register rules are:

		1. <i>undefined</i>

		A register that has this rule has no recoverable value in the previous
		frame. The previous value of this register is the undefined location
		description (see [2.5.4.4.2 Undefined Location Description
		Operations](#undefined-location-description-operations)).

		<i>By convention, the register is not preserved by a callee.</i>

		2. <i>same value</i>

		This register has not been modified from the previous caller frame.

		If the current frame is the top frame, then the previous value of this
		register is the location description L that specifies one register location
		description SL. SL specifies the register location storage that corresponds
		to the register with a bit offset of 0 for the current thread.

		If the current frame is not the top frame, then the previous value of this
		register is the location description obtained using the call frame
		information for the callee frame and callee program location invoked by the
		current caller frame for the same register.

		<i>By convention, the register is preserved by the callee, but the callee
		has not modified it.</i>

		3. <i>offset(N)</i>

		N is a signed byte offset. The previous value of this register is saved at
		the location description L. Where L is the location description of the
		current CFA (see [2.5.4 DWARF Operation
		Expressions](#dwarf-operation-expressions)) updated with the bit offset N
		scaled by 8 (the byte size).

		4. <i>val_offset(N)</i>

		N is a signed byte offset. The previous value of this register is the memory
		byte address of the location description L. Where L is the location
		description of the current CFA (see [2.5.4 DWARF Operation
		Expressions](#dwarf-operation-expressions)) updated with the bit offset N
		scaled by 8 (the byte size).

		The DWARF is ill-formed if the CFA location description is not a memory byte
		address location description, or if the register size does not match the
		size of an address in the target architecture default address space.

		<i>Since the CFA location description is required to be a memory byte
		address location description, the value of val_offset(N) will also be a
		memory byte address location description since it is offsetting the CFA
		location description by N bytes. Furthermore, the value of val_offset(N)
		will be a memory byte address in the target architecture default address
		space.</i>

		> NOTE: Should DWARF allow the address size to be a different size to the
		> size of the register? Requiring them to be the same bit size avoids any
		> issue of conversion as the bit contents of the register is simply
		> interpreted as a value of the address.
		>
		> GDB has a per register hook that allows a target specific conversion on a
		> register by register basis. It defaults to truncation of bigger registers,
		> and to actually reading bytes from the next register (or reads out of
		> bounds for the last register) for smaller registers. There are no GDB
		> tests that read a register out of bounds (except an illegal hand written
		> assembly test).

		5. <i>register(R)</i>

		This register has been stored in another register numbered R.

		The previous value of this register is the location description obtained
		using the call frame information for the current frame and current program
		location for register R.

		The DWARF is ill-formed if the size of this register does not match the size
		of register R or if there is a cyclic dependency in the call frame
		information.

		> NOTE: Should this also allow R to be larger than this register? If so is
		> the value stored in the low order bits and it is undefined what is stored
		> in the extra upper bits?

		6. <i>expression(E)</i>

		The previous value of this register is located at the location description
		produced by evaluating the DWARF operation expression E (see [2.5.4 DWARF
		Operation Expressions](#dwarf-operation-expressions)).

		E is evaluated with the current context, except the result kind is a
		location description, the compilation unit is unspecified, the object is
		unspecified, and an initial stack comprising the location description of the
		current CFA (see [2.5.4 DWARF Operation
		Expressions](#dwarf-operation-expressions)).

		7. <i>val_expression(E)</i>

		The previous value of this register is the value produced by evaluating the
		DWARF operation expression E (see [2.5.4 DWARF Operation
		Expressions](#dwarf-operation-expressions)).

		E is evaluated with the current context, except the result kind is a
		value, the compilation unit is unspecified, the object is
		unspecified, and an initial stack comprising the location
		description of the current CFA (see [2.5.4 DWARF Operation Expressions](#dwarf-operation-expressions)).

		The DWARF is ill-formed if the resulting value type size does not match the
		register size.

		> NOTE: This has limited usefulness as the DWARF expression E can only
		> produce values up to the size of the generic type. This is due to not
		> allowing any operations that specify a type in a CFI operation expression.
		> This makes it unusable for registers that are larger than the generic
		> type. However, <i>expression(E)</i> can be used to create an implicit
		> location description of any size.

		8. <i>architectural</i>

		The rule is defined externally to this specification by the augmenter.

		A Common Information Entry (CIE) holds information that is shared among many
		Frame Description Entries (FDE). There is at least one CIE in every non-empty
		`.debug_frame` section. A CIE contains the following fields, in order:

		1. `length` (initial length)

		A constant that gives the number of bytes of the CIE structure, not
		including the length field itself. The size of the length field plus the
		value of length must be an integral multiple of the address size specified
		in the `address_size` field.

		2. `CIE_id` (4 or 8 bytes, see [7.4 32-Bit and 64-Bit DWARF
		Formats](#32-bit-and-64-bit-dwarf-formats))

		A constant that is used to distinguish CIEs from FDEs.

		In the 32-bit DWARF format, the value of the CIE id in the CIE header is
		0xffffffff; in the 64-bit DWARF format, the value is 0xffffffffffffffff.

		3. `version` (ubyte)

		A version number. This number is specific to the call frame information and
		is independent of the DWARF version number.

		The value of the CIE version number is 4.

		> NOTE: Would this be increased to 5 to reflect the changes in these
		> extensions?

		4. `augmentation` (sequence of UTF-8 characters)

		A null-terminated UTF-8 string that identifies the augmentation to this CIE
		or to the FDEs that use it. If a reader encounters an augmentation string
		that is unexpected, then only the following fields can be read:

		- CIE: length, CIE_id, version, augmentation
		- FDE: length, CIE_pointer, initial_location, address_range

		If there is no augmentation, this value is a zero byte.

		<i>The augmentation string allows users to indicate that there is additional
		vendor and target architecture specific information in the CIE or FDE which
		is needed to virtually unwind a stack frame. For example, this might be
		information about dynamically allocated data which needs to be freed on exit
		from the routine.</i>

		<i>Because the `.debug_frame` section is useful independently of any
		`.debug_info` section, the augmentation string always uses UTF-8
		encoding.</i>

		5. `address_size` (ubyte)

		The size of a target address in this CIE and any FDEs that use it, in bytes.
		If a compilation unit exists for this frame, its address size must match the
		address size here.

		6. `segment_selector_size` (ubyte)

		The size of a segment selector in this CIE and any FDEs that use it, in
		bytes.

		7. `code_alignment_factor` (unsigned LEB128)

		A constant that is factored out of all advance location instructions (see
		[6.4.2.1 Row Creation Instructions](#row-creation-instructions)). The
		resulting value is `(operand * code_alignment_factor)`.

		8. `data_alignment_factor` (signed LEB128)

		A constant that is factored out of certain offset instructions (see [6.4.2.2
		CFA Definition Instructions](#cfa-definition-instructions) and [6.4.2.3
		Register Rule Instructions](#register-rule-instructions)). The
		resulting value is `(operand * data_alignment_factor)`.

		9. `return_address_register` (unsigned LEB128)

		An unsigned LEB128 constant that indicates which column in the rule table
		represents the return address of the subprogram. Note that this column might
		not correspond to an actual machine register.

		The value of the return address register is used to determine the program
		location of the caller frame. The program location of the top frame is the
		target architecture program counter value of the current thread.

		10. `initial_instructions` (array of ubyte)

		A sequence of rules that are interpreted to create the initial setting of
		each column in the table.

		The default rule for all columns before interpretation of the initial
		instructions is the undefined rule. However, an ABI authoring body or a
		compilation system authoring body may specify an alternate default value for
		any or all columns.

		11. `padding` (array of ubyte)

		Enough `DW_CFA_nop` instructions to make the size of this entry match the
		length value above.

		An FDE contains the following fields, in order:

		1. `length` (initial length)

		A constant that gives the number of bytes of the header and instruction
		stream for this subprogram, not including the length field itself. The size
		of the length field plus the value of length must be an integral multiple of
		the address size.

		2. `CIE_pointer` (4 or 8 bytes, see [7.4 32-Bit and 64-Bit DWARF
		Formats](#32-bit-and-64-bit-dwarf-formats))

		A constant offset into the `.debug_frame` section that denotes the CIE that
		is associated with this FDE.

		3. `initial_location` (segment selector and target address)

		The address of the first location associated with this table entry. If the
		segment_selector_size field of this FDE's CIE is non-zero, the initial
		location is preceded by a segment selector of the given length.

		4. `address_range` (target address)

		The number of bytes of program instructions described by this entry.

		5. `instructions` (array of ubyte)

		A sequence of table defining instructions that are described in [6.4.2 Call
		Frame Instructions](#call-frame-instructions).

		6. `padding` (array of ubyte)

		Enough `DW_CFA_nop` instructions to make the size of this entry match the
		length value above.

		#### 6.4.2 Call Frame Instructions

		Some call frame instructions have operands that are encoded as DWARF operation
		expressions E (see [2.5.4 DWARF Operation
		Expressions](#dwarf-operation-expressions)). The DWARF operations that can
		be used in E have the following restrictions:

		- `DW_OP_addrx`, `DW_OP_call2`, `DW_OP_call4`, `DW_OP_call_ref`,
		`DW_OP_const_type`, `DW_OP_constx`, `DW_OP_convert`, `DW_OP_deref_type`,
		`DW_OP_fbreg`, `DW_OP_implicit_pointer`, `DW_OP_regval_type`,
		`DW_OP_reinterpret`, and `DW_OP_xderef_type` operations are not allowed
		because the call frame information must not depend on other debug sections.
		- `DW_OP_push_object_address` is not allowed because there is no object context
		to provide a value to push.
		- `DW_OP_call_frame_cfa` and `DW_OP_entry_value` are not allowed because their
		use would be circular.

		<i>Call frame instructions to which these restrictions apply include
		`DW_CFA_def_cfa_expression`, `DW_CFA_expression`, and
		`DW_CFA_val_expression`.</i>

		##### 6.4.2.1 Row Creation Instructions

		> NOTE: These instructions are the same as in DWARF Version 5 section 6.4.2.1.

		##### 6.4.2.2 CFA Definition Instructions

		1. `DW_CFA_def_cfa`

		The `DW_CFA_def_cfa` instruction takes two unsigned LEB128 operands
		representing a register number R and a (non-factored) byte displacement B.
		The required action is to define the current CFA rule to be the result of
		evaluating the DWARF operation expression `DW_OP_bregx R, B` as a location
		description.

		2. `DW_CFA_def_cfa_sf`

		The `DW_CFA_def_cfa_sf` instruction takes two operands: an unsigned LEB128
		value representing a register number R and a signed LEB128 factored byte
		displacement B. The required action is to define the current CFA rule to be
		the result of evaluating the DWARF operation expression `DW_OP_bregx R, B *
		data_alignment_factor` as a location description.

		<i>The action is the same as `DW_CFA_def_cfa`, except that the second
		operand is signed and factored.</i>

		3. `DW_CFA_def_cfa_register`

		The `DW_CFA_def_cfa_register` instruction takes a single unsigned LEB128
		operand representing a register number R. The required action is to define
		the current CFA rule to be the result of evaluating the DWARF operation
		expression `DW_OP_bregx R, B` as a location description. B is the old CFA
		byte displacement.

		If the subprogram has no current CFA rule, or the rule was defined by a
		`DW_CFA_def_cfa_expression` instruction, then the DWARF is ill-formed.

		4. `DW_CFA_def_cfa_offset`

		The `DW_CFA_def_cfa_offset` instruction takes a single unsigned LEB128
		operand representing a (non-factored) byte displacement B. The required
		action is to define the current CFA rule to be the result of evaluating the
		DWARF operation expression `DW_OP_bregx R, B` as a location description. R
		is the old CFA register number.

		If the subprogram has no current CFA rule, or the rule was defined by a
		`DW_CFA_def_cfa_expression` instruction, then the DWARF is ill-formed.

		5. `DW_CFA_def_cfa_offset_sf`

		The `DW_CFA_def_cfa_offset_sf` instruction takes a signed LEB128 operand
		representing a factored byte displacement B. The required action is to
		define the current CFA rule to be the result of evaluating the DWARF
		operation expression `DW_OP_bregx R, B * data_alignment_factor` as a
		location description. R is the old CFA register number.

		If the subprogram has no current CFA rule, or the rule was defined by a
		`DW_CFA_def_cfa_expression` instruction, then the DWARF is ill-formed.

		<i>The action is the same as `DW_CFA_def_cfa_offset`, except that the
		operand is signed and factored.</i>

		6. `DW_CFA_def_cfa_expression`

		The `DW_CFA_def_cfa_expression` instruction takes a single operand encoded
		as a `DW_FORM_exprloc` value representing a DWARF operation expression E.
		The required action is to define the current CFA rule to be the result of
		evaluating E with the current context, except the result kind is a location
		description, the compilation unit is unspecified, the object is unspecified,
		and an empty initial stack.

		<i>See [6.4.2 Call Frame Instructions](#call-frame-instructions)
		regarding restrictions on the DWARF expression operations that can be used
		in E.</i>

		The DWARF is ill-formed if the result of evaluating E is not a memory byte
		address location description.

		##### 6.4.2.3 Register Rule Instructions

		1. `DW_CFA_undefined`

		The `DW_CFA_undefined` instruction takes a single unsigned LEB128 operand
		that represents a register number R. The required action is to set the rule
		for the register specified by R to `undefined`.

		2. `DW_CFA_same_value`

		The `DW_CFA_same_value` instruction takes a single unsigned LEB128 operand
		that represents a register number R. The required action is to set the rule
		for the register specified by R to `same value`.

		3. `DW_CFA_offset`

		The `DW_CFA_offset` instruction takes two operands: a register number R
		(encoded with the opcode) and an unsigned LEB128 constant representing a
		factored displacement B. The required action is to change the rule for the
		register specified by R to be an <i>offset(B * data_alignment_factor)</i>
		rule.

		> NOTE: Seems this should be named `DW_CFA_offset_uf` since the offset is
		> unsigned factored.

		4. `DW_CFA_offset_extended`

		The `DW_CFA_offset_extended` instruction takes two unsigned LEB128 operands
		representing a register number R and a factored displacement B. This
		instruction is identical to `DW_CFA_offset`, except for the encoding and
		size of the register operand.

		> NOTE: Seems this should be named `DW_CFA_offset_extended_uf` since the
		> displacement is unsigned factored.

		5. `DW_CFA_offset_extended_sf`

		The `DW_CFA_offset_extended_sf` instruction takes two operands: an unsigned
		LEB128 value representing a register number R and a signed LEB128 factored
		displacement B. This instruction is identical to `DW_CFA_offset_extended`,
		except that B is signed.

		6. `DW_CFA_val_offset`

		The `DW_CFA_val_offset` instruction takes two unsigned LEB128 operands
		representing a register number R and a factored displacement B. The required
		action is to change the rule for the register indicated by R to be a
		<i>val_offset(B * data_alignment_factor)</i> rule.

		> NOTE: Seems this should be named `DW_CFA_val_offset_uf` since the
		displacement is unsigned factored.

		7. `DW_CFA_val_offset_sf`

		The `DW_CFA_val_offset_sf` instruction takes two operands: an unsigned
		LEB128 value representing a register number R and a signed LEB128 factored
		displacement B. This instruction is identical to `DW_CFA_val_offset`, except
		that B is signed.

		8. `DW_CFA_register`

		The `DW_CFA_register` instruction takes two unsigned LEB128 operands
		representing register numbers R1 and R2 respectively. The required action is
		to set the rule for the register specified by R1 to be a <i>register(R2)</i>
		rule.

		9. `DW_CFA_expression`

		The `DW_CFA_expression` instruction takes two operands: an unsigned LEB128
		value representing a register number R, and a `DW_FORM_block` value
		representing a DWARF operation expression E. The required action is to
		change the rule for the register specified by R to be an
		<i>expression(E)</i> rule.

		<i>That is, E computes the location description where the register value can
		be retrieved.</i>

		<i>See [6.4.2 Call Frame Instructions](#call-frame-instructions)
		regarding restrictions on the DWARF expression operations that can be used
		in E.</i>

		10. `DW_CFA_val_expression`

		The `DW_CFA_val_expression` instruction takes two operands: an unsigned
		LEB128 value representing a register number R, and a `DW_FORM_block` value
		representing a DWARF operation expression E. The required action is to
		change the rule for the register specified by R to be a
		<i>val_expression(E)</i> rule.

		<i>That is, E computes the value of register R.</i>

		<i>See [6.4.2 Call Frame Instructions](#call-frame-instructions)
		regarding restrictions on the DWARF expression operations that can be used
		in E.</i>

		If the result of evaluating E is not a value with a base type size that
		matches the register size, then the DWARF is ill-formed.

		11. `DW_CFA_restore`

		The `DW_CFA_restore` instruction takes a single operand (encoded with the
		opcode) that represents a register number R. The required action is to
		change the rule for the register specified by R to the rule assigned it by
		the `initial_instructions` in the CIE.

		12. `DW_CFA_restore_extended`

		The `DW_CFA_restore_extended` instruction takes a single unsigned LEB128
		operand that represents a register number R. This instruction is identical
		to `DW_CFA_restore`, except for the encoding and size of the register
		operand.

		##### 6.4.2.4 Row State Instructions

		> NOTE: These instructions are the same as in DWARF Version 5 section 6.4.2.4.

		##### 6.4.2.5 Padding Instruction

		> NOTE: These instructions are the same as in DWARF Version 5 section 6.4.2.5.

		#### 6.4.3 Call Frame Instruction Usage

		> NOTE: The same as in DWARF Version 5 section 6.4.3.

		#### 6.4.4 Call Frame Calling Address

		> NOTE: The same as in DWARF Version 5 section 6.4.4.

		## 7 Data Representation

		> NOTE: This section provides changes to existing debugger information entry
		> attributes. These would be incorporated into the corresponding DWARF Version 5
		> chapter 7 sections.

		### 7.4 32-Bit and 64-Bit DWARF Formats

		> NOTE: This augments DWARF Version 5 section 7.4 list item 3's table.

		Form Role
		------------------------ --------------------------------------
		DW_OP_implicit_pointer offset in `.debug_info`

		### 7.5 Format of Debugging Information

		#### 7.5.5 Classes and Forms

		> NOTE: The same as in DWARF Version 5 section 7.5.5.

		### 7.7 DWARF Expressions

		> NOTE: Rename DWARF Version 5 section 7.7 to reflect the unification of
		> location descriptions into DWARF expressions.

		#### 7.7.1 Operation Expressions

		> NOTE: Rename DWARF Version 5 section 7.7.1 and delete section 7.7.2 to reflect
		> the unification of location descriptions into DWARF expressions.

		#### 7.7.3 Location List Expressions

		> NOTE: Rename DWARF Version 5 section 7.7.3 to reflect that location lists are
		> a kind of DWARF expression.

# Further Information		# Further Information

The following references provide additional information on the extension.		The following references provide additional information on the extension.

Slides and a video of a presentation at the Linux Plumbers Conference 2021		Slides and a video of a presentation at the Linux Plumbers Conference 2021
related to this extension are available.		related to this extension are available.

The LLVM compiler extension includes possible normative text changes for this		The LLVM compiler extension includes the operations mentioned in the motivating
extension as well as the operations mentioned in the motivating examples. It		examples. It also covers other extensions needed for heterogeneous devices.
also covers other extensions needed for heterogeneous devices.

- DWARF extensions for optimized SIMT/SIMD (GPU) debugging - Linux Plumbers Conference 2021		- DWARF extensions for optimized SIMT/SIMD (GPU) debugging - Linux Plumbers Conference 2021
- [Video](https://www.youtube.com/watch?v=QiR0ra0ymEY&t=10015s)		- [Video](https://www.youtube.com/watch?v=QiR0ra0ymEY&t=10015s)
- [Slides](https://linuxplumbersconf.org/event/11/contributions/1012/attachments/798/1505/DWARF_Extensions_for_Optimized_SIMT-SIMD_GPU_Debugging-LPC2021.pdf)		- [Slides](https://linuxplumbersconf.org/event/11/contributions/1012/attachments/798/1505/DWARF_Extensions_for_Optimized_SIMT-SIMD_GPU_Debugging-LPC2021.pdf)
- [DWARF Extensions For Heterogeneous Debugging](https://llvm.org/docs/AMDGPUDwarfExtensionsForHeterogeneousDebugging.html)		- [DWARF Extensions For Heterogeneous Debugging](https://llvm.org/docs/AMDGPUDwarfExtensionsForHeterogeneousDebugging.html)

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