[DebugInfo][Docs] Improve code formatting in instruction referencing doc

[jryans]

Summary

This adds code blocks and inline code formatting to improve the readability of
the instruction referencing doc.

[Orlando]

Hi @jryans, thanks for this update. I'm not particularly familiar with reStructuredText formatting so I've just been looking at an existing doc (https://github.com/llvm/llvm-project/blob/main/llvm/docs/LangRef.rst) to guide my review.

llvm/docs/InstrRefDebugInfo.md

25–28

I'm not particularly familiar with reStructuredText formatting at all - do triple backticks format a code block? I can't see them used in https://github.com/llvm/llvm-project/blob/main/llvm/docs/LangRef.rst. It looks like that document uses .. code-block::, as in:

.. code-block:: llvm

%2 = add i32 %0, %1
call void @llvm.dbg.value(metadata i32 %2,

48

It looks like double backticks (rather than single) are used to format code inline in reStructuredText.

[jryans]

Thanks for taking a look @Orlando!

Unlike most historical LLVM docs, this file is using Markdown, which ends up rendered together with the reStructuredText docs by Sphinx. Markdown is recommended for new docs.

With Markdown in mind, please take another look when you have time. 🙂

[Orlando]

In D126767#3562844, @jryans wrote:

Thanks for taking a look @Orlando!

Unlike most historical LLVM docs, this file is using Markdown, which ends up rendered together with the reStructuredText docs by Sphinx. Markdown is recommended for new docs.

With Markdown in mind, please take another look when you have time. 🙂

Thanks for the link, I had completely missed that (and managed to miss the .md file extension - oops!). Yep this LGTM, thanks.

[jryans]

Thanks for the review! 😄

Since this is a pretty clear improvement and docs-only, I'll go ahead and land it now without waiting for further comments.

llvm/docs/InstrRefDebugInfo.md

 
 # Solution: instruction referencing
 
 Rather than identify the virtual register that a variable value resides in,
 instead in instruction referencing mode, LLVM refers to the machine instruction
 and operand position that the value is defined in. Consider the LLVM IR way of
 referring to instruction values:
 
-  %2 = add i32 %0, %1
-  call void @llvm.dbg.value(metadata i32 %2,
+```llvm
+```
 
 In LLVM IR, the IR Value is synonymous with the instruction that computes the
 value, to the extent that in memory a Value is a pointer to the computing
 instruction. Instruction referencing implements this relationship in the
 codegen backend of LLVM, after instruction selection. Consider the X86 assembly
 below and instruction referencing debug info, corresponding to the earlier
 LLVM IR:
 
-  %2:gr32 = ADD32rr %0, %1, implicit-def $eflags, debug-instr-number 1
-  DBG_INSTR_REF 1, 0, !123, !456, debug-location !789
+```text
+```
 
-While the function remains in SSA form, virtual register %2 is sufficient to
+While the function remains in SSA form, virtual register `%2` is sufficient to
 identify the value computed by the instruction -- however the function
 eventually leaves SSA form, and register optimisations will obscure which
 register the desired value is in. Instead, a more consistent way of identifying
-the instruction's value is to refer to the MachineOperand where the value is
-defined: independently of which register is defined by that MachineOperand. In
-the code above, the DBG_INSTR_REF instruction refers to instruction number one,
-operand zero, while the ADD32rr has a debug-instr-number attribute attached
-indicating that it is instruction number one.
+the instruction's value is to refer to the `MachineOperand` where the value is
+defined: independently of which register is defined by that `MachineOperand`. In
+the code above, the `DBG_INSTR_REF` instruction refers to instruction number
+one, operand zero, while the `ADD32rr` has a `debug-instr-number` attribute
+attached indicating that it is instruction number one.
 
 De-coupling variable locations from registers avoids difficulties involving
 register allocation and optimisation, but requires additional instrumentation
 when the instructions are optimised instead. Optimisations that replace
 instructions with optimised versions that compute the same value must either
 preserve the instruction number, or record a substitution from the old
 instruction / operand number pair to the new instruction / operand pair -- see
-MachineFunction::substituteDebugValuesForInst. If debug info maintenance is not
-performed, or an instruction is eliminated as dead code, the variable location
-is safely dropped and marked "optimised out". The exception is instructions
-that are mutated rather than replaced, which always need debug info
+`MachineFunction::substituteDebugValuesForInst`. If debug info maintenance is
+not performed, or an instruction is eliminated as dead code, the variable
+location is safely dropped and marked "optimised out". The exception is
+instructions that are mutated rather than replaced, which always need debug info
 maintenance.
 
 # Register allocator considerations
 
 When the register allocator runs, debugging instructions do not directly refer
 to any virtual registers, and thus there is no need for expensive location
-maintenance during regalloc (i.e., LiveDebugVariables). Debug instructions are
+maintenance during regalloc (i.e. `LiveDebugVariables`). Debug instructions are
 unlinked from the function, then linked back in after register allocation
 completes.
 
-The exception is PHI instructions: these become implicit definitions at control
-flow merges once regalloc finishes, and any debug numbers attached to PHI
-instructions are lost. To circumvent this, debug numbers of PHIs are recorded
-at the start of register allocation (phi-node-elimination), then DBG_PHI
-instructions are inserted after regalloc finishes. This requires some
+The exception is `PHI` instructions: these become implicit definitions at
+control flow merges once regalloc finishes, and any debug numbers attached to
+`PHI` instructions are lost. To circumvent this, debug numbers of `PHI`s are
+recorded at the start of register allocation (`phi-node-elimination`), then
+`DBG_PHI` instructions are inserted after regalloc finishes. This requires some
 maintenance of which register a variable is located in during regalloc, but at
 single positions (block entry points) rather than ranges of instructions.
 
 An example, before regalloc:
 
-  bb.2:
-    %2 = PHI %1, %bb.0, %2, %bb.1, debug-instr-number 1
+```text
+```
 
 After:
 
-  bb.2:
-    DBG_PHI $rax, 1
+```text
+```
 
-# LiveDebugValues
+# `LiveDebugValues`
 
 After optimisations and code layout complete, information about variable
 values must be translated into variable locations, i.e. registers and stack
-slots. This is performed in the [LiveDebugValues pass][LiveDebugValues], where
+slots. This is performed in the [LiveDebugValues pass][`LiveDebugValues`], where
 the debug instructions and machine code are separated out into two independent
 functions:
  * One that assigns values to variable names,
  * One that assigns values to machine registers and stack slots.
 
-LLVM's existing SSA tools are used to place PHIs for each function, between
+LLVM's existing SSA tools are used to place `PHI`s for each function, between
 variable values and the values contained in machine locations, with value
-propagation eliminating any un-necessary PHIs. The two can then be joined up
+propagation eliminating any unnecessary `PHI`s. The two can then be joined up
 to map variables to values, then values to locations, for each instruction in
 the function.
 
 Key to this process is being able to identify the movement of values between
 registers and stack locations, so that the location of values can be preserved
 for the full time that they are resident in the machine.
 
 # Required target support and transition guide
 
 Instruction referencing will work on any target, but likely with poor coverage.
 Supporting instruction referencing well requires:
- * Target hooks to be implemented to allow LiveDebugValues to follow values through the machine,
- * Target-specific optimisations to be instrumented, to preserve instruction numbers.
+ * Target hooks to be implemented to allow `LiveDebugValues` to follow values
+   through the machine,
+ * Target-specific optimisations to be instrumented, to preserve instruction
+   numbers.
 
 ## Target hooks
 
-TargetInstrInfo::isCopyInstrImpl must be implemented to recognise any
-instructions that are copy-like -- LiveDebugValues uses this to identify when
+`TargetInstrInfo::isCopyInstrImpl` must be implemented to recognise any
+instructions that are copy-like -- `LiveDebugValues` uses this to identify when
 values move between registers.
 
-TargetInstrInfo::isLoadFromStackSlotPostFE and
-TargetInstrInfo::isStoreToStackSlotPostFE are needed to identify spill and
+`TargetInstrInfo::isLoadFromStackSlotPostFE` and
+`TargetInstrInfo::isStoreToStackSlotPostFE` are needed to identify spill and
 restore instructions. Each should return the destination or source register
-respectively. LiveDebugValues will track the movement of a value from / to
+respectively. `LiveDebugValues` will track the movement of a value from / to
 the stack slot. In addition, any instruction that writes to a stack spill
-should have a MachineMemoryOperand attached, so that LiveDebugValues can
+should have a `MachineMemoryOperand` attached, so that `LiveDebugValues` can
 recognise that a slot has been clobbered.
 
 ## Target-specific optimisation instrumentation
 
-Optimisations come in two flavours: those that mutate a MachineInstr to make
+Optimisations come in two flavours: those that mutate a `MachineInstr` to make
 it do something different, and those that create a new instruction to replace
 the operation of the old.
 
 The former _must_ be instrumented -- the relevant question is whether any
 register def in any operand will produce a different value, as a result of the
-mutation. If the answer is yes, then there is a risk that a DBG_INSTR_REF
+mutation. If the answer is yes, then there is a risk that a `DBG_INSTR_REF`
 instruction referring to that operand will end up assigning the different
 value to a variable, presenting the debugging developer with an unexpected
-variable value. In such scenarios, call MachineInstr::dropDebugNumber() on the
-mutated instruction to erase its instruction number. Any DBG_INSTR_REF
+variable value. In such scenarios, call `MachineInstr::dropDebugNumber()` on the
+mutated instruction to erase its instruction number. Any `DBG_INSTR_REF`
 referring to it will produce an empty variable location instead, that appears
 as "optimised out" in the debugger.
 
 For the latter flavour of optimisation, to increase coverage you should record
 an instruction number substitution: a mapping from the old instruction number /
 operand pair to new instruction number / operand pair. Consider if we replace
 a three-address add instruction with a two-address add:
 
-  %2:gr32 = ADD32rr %0, %1, debug-instr-number 1
+```text
+```
 
 becomes
 
-  %2:gr32 = ADD32rr %0(tied-def 0), %1, debug-instr-number 2
+```text
+```
 
 With a substitution from "instruction number 1 operand 0" to "instruction number
-2 operand 0" recorded in the MachineFunction. In LiveDebugValues, DBG_INSTR_REFs
-will be mapped through the substitution table to find the most recent
-instruction number / operand number of the value it refers to.
+2 operand 0" recorded in the `MachineFunction`. In `LiveDebugValues`,
+`DBG_INSTR_REF`s will be mapped through the substitution table to find the most
+recent instruction number / operand number of the value it refers to.
 
-Use MachineFunction::substituteDebugValuesForInst to automatically produce
+Use `MachineFunction::substituteDebugValuesForInst` to automatically produce
 substitutions between an old and new instruction. It assumes that any operand
 that is a def in the old instruction is a def in the new instruction at the
 same operand position. This works most of the time, for example in the example
 above.
 
 If operand numbers do not line up between the old and new instruction, use
-MachineInstr::getDebugInstrNum to acquire the instruction number for the new
-instruction, and MachineFunction::makeDebugValueSubstitution to record the
+`MachineInstr::getDebugInstrNum` to acquire the instruction number for the new
+instruction, and `MachineFunction::makeDebugValueSubstitution` to record the
 mapping between register definitions in the old and new instructions. If some
 values computed by the old instruction are no longer computed by the new
-instruction, record no substitution -- LiveDebugValues will safely drop the
+instruction, record no substitution -- `LiveDebugValues` will safely drop the
 now unavailable variable value.
 
-Should your target clone instructions, much the same as the TailDuplicator
+Should your target clone instructions, much the same as the `TailDuplicator`
 optimisation pass, do not attempt to preserve the instruction numbers or
-record any substitutions. MachineFunction::CloneMachineInstr should drop the
+record any substitutions. `MachineFunction::CloneMachineInstr` should drop the
 instruction number of any cloned instruction, to avoid duplicate numbers
-appearing to LiveDebugValues. Dealing with duplicated instructions is a
+appearing to `LiveDebugValues`. Dealing with duplicated instructions is a
 natural extension to instruction referencing that's currently unimplemented.
 
 [LiveDebugValues]: SourceLevelDebugging.html#livedebugvalues-expansion-of-variable-locations