[IR] Introduce a dereferenceable_or_null(N) attribute.

[sanjoy]

Summary

If a pointer is marked as dereferenceable_or_null(N), LLVM assumes it
is either null or dereferenceable(N) or both. This change only
introduces the attribute and adds a token test case for the llvm-as
/ llvm-dis. It does not hook up other parts of the optimizer to
actually exploit the attribute -- those changes will come later.

For pointers in address space 0, dereferenceable(N) is now exactly
equivalent to dereferenceable_or_null(N) && nonnull. For other
address spaces, dereferenceable(N) is potentially weaker than
dereferenceable_or_null(N) && nonnull (since we could have a null
dereferenceable(N) pointer).

The motivating case for this change is Java (and other managed
languages), where pointers are either null or dereferenceable up to
some usually known-at-compile-time constant offset.

[sanjoy]

For context -- this was discussed on llvmdev some time ago:
http://article.gmane.org/gmane.comp.compilers.llvm.devel/82186

[nicholas]

Sanjoy Das wrote:

Hi hfinkel, rafael,

If a pointer is marked as dereferenceable_xor_null(N), LLVM assumes it
is exactly one of { null, dereferenceable(N) }.

Painting the bike shed ... I dislike the name so much I'd rather you
changed "dereferenceable" to mean this, and then allow people to apply
"nonnull" with dereferenceable to get the current meaning of
deferenceable. Changing the meaning of the existing attribute is less
bad. (I also think that it's strict a loosening of the guarantees that
dereferenceable offers, so using older .ll/.bc files with newer llvm
will work, with loss of optimization opportunity in the worst case.
That's fine.)

This change only

introduces the attribute and adds a token test case for the llvm-as /
llvm-dis. It does not hook up other parts of the optimizer to
actually exploit the attribute -- those changes will come later.

For pointers in address space 0, dereferenceable(N) is now exactly
equivalent to dereferenceable_xor_null(N)&& nonnull. For other
address spaces, dereferenceable(N) is potentially weaker than
dereferenceable_xor_null(N)&& nonnull (since we could have a null
dereferenceable(N) pointer).

The motivating case for this change is Java (and other managed
languages), where pointers are either null or dereferenceable up to
some usually known-at-compile-time constant offset.

Also pointers returned by malloc. LibCallSimplifier::optimizeCall
annotates known functions with attributes, please add that there to
anything that it applies to.

http://reviews.llvm.org/D8650

Files:

docs/LangRef.rst
include/llvm-c/Core.h
include/llvm/Bitcode/LLVMBitCodes.h
include/llvm/IR/Attributes.h
include/llvm/IR/Function.h
include/llvm/IR/Instructions.h
lib/AsmParser/LLLexer.cpp
lib/AsmParser/LLParser.cpp
lib/AsmParser/LLParser.h
lib/AsmParser/LLToken.h
lib/Bitcode/Reader/BitcodeReader.cpp
lib/Bitcode/Writer/BitcodeWriter.cpp
lib/IR/AttributeImpl.h
lib/IR/Attributes.cpp
lib/IR/Function.cpp
lib/IR/Instructions.cpp
test/Assembler/deref-xor-null.ll

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[hfinkel]

• Original Message -----

From: "Nick Lewycky" <[e-mail]>
To: [e-mail], "rafael espindola" <[e-mail]>
Cc: [e-mail], [e-mail]
Sent: Friday, March 27, 2015 12:24:00 AM
Subject: Re: [PATCH] [IR] Introduce a dereferenceable_xor_null(N) attribute.

Sanjoy Das wrote:

Hi hfinkel, rafael,

If a pointer is marked as dereferenceable_xor_null(N), LLVM assumes
it

is exactly one of { null, dereferenceable(N) }.

Painting the bike shed ... I dislike the name so much I'd rather you

FWIW, I dislike this name as well, specifically the 'xor' part. Honestly, I think that dereferenceable_or_null is clear enough, but I can understand how some might find this confusing. Maybe dereferenceable_unless_null? Or just dereferenceable_if_not_null? A null pointer is address space 0 is not dereferenceable (regardless of this attribute), but what about not in address space zero? Normally, being null does not convey any information about being dereferenceable, but with this attribute, do you intend to change that?

My preference for using a different name was for correspondence with the API. It would be odd, I think, if adding dereferenceable to a pointer value was insufficient to cause isDereferenceablePointer to return true. This is not a super-strong preference ;)

-Hal

changed "dereferenceable" to mean this, and then allow people to
apply
"nonnull" with dereferenceable to get the current meaning of
deferenceable. Changing the meaning of the existing attribute is less
bad. (I also think that it's strict a loosening of the guarantees
that
dereferenceable offers, so using older .ll/.bc files with newer llvm
will work, with loss of optimization opportunity in the worst case.
That's fine.)

This change only

introduces the attribute and adds a token test case for the
llvm-as /

llvm-dis. It does not hook up other parts of the optimizer to

actually exploit the attribute -- those changes will come later.

For pointers in address space 0, dereferenceable(N) is now
exactly

equivalent to dereferenceable_xor_null(N)&& nonnull. For
other

address spaces, dereferenceable(N) is potentially weaker than

dereferenceable_xor_null(N)&& nonnull (since we could have a
null

dereferenceable(N) pointer).

The motivating case for this change is Java (and other managed

languages), where pointers are either null or dereferenceable up
to

some usually known-at-compile-time constant offset.

Also pointers returned by malloc. LibCallSimplifier::optimizeCall
annotates known functions with attributes, please add that there to
anything that it applies to.

http://reviews.llvm.org/D8650

Files:

docs/LangRef.rst

include/llvm-c/Core.h

include/llvm/Bitcode/LLVMBitCodes.h

include/llvm/IR/Attributes.h

include/llvm/IR/Function.h

include/llvm/IR/Instructions.h

lib/AsmParser/LLLexer.cpp

lib/AsmParser/LLParser.cpp

lib/AsmParser/LLParser.h

lib/AsmParser/LLToken.h

lib/Bitcode/Reader/BitcodeReader.cpp

lib/Bitcode/Writer/BitcodeWriter.cpp

lib/IR/AttributeImpl.h

lib/IR/Attributes.cpp

lib/IR/Function.cpp

lib/IR/Instructions.cpp

test/Assembler/deref-xor-null.ll

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[sanjoy]

Rename attribute (and all related things) to dereferenceable_or_null.

• s/xor/or/

[sanjoy]

Super-gentle ping.

[sanjoy]

ping!

[hfinkel]

docs/LangRef.rst

1017

Please explicitly note that it can only be both when not in addrspace(0). When not in addrspace(0), does this attribute imply that null is not dereferenceable (even if it would be otherwise)?

On the other hand, if dereferenceable_or_null(n) is equivalent to dereferenceable(n) for non-addrspace(0) pointers, maybe we should disallow it to avoid confusion.

lib/AsmParser/LLParser.cpp

1564

Please refactor this so it shares common logic with dereferenceable(n).

lib/IR/Attributes.cpp

302

We now have three essentially-identical blocks of code here. Please add some lambda function and combine them instead.

test/Assembler/deref-or-null.ll

1 ↗

(On Diff #23235)

Don't add another separate test for this. Just add this to test/Bitcode/attributes.ll (along with the tests for dereferenceable).

[sanjoy]

docs/LangRef.rst

1017

Please explicitly note that it can only be both when not in addrspace(0).

Done.

When not in addrspace(0), does this attribute imply that null is not dereferenceable (even if it would be otherwise)?

No, it just means that the pointer is at least one of "null" or "dereferenceable(<n>)" (I've changed the language to make this clearer). So you cannot replace "load i32, i32 aspace(42)* T" where T is known to be "null" and also deref_or_null with "unreachable".

lib/AsmParser/LLParser.cpp

1564

Done!

lib/IR/Attributes.cpp

302

Done!

test/Assembler/deref-or-null.ll

1 ↗

(On Diff #23235)

Done!

[sanjoy]

Address Hal's review.

[sanjoy]

ping!

[hfinkel]

LGTM.

test/Bitcode/attributes.ll

249

Please add the attribute on the return value to provide coverage on that.

docs/LangRef.rst

``dereferenceable(<n>)``
     is dereferenceable can be loaded from speculatively without a risk of
     trapping. The number of bytes known to be dereferenceable must be provided
     in parentheses. It is legal for the number of bytes to be less than the
     size of the pointee type. The ``nonnull`` attribute does not imply
     dereferenceability (consider a pointer to one element past the end of an
     array), however ``dereferenceable(<n>)`` does imply ``nonnull`` in
     ``addrspace(0)`` (which is the default address space).
 
+``dereferenceable_or_null(<n>)``
+    This indicates that the parameter or return value isn't both
+    non-null and non-dereferenceable (up to ``<n>`` bytes) at the same
+    time.  All non-null pointers tagged with
+    ``dereferenceable_or_null(<n>)`` are ``dereferenceable(<n>)``.
+    For address space 0 ``dereferenceable_or_null(<n>)`` implies that
+    a pointer is exactly one of ``dereferenceable(<n>)`` or ``null``,
+    and in other address spaces ``dereferenceable_or_null(<n>)``
+    implies that a pointer is at least one of ``dereferenceable(<n>)``
+    or ``null`` (i.e. it may be both ``null`` and
+    ``dereferenceable(<n>)``).  This attribute may only be applied to
+    pointer typed parameters.
+
 .. _gc:
 
 Garbage Collector Strategy Names
 --------------------------------
 
 Each function may specify a garbage collector strategy name, which is simply a
 string:
 

include/llvm-c/Core.h

typedef enum {
     LLVMAddressSafety = 1ULL << 32,
     LLVMStackProtectStrongAttribute = 1ULL<<33,
     LLVMCold = 1ULL << 34,
     LLVMOptimizeNone = 1ULL << 35,
     LLVMInAllocaAttribute = 1ULL << 36,
     LLVMNonNullAttribute = 1ULL << 37,
     LLVMJumpTableAttribute = 1ULL << 38,
     LLVMDereferenceableAttribute = 1ULL << 39,
+    LLVMDereferenceableOrNullAttribute = 1ULL << 40,
     */
 } LLVMAttribute;
 
 typedef enum {
   /* Terminator Instructions */
   LLVMRet            = 1,
   LLVMBr             = 2,
   LLVMSwitch         = 3,

include/llvm/Bitcode/LLVMBitCodes.h

enum AttributeKindCodes {
     ATTR_KIND_UW_TABLE = 33,
     ATTR_KIND_Z_EXT = 34,
     ATTR_KIND_BUILTIN = 35,
     ATTR_KIND_COLD = 36,
     ATTR_KIND_OPTIMIZE_NONE = 37,
     ATTR_KIND_IN_ALLOCA = 38,
     ATTR_KIND_NON_NULL = 39,
     ATTR_KIND_JUMP_TABLE = 40,
-    ATTR_KIND_DEREFERENCEABLE = 41
+    ATTR_KIND_DEREFERENCEABLE = 41,
+    ATTR_KIND_DEREFERENCEABLE_OR_NULL = 42
   };
 
   enum ComdatSelectionKindCodes {
     COMDAT_SELECTION_KIND_ANY = 1,
     COMDAT_SELECTION_KIND_EXACT_MATCH = 2,
     COMDAT_SELECTION_KIND_LARGEST = 3,
     COMDAT_SELECTION_KIND_NO_DUPLICATES = 4,
     COMDAT_SELECTION_KIND_SAME_SIZE = 5,
   };
 
 } // End bitc namespace
 } // End llvm namespace
 
 #endif

include/llvm/IR/Attributes.h

enum AttrKind {
     NoCapture,             ///< Function creates no aliases of pointer
     NoDuplicate,           ///< Call cannot be duplicated
     NoImplicitFloat,       ///< Disable implicit floating point insts
     NoInline,              ///< inline=never
     NonLazyBind,           ///< Function is called early and/or
                            ///< often, so lazy binding isn't worthwhile
     NonNull,               ///< Pointer is known to be not null
     Dereferenceable,       ///< Pointer is known to be dereferenceable
+    DereferenceableOrNull, ///< Pointer is either null or dereferenceable
     NoRedZone,             ///< Disable redzone
     NoReturn,              ///< Mark the function as not returning
     NoUnwind,              ///< Function doesn't unwind stack
     OptimizeForSize,       ///< opt_size
     OptimizeNone,          ///< Function must not be optimized.
     ReadNone,              ///< Function does not access memory
     ReadOnly,              ///< Function only reads from memory
     Returned,              ///< Return value is always equal to this argument
static Attribute get(LLVMContext &Context, StringRef Kind,
                        StringRef Val = StringRef());
 
   /// \brief Return a uniquified Attribute object that has the specific
   /// alignment set.
   static Attribute getWithAlignment(LLVMContext &Context, uint64_t Align);
   static Attribute getWithStackAlignment(LLVMContext &Context, uint64_t Align);
   static Attribute getWithDereferenceableBytes(LLVMContext &Context,
                                               uint64_t Bytes);
+  static Attribute getWithDereferenceableOrNullBytes(LLVMContext &Context,
+                                                     uint64_t Bytes);
 
   //===--------------------------------------------------------------------===//
   // Attribute Accessors
   //===--------------------------------------------------------------------===//
 
   /// \brief Return true if the attribute is an Attribute::AttrKind type.
   bool isEnumAttribute() const;
 
public:
   /// \brief Returns the stack alignment field of an attribute as a byte
   /// alignment value.
   unsigned getStackAlignment() const;
 
   /// \brief Returns the number of dereferenceable bytes from the
   /// dereferenceable attribute (or zero if unknown).
   uint64_t getDereferenceableBytes() const;
 
+  /// \brief Returns the number of dereferenceable_or_null bytes from the
+  /// dereferenceable_or_null attribute (or zero if unknown).
+  uint64_t getDereferenceableOrNullBytes() const;
+
   /// \brief The Attribute is converted to a string of equivalent mnemonic. This
   /// is, presumably, for writing out the mnemonics for the assembly writer.
   std::string getAsString(bool InAttrGrp = false) const;
 
   /// \brief Equality and non-equality operators.
   bool operator==(Attribute A) const { return pImpl == A.pImpl; }
   bool operator!=(Attribute A) const { return pImpl != A.pImpl; }
 
public:
   AttributeSet removeAttributes(LLVMContext &C, unsigned Index, 
                                 AttributeSet Attrs) const;
 
   /// \brief Add the dereferenceable attribute to the attribute set at the given
   /// index. Since attribute sets are immutable, this returns a new set.
   AttributeSet addDereferenceableAttr(LLVMContext &C, unsigned Index,
                                       uint64_t Bytes) const;
 
+  /// \brief Add the dereferenceable_or_null attribute to the attribute set at
+  /// the given index. Since attribute sets are immutable, this returns a new
+  /// set.
+  AttributeSet addDereferenceableOrNullAttr(LLVMContext &C, unsigned Index,
+                                            uint64_t Bytes) const;
+
   //===--------------------------------------------------------------------===//
   // AttributeSet Accessors
   //===--------------------------------------------------------------------===//
 
   /// \brief Retrieve the LLVM context.
   LLVMContext &getContext() const;
 
   /// \brief The attributes for the specified index are returned.
public:
   unsigned getParamAlignment(unsigned Index) const;
 
   /// \brief Get the stack alignment.
   unsigned getStackAlignment(unsigned Index) const;
 
   /// \brief Get the number of dereferenceable bytes (or zero if unknown).
   uint64_t getDereferenceableBytes(unsigned Index) const;
 
+  /// \brief Get the number of dereferenceable_or_null bytes (or zero if
+  /// unknown).
+  uint64_t getDereferenceableOrNullBytes(unsigned Index) const;
+
   /// \brief Return the attributes at the index as a string.
   std::string getAsString(unsigned Index, bool InAttrGrp = false) const;
 
   typedef ArrayRef<Attribute>::iterator iterator;
 
   iterator begin(unsigned Slot) const;
   iterator end(unsigned Slot) const;
 

 /// value, however, is not. So this can be used as a quick way to test for
 /// equality, presence of attributes, etc.
 class AttrBuilder {
   std::bitset<Attribute::EndAttrKinds> Attrs;
   std::map<std::string, std::string> TargetDepAttrs;
   uint64_t Alignment;
   uint64_t StackAlignment;
   uint64_t DerefBytes;
+  uint64_t DerefOrNullBytes;
 public:
   AttrBuilder() : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {}
   explicit AttrBuilder(uint64_t Val)
     : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {
     addRawValue(Val);
   }
   AttrBuilder(const Attribute &A)
     : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {
public:
 
   /// \brief Retrieve the stack alignment attribute, if it exists.
   uint64_t getStackAlignment() const { return StackAlignment; }
 
   /// \brief Retrieve the number of dereferenceable bytes, if the dereferenceable
   /// attribute exists (zero is returned otherwise).
   uint64_t getDereferenceableBytes() const { return DerefBytes; }
 
+  /// \brief Retrieve the number of dereferenceable_or_null bytes, if the
+  /// dereferenceable_or_null attribute exists (zero is returned otherwise).
+  uint64_t getDereferenceableOrNullBytes() const { return DerefOrNullBytes; }
+
   /// \brief This turns an int alignment (which must be a power of 2) into the
   /// form used internally in Attribute.
   AttrBuilder &addAlignmentAttr(unsigned Align);
 
   /// \brief This turns an int stack alignment (which must be a power of 2) into
   /// the form used internally in Attribute.
   AttrBuilder &addStackAlignmentAttr(unsigned Align);
 
   /// \brief This turns the number of dereferenceable bytes into the form used
   /// internally in Attribute.
   AttrBuilder &addDereferenceableAttr(uint64_t Bytes);
 
+  /// \brief This turns the number of dereferenceable_or_null bytes into the
+  /// form used internally in Attribute.
+  AttrBuilder &addDereferenceableOrNullAttr(uint64_t Bytes);
+
   /// \brief Return true if the builder contains no target-independent
   /// attributes.
   bool empty() const { return Attrs.none(); }
 
   // Iterators for target-dependent attributes.
   typedef std::pair<std::string, std::string>                td_type;
   typedef std::map<std::string, std::string>::iterator       td_iterator;
   typedef std::map<std::string, std::string>::const_iterator td_const_iterator;

include/llvm/IR/Function.h

public:
   void addAttributes(unsigned i, AttributeSet attrs);
 
   /// @brief removes the attributes from the list of attributes.
   void removeAttributes(unsigned i, AttributeSet attr);
 
   /// @brief adds the dereferenceable attribute to the list of attributes.
   void addDereferenceableAttr(unsigned i, uint64_t Bytes);
 
+  /// @brief adds the dereferenceable_or_null attribute to the list of
+  /// attributes.
+  void addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes);
+
   /// @brief Extract the alignment for a call or parameter (0=unknown).
   unsigned getParamAlignment(unsigned i) const {
     return AttributeSets.getParamAlignment(i);
   }
 
   /// @brief Extract the number of dereferenceable bytes for a call or
   /// parameter (0=unknown).
   uint64_t getDereferenceableBytes(unsigned i) const {

include/llvm/IR/Instructions.h

public:
   void addAttribute(unsigned i, Attribute::AttrKind attr);
 
   /// removeAttribute - removes the attribute from the list of attributes.
   void removeAttribute(unsigned i, Attribute attr);
 
   /// \brief adds the dereferenceable attribute to the list of attributes.
   void addDereferenceableAttr(unsigned i, uint64_t Bytes);
 
+  /// \brief adds the dereferenceable_or_null attribute to the list of
+  /// attributes.
+  void addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes);
+
   /// \brief Determine whether this call has the given attribute.
   bool hasFnAttr(Attribute::AttrKind A) const {
     assert(A != Attribute::NoBuiltin &&
            "Use CallInst::isNoBuiltin() to check for Attribute::NoBuiltin");
     return hasFnAttrImpl(A);
   }
 
   /// \brief Determine whether the call or the callee has the given attributes.
public:
   void addAttribute(unsigned i, Attribute::AttrKind attr);
 
   /// removeAttribute - removes the attribute from the list of attributes.
   void removeAttribute(unsigned i, Attribute attr);
 
   /// \brief adds the dereferenceable attribute to the list of attributes.
   void addDereferenceableAttr(unsigned i, uint64_t Bytes);
 
+  /// \brief adds the dereferenceable_or_null attribute to the list of
+  /// attributes.
+  void addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes);
+
   /// \brief Determine whether this call has the given attribute.
   bool hasFnAttr(Attribute::AttrKind A) const {
     assert(A != Attribute::NoBuiltin &&
            "Use CallInst::isNoBuiltin() to check for Attribute::NoBuiltin");
     return hasFnAttrImpl(A);
   }
 
   /// \brief Determine whether the call or the callee has the given attributes.

lib/AsmParser/LLLexer.cpp

#define KEYWORD(STR)                                                           \
   KEYWORD(attributes);
 
   KEYWORD(alwaysinline);
   KEYWORD(builtin);
   KEYWORD(byval);
   KEYWORD(inalloca);
   KEYWORD(cold);
   KEYWORD(dereferenceable);
+  KEYWORD(dereferenceable_or_null);
   KEYWORD(inlinehint);
   KEYWORD(inreg);
   KEYWORD(jumptable);
   KEYWORD(minsize);
   KEYWORD(naked);
   KEYWORD(nest);
   KEYWORD(noalias);
   KEYWORD(nobuiltin);

lib/AsmParser/LLParser.h

private:
     bool ParseOptionalLinkage(unsigned &Linkage, bool &HasLinkage);
     bool ParseOptionalLinkage(unsigned &Linkage) {
       bool HasLinkage; return ParseOptionalLinkage(Linkage, HasLinkage);
     }
     bool ParseOptionalVisibility(unsigned &Visibility);
     bool ParseOptionalDLLStorageClass(unsigned &DLLStorageClass);
     bool ParseOptionalCallingConv(unsigned &CC);
     bool ParseOptionalAlignment(unsigned &Alignment);
-    bool ParseOptionalDereferenceableBytes(uint64_t &Bytes);
+    bool ParseOptionalDerefAttrBytes(lltok::Kind AttrKind, uint64_t &Bytes);
     bool ParseScopeAndOrdering(bool isAtomic, SynchronizationScope &Scope,
                                AtomicOrdering &Ordering);
     bool ParseOrdering(AtomicOrdering &Ordering);
     bool ParseOptionalStackAlignment(unsigned &Alignment);
     bool ParseOptionalCommaAlign(unsigned &Alignment, bool &AteExtraComma);
     bool ParseOptionalCommaInAlloca(bool &IsInAlloca);
     bool ParseIndexList(SmallVectorImpl<unsigned> &Indices,bool &AteExtraComma);
     bool ParseIndexList(SmallVectorImpl<unsigned> &Indices) {

lib/AsmParser/LLParser.cpp

while (true) {
     case lltok::kw_signext:
     case lltok::kw_zeroext:
       HaveError |=
         Error(Lex.getLoc(),
               "invalid use of attribute on a function");
       break;
     case lltok::kw_byval:
     case lltok::kw_dereferenceable:
+    case lltok::kw_dereferenceable_or_null:
     case lltok::kw_inalloca:
     case lltok::kw_nest:
     case lltok::kw_noalias:
     case lltok::kw_nocapture:
     case lltok::kw_nonnull:
     case lltok::kw_returned:
     case lltok::kw_sret:
       HaveError |=
case lltok::kw_align: {
       if (ParseOptionalAlignment(Alignment))
         return true;
       B.addAlignmentAttr(Alignment);
       continue;
     }
     case lltok::kw_byval:           B.addAttribute(Attribute::ByVal); break;
     case lltok::kw_dereferenceable: {
       uint64_t Bytes;
-      if (ParseOptionalDereferenceableBytes(Bytes))
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable, Bytes))
         return true;
       B.addDereferenceableAttr(Bytes);
       continue;
     }
+    case lltok::kw_dereferenceable_or_null: {
+      uint64_t Bytes;
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable_or_null, Bytes))
+        return true;
+      B.addDereferenceableOrNullAttr(Bytes);
+      continue;
+    }
     case lltok::kw_inalloca:        B.addAttribute(Attribute::InAlloca); break;
     case lltok::kw_inreg:           B.addAttribute(Attribute::InReg); break;
     case lltok::kw_nest:            B.addAttribute(Attribute::Nest); break;
     case lltok::kw_noalias:         B.addAttribute(Attribute::NoAlias); break;
     case lltok::kw_nocapture:       B.addAttribute(Attribute::NoCapture); break;
     case lltok::kw_nonnull:         B.addAttribute(Attribute::NonNull); break;
     case lltok::kw_readnone:        B.addAttribute(Attribute::ReadNone); break;
     case lltok::kw_readonly:        B.addAttribute(Attribute::ReadOnly); break;
bool LLParser::ParseOptionalReturnAttrs(AttrBuilder &B) {
 
   while (1) {
     lltok::Kind Token = Lex.getKind();
     switch (Token) {
     default:  // End of attributes.
       return HaveError;
     case lltok::kw_dereferenceable: {
       uint64_t Bytes;
-      if (ParseOptionalDereferenceableBytes(Bytes))
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable, Bytes))
         return true;
       B.addDereferenceableAttr(Bytes);
       continue;
     }
+    case lltok::kw_dereferenceable_or_null: {
+      uint64_t Bytes;
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable_or_null, Bytes))
+        return true;
+      B.addDereferenceableOrNullAttr(Bytes);
+      continue;
+    }
     case lltok::kw_inreg:           B.addAttribute(Attribute::InReg); break;
     case lltok::kw_noalias:         B.addAttribute(Attribute::NoAlias); break;
     case lltok::kw_nonnull:         B.addAttribute(Attribute::NonNull); break;
     case lltok::kw_signext:         B.addAttribute(Attribute::SExt); break;
     case lltok::kw_zeroext:         B.addAttribute(Attribute::ZExt); break;
 
     // Error handling.
     case lltok::kw_align:
bool LLParser::ParseOptionalAlignment(unsigned &Alignment) {
   if (ParseUInt32(Alignment)) return true;
   if (!isPowerOf2_32(Alignment))
     return Error(AlignLoc, "alignment is not a power of two");
   if (Alignment > Value::MaximumAlignment)
     return Error(AlignLoc, "huge alignments are not supported yet");
   return false;
 }
 
-/// ParseOptionalDereferenceableBytes
+/// ParseOptionalDerefAttrBytes
 ///   ::= /* empty */
-///   ::= 'dereferenceable' '(' 4 ')'
-bool LLParser::ParseOptionalDereferenceableBytes(uint64_t &Bytes) {
+///   ::= AttrKind '(' 4 ')'
+///
+/// where AttrKind is either 'dereferenceable' or 'dereferenceable_or_null'.
+bool LLParser::ParseOptionalDerefAttrBytes(lltok::Kind AttrKind,
+                                           uint64_t &Bytes) {
+  assert((AttrKind == lltok::kw_dereferenceable ||
+          AttrKind == lltok::kw_dereferenceable_or_null) &&
+         "contract!");
+
   Bytes = 0;
-  if (!EatIfPresent(lltok::kw_dereferenceable))
+  if (!EatIfPresent(AttrKind))
     return false;
   LocTy ParenLoc = Lex.getLoc();
   if (!EatIfPresent(lltok::lparen))
     return Error(ParenLoc, "expected '('");
   LocTy DerefLoc = Lex.getLoc();
   if (ParseUInt64(Bytes)) return true;
   ParenLoc = Lex.getLoc();
   if (!EatIfPresent(lltok::rparen))
     return Error(ParenLoc, "expected ')'");
   if (!Bytes)
     return Error(DerefLoc, "dereferenceable bytes must be non-zero");
   return false;
 }
 
 /// ParseOptionalCommaAlign
 ///   ::=
 ///   ::= ',' align 4
 ///
 /// This returns with AteExtraComma set to true if it ate an excess comma at the
 /// end.
 bool LLParser::ParseOptionalCommaAlign(unsigned &Alignment,
                                        bool &AteExtraComma) {
   AteExtraComma = false;
   while (EatIfPresent(lltok::comma)) {
     // Metadata at the end is an early exit.
     if (Lex.getKind() == lltok::MetadataVar) {

lib/AsmParser/LLToken.h

enum Kind {
     kw_attributes,
     kw_alwaysinline,
     kw_sanitize_address,
     kw_builtin,
     kw_byval,
     kw_inalloca,
     kw_cold,
     kw_dereferenceable,
+    kw_dereferenceable_or_null,
     kw_inlinehint,
     kw_inreg,
     kw_jumptable,
     kw_minsize,
     kw_naked,
     kw_nest,
     kw_noalias,
     kw_nobuiltin,

lib/Bitcode/Reader/BitcodeReader.cpp

static Attribute::AttrKind GetAttrFromCode(uint64_t Code) {
   case bitc::ATTR_KIND_NO_INLINE:
     return Attribute::NoInline;
   case bitc::ATTR_KIND_NON_LAZY_BIND:
     return Attribute::NonLazyBind;
   case bitc::ATTR_KIND_NON_NULL:
     return Attribute::NonNull;
   case bitc::ATTR_KIND_DEREFERENCEABLE:
     return Attribute::Dereferenceable;
+  case bitc::ATTR_KIND_DEREFERENCEABLE_OR_NULL:
+    return Attribute::DereferenceableOrNull;
   case bitc::ATTR_KIND_NO_RED_ZONE:
     return Attribute::NoRedZone;
   case bitc::ATTR_KIND_NO_RETURN:
     return Attribute::NoReturn;
   case bitc::ATTR_KIND_NO_UNWIND:
     return Attribute::NoUnwind;
   case bitc::ATTR_KIND_OPTIMIZE_FOR_SIZE:
     return Attribute::OptimizeForSize;
case bitc::PARAMATTR_GRP_CODE_ENTRY: { // ENTRY: [grpid, idx, a0, a1, ...]
           if (std::error_code EC = ParseAttrKind(Record[++i], &Kind))
             return EC;
           if (Kind == Attribute::Alignment)
             B.addAlignmentAttr(Record[++i]);
           else if (Kind == Attribute::StackAlignment)
             B.addStackAlignmentAttr(Record[++i]);
           else if (Kind == Attribute::Dereferenceable)
             B.addDereferenceableAttr(Record[++i]);
+          else if (Kind == Attribute::DereferenceableOrNull)
+            B.addDereferenceableOrNullAttr(Record[++i]);
         } else {                     // String attribute
           assert((Record[i] == 3 || Record[i] == 4) &&
                  "Invalid attribute group entry");
           bool HasValue = (Record[i++] == 4);
           SmallString<64> KindStr;
           SmallString<64> ValStr;
 
           while (Record[i] != 0 && i != e)

lib/Bitcode/Writer/BitcodeWriter.cpp

static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
   case Attribute::NoInline:
     return bitc::ATTR_KIND_NO_INLINE;
   case Attribute::NonLazyBind:
     return bitc::ATTR_KIND_NON_LAZY_BIND;
   case Attribute::NonNull:
     return bitc::ATTR_KIND_NON_NULL;
   case Attribute::Dereferenceable:
     return bitc::ATTR_KIND_DEREFERENCEABLE;
+  case Attribute::DereferenceableOrNull:
+    return bitc::ATTR_KIND_DEREFERENCEABLE_OR_NULL;
   case Attribute::NoRedZone:
     return bitc::ATTR_KIND_NO_RED_ZONE;
   case Attribute::NoReturn:
     return bitc::ATTR_KIND_NO_RETURN;
   case Attribute::NoUnwind:
     return bitc::ATTR_KIND_NO_UNWIND;
   case Attribute::OptimizeForSize:
     return bitc::ATTR_KIND_OPTIMIZE_FOR_SIZE;

lib/IR/AttributeImpl.h

 
 class IntAttributeImpl : public EnumAttributeImpl {
   void anchor() override;
   uint64_t Val;
 
 public:
   IntAttributeImpl(Attribute::AttrKind Kind, uint64_t Val)
       : EnumAttributeImpl(IntAttrEntry, Kind), Val(Val) {
-    assert(
-        (Kind == Attribute::Alignment || Kind == Attribute::StackAlignment ||
-         Kind == Attribute::Dereferenceable) &&
+    assert((Kind == Attribute::Alignment || Kind == Attribute::StackAlignment ||
+            Kind == Attribute::Dereferenceable ||
+            Kind == Attribute::DereferenceableOrNull) &&
            "Wrong kind for int attribute!");
   }
 
   uint64_t getValue() const { return Val; }
 };
 
 class StringAttributeImpl : public AttributeImpl {
   virtual void anchor();
   std::string Kind;

lib/IR/Attributes.cpp

 }
 
 Attribute Attribute::getWithDereferenceableBytes(LLVMContext &Context,
                                                 uint64_t Bytes) {
   assert(Bytes && "Bytes must be non-zero.");
   return get(Context, Dereferenceable, Bytes);
 }
 
+Attribute Attribute::getWithDereferenceableOrNullBytes(LLVMContext &Context,
+                                                       uint64_t Bytes) {
+  assert(Bytes && "Bytes must be non-zero.");
+  return get(Context, DereferenceableOrNull, Bytes);
+}
+
 //===----------------------------------------------------------------------===//
 // Attribute Accessor Methods
 //===----------------------------------------------------------------------===//
 
 bool Attribute::isEnumAttribute() const {
   return pImpl && pImpl->isEnumAttribute();
 }
 

 /// This returns the number of dereferenceable bytes.
 uint64_t Attribute::getDereferenceableBytes() const {
   assert(hasAttribute(Attribute::Dereferenceable) &&
          "Trying to get dereferenceable bytes from "
          "non-dereferenceable attribute!");
   return pImpl->getValueAsInt();
 }
 
+uint64_t Attribute::getDereferenceableOrNullBytes() const {
+  assert(hasAttribute(Attribute::DereferenceableOrNull) &&
+         "Trying to get dereferenceable bytes from "
+         "non-dereferenceable attribute!");
+  return pImpl->getValueAsInt();
+}
+
 std::string Attribute::getAsString(bool InAttrGrp) const {
   if (!pImpl) return "";
 
   if (hasAttribute(Attribute::SanitizeAddress))
     return "sanitize_address";
   if (hasAttribute(Attribute::AlwaysInline))
     return "alwaysinline";
   if (hasAttribute(Attribute::Builtin))
std::string Attribute::getAsString(bool InAttrGrp) const {
   if (hasAttribute(Attribute::Alignment)) {
     std::string Result;
     Result += "align";
     Result += (InAttrGrp) ? "=" : " ";
     Result += utostr(getValueAsInt());
     return Result;
   }
 
-  if (hasAttribute(Attribute::StackAlignment)) {
+  auto AttrWithBytesToString = [&](const char *Name) {
     std::string Result;
-    Result += "alignstack";
+    Result += Name;
     if (InAttrGrp) {
       Result += "=";
       Result += utostr(getValueAsInt());
     } else {
       Result += "(";
       Result += utostr(getValueAsInt());
       Result += ")";
     }
     return Result;
-  }
+  };
 
-  if (hasAttribute(Attribute::Dereferenceable)) {
-    std::string Result;
-    Result += "dereferenceable";
-    if (InAttrGrp) {
-      Result += "=";
-      Result += utostr(getValueAsInt());
-    } else {
-      Result += "(";
-      Result += utostr(getValueAsInt());
-      Result += ")";
-    }
-    return Result;
-  }
+  if (hasAttribute(Attribute::StackAlignment))
+    return AttrWithBytesToString("alignstack");
+
+  if (hasAttribute(Attribute::Dereferenceable))
+    return AttrWithBytesToString("dereferenceable");
+
+  if (hasAttribute(Attribute::DereferenceableOrNull))
+    return AttrWithBytesToString("dereferenceable_or_null");
 
   // Convert target-dependent attributes to strings of the form:
   //
   //   "kind"
   //   "kind" = "value"
   //
   if (isStringAttribute()) {
     std::string Result;
     Result += (Twine('"') + getKindAsString() + Twine('"')).str();
 
uint64_t AttributeImpl::getAttrMask(Attribute::AttrKind Val) {
   case Attribute::Cold:            return 1ULL << 40;
   case Attribute::Builtin:         return 1ULL << 41;
   case Attribute::OptimizeNone:    return 1ULL << 42;
   case Attribute::InAlloca:        return 1ULL << 43;
   case Attribute::NonNull:         return 1ULL << 44;
   case Attribute::JumpTable:       return 1ULL << 45;
   case Attribute::Dereferenceable:
     llvm_unreachable("dereferenceable attribute not supported in raw format");
+    break;
+  case Attribute::DereferenceableOrNull:
+    llvm_unreachable("dereferenceable_or_null attribute not supported in raw "
+                     "format");
+    break;
   }
   llvm_unreachable("Unsupported attribute type");
 }
 
 //===----------------------------------------------------------------------===//
 // AttributeSetNode Definition
 //===----------------------------------------------------------------------===//
 
if (Kind == Attribute::Alignment)
                                      getWithAlignment(C, B.getAlignment())));
     else if (Kind == Attribute::StackAlignment)
       Attrs.push_back(std::make_pair(Index, Attribute::
                               getWithStackAlignment(C, B.getStackAlignment())));
     else if (Kind == Attribute::Dereferenceable)
       Attrs.push_back(std::make_pair(Index,
                                      Attribute::getWithDereferenceableBytes(C,
                                        B.getDereferenceableBytes())));
+    else if (Kind == Attribute::DereferenceableOrNull)
+      Attrs.push_back(
+          std::make_pair(Index, Attribute::getWithDereferenceableOrNullBytes(
+                                    C, B.getDereferenceableOrNullBytes())));
     else
       Attrs.push_back(std::make_pair(Index, Attribute::get(C, Kind)));
   }
 
   // Add target-dependent (string) attributes.
   for (const AttrBuilder::td_type &TDA : B.td_attrs())
     Attrs.push_back(
         std::make_pair(Index, Attribute::get(C, TDA.first, TDA.second)));

 
 AttributeSet AttributeSet::addDereferenceableAttr(LLVMContext &C, unsigned Index,
                                                   uint64_t Bytes) const {
   llvm::AttrBuilder B;
   B.addDereferenceableAttr(Bytes);
   return addAttributes(C, Index, AttributeSet::get(C, Index, B));
 }
 
+AttributeSet AttributeSet::addDereferenceableOrNullAttr(LLVMContext &C,
+                                                        unsigned Index,
+                                                        uint64_t Bytes) const {
+  llvm::AttrBuilder B;
+  B.addDereferenceableOrNullAttr(Bytes);
+  return addAttributes(C, Index, AttributeSet::get(C, Index, B));
+}
+
 //===----------------------------------------------------------------------===//
 // AttributeSet Accessor Methods
 //===----------------------------------------------------------------------===//
 
 LLVMContext &AttributeSet::getContext() const {
   return pImpl->getContext();
 }
 
void AttributeSet::dump() const {
   dbgs() << "]\n";
 }
 
 //===----------------------------------------------------------------------===//
 // AttrBuilder Method Implementations
 //===----------------------------------------------------------------------===//
 
 AttrBuilder::AttrBuilder(AttributeSet AS, unsigned Index)
-  : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {
+    : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0),
+      DerefOrNullBytes(0) {
   AttributeSetImpl *pImpl = AS.pImpl;
   if (!pImpl) return;
 
   for (unsigned I = 0, E = pImpl->getNumAttributes(); I != E; ++I) {
     if (pImpl->getSlotIndex(I) != Index) continue;
 
     for (AttributeSetImpl::iterator II = pImpl->begin(I),
            IE = pImpl->end(I); II != IE; ++II)
       addAttribute(*II);
 
     break;
   }
 }
 
 void AttrBuilder::clear() {
   Attrs.reset();
-  Alignment = StackAlignment = DerefBytes = 0;
+  Alignment = StackAlignment = DerefBytes = DerefOrNullBytes = 0;
 }
 
 AttrBuilder &AttrBuilder::addAttribute(Attribute::AttrKind Val) {
   assert((unsigned)Val < Attribute::EndAttrKinds && "Attribute out of range!");
   assert(Val != Attribute::Alignment && Val != Attribute::StackAlignment &&
          Val != Attribute::Dereferenceable &&
          "Adding integer attribute without adding a value!");
   Attrs[Val] = true;
AttrBuilder &AttrBuilder::addAttribute(Attribute Attr) {
   Attrs[Kind] = true;
 
   if (Kind == Attribute::Alignment)
     Alignment = Attr.getAlignment();
   else if (Kind == Attribute::StackAlignment)
     StackAlignment = Attr.getStackAlignment();
   else if (Kind == Attribute::Dereferenceable)
     DerefBytes = Attr.getDereferenceableBytes();
+  else if (Kind == Attribute::DereferenceableOrNull)
+    DerefOrNullBytes = Attr.getDereferenceableOrNullBytes();
   return *this;
 }
 
 AttrBuilder &AttrBuilder::addAttribute(StringRef A, StringRef V) {
   TargetDepAttrs[A] = V;
   return *this;
 }
 
 AttrBuilder &AttrBuilder::removeAttribute(Attribute::AttrKind Val) {
   assert((unsigned)Val < Attribute::EndAttrKinds && "Attribute out of range!");
   Attrs[Val] = false;
 
   if (Val == Attribute::Alignment)
     Alignment = 0;
   else if (Val == Attribute::StackAlignment)
     StackAlignment = 0;
   else if (Val == Attribute::Dereferenceable)
     DerefBytes = 0;
+  else if (Val == Attribute::DereferenceableOrNull)
+    DerefOrNullBytes = 0;
 
   return *this;
 }
 
 AttrBuilder &AttrBuilder::removeAttributes(AttributeSet A, uint64_t Index) {
   unsigned Slot = ~0U;
   for (unsigned I = 0, E = A.getNumSlots(); I != E; ++I)
     if (A.getSlotIndex(I) == Index) {
if (Attr.isEnumAttribute() || Attr.isIntAttribute()) {
       Attrs[Kind] = false;
 
       if (Kind == Attribute::Alignment)
         Alignment = 0;
       else if (Kind == Attribute::StackAlignment)
         StackAlignment = 0;
       else if (Kind == Attribute::Dereferenceable)
         DerefBytes = 0;
+      else if (Kind == Attribute::DereferenceableOrNull)
+        DerefOrNullBytes = 0;
     } else {
       assert(Attr.isStringAttribute() && "Invalid attribute type!");
       std::map<std::string, std::string>::iterator
         Iter = TargetDepAttrs.find(Attr.getKindAsString());
       if (Iter != TargetDepAttrs.end())
         TargetDepAttrs.erase(Iter);
     }
   }

 AttrBuilder &AttrBuilder::addDereferenceableAttr(uint64_t Bytes) {
   if (Bytes == 0) return *this;
 
   Attrs[Attribute::Dereferenceable] = true;
   DerefBytes = Bytes;
   return *this;
 }
 
+AttrBuilder &AttrBuilder::addDereferenceableOrNullAttr(uint64_t Bytes) {
+  if (Bytes == 0)
+    return *this;
+
+  Attrs[Attribute::DereferenceableOrNull] = true;
+  DerefOrNullBytes = Bytes;
+  return *this;
+}
+
 AttrBuilder &AttrBuilder::merge(const AttrBuilder &B) {
   // FIXME: What if both have alignments, but they don't match?!
   if (!Alignment)
     Alignment = B.Alignment;
 
   if (!StackAlignment)
     StackAlignment = B.StackAlignment;
 

 }
 
 AttrBuilder &AttrBuilder::addRawValue(uint64_t Val) {
   // FIXME: Remove this in 4.0.
   if (!Val) return *this;
 
   for (Attribute::AttrKind I = Attribute::None; I != Attribute::EndAttrKinds;
        I = Attribute::AttrKind(I + 1)) {
-    if (I == Attribute::Dereferenceable)
+    if (I == Attribute::Dereferenceable ||
+        I == Attribute::DereferenceableOrNull)
       continue;
     if (uint64_t A = (Val & AttributeImpl::getAttrMask(I))) {
       Attrs[I] = true;
  
       if (I == Attribute::Alignment)
         Alignment = 1ULL << ((A >> 16) - 1);
       else if (I == Attribute::StackAlignment)
         StackAlignment = 1ULL << ((A >> 26)-1);
AttributeSet AttributeFuncs::typeIncompatible(Type *Ty, uint64_t Index) {
   if (!Ty->isPointerTy())
     // Attribute that only apply to pointers.
     Incompatible.addAttribute(Attribute::ByVal)
       .addAttribute(Attribute::Nest)
       .addAttribute(Attribute::NoAlias)
       .addAttribute(Attribute::NoCapture)
       .addAttribute(Attribute::NonNull)
       .addDereferenceableAttr(1) // the int here is ignored
+      .addDereferenceableOrNullAttr(1) // the int here is ignored
       .addAttribute(Attribute::ReadNone)
       .addAttribute(Attribute::ReadOnly)
       .addAttribute(Attribute::StructRet)
       .addAttribute(Attribute::InAlloca);
 
   return AttributeSet::get(Ty->getContext(), Index, Incompatible);
 }

lib/IR/Function.cpp

 }
 
 void Function::addDereferenceableAttr(unsigned i, uint64_t Bytes) {
   AttributeSet PAL = getAttributes();
   PAL = PAL.addDereferenceableAttr(getContext(), i, Bytes);
   setAttributes(PAL);
 }
 
+void Function::addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes) {
+  AttributeSet PAL = getAttributes();
+  PAL = PAL.addDereferenceableOrNullAttr(getContext(), i, Bytes);
+  setAttributes(PAL);
+}
+
 // Maintain the GC name for each function in an on-the-side table. This saves
 // allocating an additional word in Function for programs which do not use GC
 // (i.e., most programs) at the cost of increased overhead for clients which do
 // use GC.
 static DenseMap<const Function*,PooledStringPtr> *GCNames;
 static StringPool *GCNamePool;
 static ManagedStatic<sys::SmartRWMutex<true> > GCLock;
 

lib/IR/Instructions.cpp

 }
 
 void CallInst::addDereferenceableAttr(unsigned i, uint64_t Bytes) {
   AttributeSet PAL = getAttributes();
   PAL = PAL.addDereferenceableAttr(getContext(), i, Bytes);
   setAttributes(PAL);
 }
 
+void CallInst::addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes) {
+  AttributeSet PAL = getAttributes();
+  PAL = PAL.addDereferenceableOrNullAttr(getContext(), i, Bytes);
+  setAttributes(PAL);
+}
+
 bool CallInst::hasFnAttrImpl(Attribute::AttrKind A) const {
   if (AttributeList.hasAttribute(AttributeSet::FunctionIndex, A))
     return true;
   if (const Function *F = getCalledFunction())
     return F->getAttributes().hasAttribute(AttributeSet::FunctionIndex, A);
   return false;
 }
 

 }
 
 void InvokeInst::addDereferenceableAttr(unsigned i, uint64_t Bytes) {
   AttributeSet PAL = getAttributes();
   PAL = PAL.addDereferenceableAttr(getContext(), i, Bytes);
   setAttributes(PAL);
 }
 
+void InvokeInst::addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes) {
+  AttributeSet PAL = getAttributes();
+  PAL = PAL.addDereferenceableOrNullAttr(getContext(), i, Bytes);
+  setAttributes(PAL);
+}
+
 LandingPadInst *InvokeInst::getLandingPadInst() const {
   return cast<LandingPadInst>(getUnwindDest()->getFirstNonPHI());
 }
 
 //===----------------------------------------------------------------------===//
 //                        ReturnInst Implementation
 //===----------------------------------------------------------------------===//
 

test/Bitcode/attributes.ll

; CHECK: define dereferenceable(18446744073709551606) i8* @f40(i8* dereferenceable(18446744073709551615) %a) {
         ret i8* %a
 }
 
 define void @f41(i8* align 32, double* align 64) {
 ; CHECK: define void @f41(i8* align 32, double* align 64) {
         ret void
 }
 
+; CHECK: define void @f42(i8* dereferenceable_or_null(8) %foo)
+define void @f42(i8* dereferenceable_or_null(8) %foo) {
+ entry:
+  ret void
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }
 ; CHECK: attributes #3 = { readonly }
 ; CHECK: attributes #4 = { noinline }
 ; CHECK: attributes #5 = { alwaysinline }
 ; CHECK: attributes #6 = { optsize }
 ; CHECK: attributes #7 = { ssp }