Add a dereferenceable attribute

This attribute indicates that the parameter or return pointer is
dereferenceable. Practically speaking, loads from such a pointer within the
associated byte range are safe to speculatively execute. Such pointer
parameters are common in source languages (C++ references, for example).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213385 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -969,6 +969,17 @@ Currently, only the following parameter attributes are defined:
     passed in is non-null, or the callee must ensure that the returned pointer 
     is non-null.
 
+``dereferenceable(<n>)``
+    This indicates that the parameter or return pointer is dereferenceable. This
+    attribute may only be applied to pointer typed parameters. A pointer that
+    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).
+
 .. _gc:
 
 Garbage Collector Names

include/llvm-c/Core.h

@@ -168,6 +168,7 @@ typedef enum {
     LLVMInAllocaAttribute = 1ULL << 36,
     LLVMNonNullAttribute = 1ULL << 37,
     LLVMJumpTableAttribute = 1ULL << 38,
+    LLVMDereferenceableAttribute = 1ULL << 39,
     */
 } LLVMAttribute;
 

include/llvm/Bitcode/LLVMBitCodes.h

@@ -374,7 +374,8 @@ namespace bitc {
     ATTR_KIND_OPTIMIZE_NONE = 37,
     ATTR_KIND_IN_ALLOCA = 38,
     ATTR_KIND_NON_NULL = 39,
-    ATTR_KIND_JUMP_TABLE = 40
+    ATTR_KIND_JUMP_TABLE = 40,
+    ATTR_KIND_DEREFERENCEABLE = 41
   };
 
   enum ComdatSelectionKindCodes {

include/llvm/IR/Argument.h

@@ -56,9 +56,15 @@ public:
   unsigned getArgNo() const;
 
   /// \brief Return true if this argument has the nonnull attribute on it in
-  /// its containing function.
+  /// its containing function. Also returns true if at least one byte is known
+  /// to be dereferenceable and the pointer is in addrspace(0).
   bool hasNonNullAttr() const;
 
+  /// \brief If this argument has the dereferenceable attribute on it in its
+  /// containing function, return the number of bytes known to be
+  /// dereferenceable. Otherwise, zero is returned.
+  uint64_t getDereferenceableBytes() const;
+
   /// \brief Return true if this argument has the byval attribute on it in its
   /// containing function.
   bool hasByValAttr() const;

include/llvm/IR/Attributes.h

@@ -88,6 +88,7 @@ public:
     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
     NoRedZone,             ///< Disable redzone
     NoReturn,              ///< Mark the function as not returning
     NoUnwind,              ///< Function doesn't unwind stack
@@ -133,6 +134,8 @@ public:
   /// 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);
 
   //===--------------------------------------------------------------------===//
   // Attribute Accessors
@@ -178,6 +181,10 @@ public:
   /// 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 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;
@@ -316,6 +323,9 @@ public:
   /// \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 Return the attributes at the index as a string.
   std::string getAsString(unsigned Index, bool InAttrGrp = false) const;
 
@@ -395,13 +405,15 @@ class AttrBuilder {
   std::map<std::string, std::string> TargetDepAttrs;
   uint64_t Alignment;
   uint64_t StackAlignment;
+  uint64_t DerefBytes;
 public:
-  AttrBuilder() : Attrs(0), Alignment(0), StackAlignment(0) {}
+  AttrBuilder() : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {}
   explicit AttrBuilder(uint64_t Val)
-    : Attrs(0), Alignment(0), StackAlignment(0) {
+    : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {
     addRawValue(Val);
   }
-  AttrBuilder(const Attribute &A) : Attrs(0), Alignment(0), StackAlignment(0) {
+  AttrBuilder(const Attribute &A)
+    : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {
     addAttribute(A);
   }
   AttrBuilder(AttributeSet AS, unsigned Idx);
@@ -455,6 +467,10 @@ 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 This turns an int alignment (which must be a power of 2) into the
   /// form used internally in Attribute.
   AttrBuilder &addAlignmentAttr(unsigned Align);
@@ -463,6 +479,10 @@ public:
   /// 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 Return true if the builder contains no target-independent
   /// attributes.
   bool empty() const { return Attrs.none(); }

include/llvm/IR/CallSite.h

@@ -217,6 +217,12 @@ public:
     CALLSITE_DELEGATE_GETTER(getParamAlignment(i));
   }
 
+  /// @brief Extract the number of dereferenceable bytes for a call or
+  /// parameter (0=unknown).
+  uint64_t getDereferenceableBytes(uint16_t i) const {
+    CALLSITE_DELEGATE_GETTER(getDereferenceableBytes(i));
+  }
+
   /// \brief Return true if the call should not be treated as a call to a
   /// builtin.
   bool isNoBuiltin() const {
@@ -302,6 +308,19 @@ public:
            paramHasAttr(ArgNo + 1, Attribute::ReadNone);
   }
 
+  /// @brief Return true if the return value is known to be not null.
+  /// This may be because it has the nonnull attribute, or because at least
+  /// one byte is dereferenceable and the pointer is in addrspace(0).
+  bool isReturnNonNull() const {
+    if (paramHasAttr(0, Attribute::NonNull))
+      return true;
+    else if (getDereferenceableBytes(0) > 0 &&
+             getType()->getPointerAddressSpace() == 0)
+      return true;
+
+    return false;
+  }
+
   /// hasArgument - Returns true if this CallSite passes the given Value* as an
   /// argument to the called function.
   bool hasArgument(const Value *Arg) const {

include/llvm/IR/Function.h

@@ -233,6 +233,12 @@ public:
     return AttributeSets.getParamAlignment(i);
   }
 
+  /// @brief Extract the number of dereferenceable bytes for a call or
+  /// parameter (0=unknown).
+  uint64_t getDereferenceableBytes(unsigned i) const {
+    return AttributeSets.getDereferenceableBytes(i);
+  }
+
   /// @brief Determine if the function does not access memory.
   bool doesNotAccessMemory() const {
     return AttributeSets.hasAttribute(AttributeSet::FunctionIndex,

include/llvm/IR/Instructions.h

@@ -1376,6 +1376,12 @@ public:
     return AttributeList.getParamAlignment(i);
   }
 
+  /// \brief Extract the number of dereferenceable bytes for a call or
+  /// parameter (0=unknown).
+  uint64_t getDereferenceableBytes(unsigned i) const {
+    return AttributeList.getDereferenceableBytes(i);
+  }
+
   /// \brief Return true if the call should not be treated as a call to a
   /// builtin.
   bool isNoBuiltin() const {
@@ -3051,6 +3057,12 @@ public:
     return AttributeList.getParamAlignment(i);
   }
 
+  /// \brief Extract the number of dereferenceable bytes for a call or
+  /// parameter (0=unknown).
+  uint64_t getDereferenceableBytes(unsigned i) const {
+    return AttributeList.getDereferenceableBytes(i);
+  }
+
   /// \brief Return true if the call should not be treated as a call to a
   /// builtin.
   bool isNoBuiltin() const {

lib/Analysis/ValueTracking.cpp

@@ -2086,7 +2086,7 @@ bool llvm::isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI) {
     return !GV->hasExternalWeakLinkage();
 
   if (ImmutableCallSite CS = V)
-    if (CS.paramHasAttr(0, Attribute::NonNull))
+    if (CS.isReturnNonNull())
       return true;
 
   // operator new never returns null.

lib/AsmParser/LLLexer.cpp

@@ -612,6 +612,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(byval);
   KEYWORD(inalloca);
   KEYWORD(cold);
+  KEYWORD(dereferenceable);
   KEYWORD(inlinehint);
   KEYWORD(inreg);
   KEYWORD(jumptable);

lib/AsmParser/LLParser.cpp

@@ -1052,6 +1052,7 @@ bool LLParser::ParseFnAttributeValuePairs(AttrBuilder &B,
               "invalid use of attribute on a function");
       break;
     case lltok::kw_byval:
+    case lltok::kw_dereferenceable:
     case lltok::kw_inalloca:
     case lltok::kw_nest:
     case lltok::kw_noalias:
@@ -1212,6 +1213,16 @@ bool LLParser::ParseUInt32(unsigned &Val) {
   return false;
 }
 
+/// ParseUInt64
+///   ::= uint64
+bool LLParser::ParseUInt64(uint64_t &Val) {
+  if (Lex.getKind() != lltok::APSInt || Lex.getAPSIntVal().isSigned())
+    return TokError("expected integer");
+  Val = Lex.getAPSIntVal().getLimitedValue();
+  Lex.Lex();
+  return false;
+}
+
 /// ParseTLSModel
 ///   := 'localdynamic'
 ///   := 'initialexec'
@@ -1284,6 +1295,13 @@ bool LLParser::ParseOptionalParamAttrs(AttrBuilder &B) {
       continue;
     }
     case lltok::kw_byval:           B.addAttribute(Attribute::ByVal); break;
+    case lltok::kw_dereferenceable: {
+      uint64_t Bytes;
+      if (ParseOptionalDereferenceableBytes(Bytes))
+        return true;
+      B.addDereferenceableAttr(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;
@@ -1341,6 +1359,13 @@ bool LLParser::ParseOptionalReturnAttrs(AttrBuilder &B) {
     switch (Token) {
     default:  // End of attributes.
       return HaveError;
+    case lltok::kw_dereferenceable: {
+      uint64_t Bytes;
+      if (ParseOptionalDereferenceableBytes(Bytes))
+        return true;
+      B.addDereferenceableAttr(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;
@@ -1606,6 +1631,26 @@ bool LLParser::ParseOptionalAlignment(unsigned &Alignment) {
   return false;
 }
 
+/// ParseOptionalDereferenceableBytes
+///   ::= /* empty */
+///   ::= 'dereferenceable' '(' 4 ')'
+bool LLParser::ParseOptionalDereferenceableBytes(uint64_t &Bytes) {
+  Bytes = 0;
+  if (!EatIfPresent(lltok::kw_dereferenceable))
+    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

lib/AsmParser/LLParser.h

@@ -202,6 +202,11 @@ namespace llvm {
       Loc = Lex.getLoc();
       return ParseUInt32(Val);
     }
+    bool ParseUInt64(uint64_t &Val);
+    bool ParseUInt64(uint64_t &Val, LocTy &Loc) {
+      Loc = Lex.getLoc();
+      return ParseUInt64(Val);
+    }
 
     bool ParseTLSModel(GlobalVariable::ThreadLocalMode &TLM);
     bool ParseOptionalThreadLocal(GlobalVariable::ThreadLocalMode &TLM);
@@ -219,6 +224,7 @@ namespace llvm {
     bool ParseOptionalDLLStorageClass(unsigned &DLLStorageClass);
     bool ParseOptionalCallingConv(CallingConv::ID &CC);
     bool ParseOptionalAlignment(unsigned &Alignment);
+    bool ParseOptionalDereferenceableBytes(uint64_t &Bytes);
     bool ParseScopeAndOrdering(bool isAtomic, SynchronizationScope &Scope,
                                AtomicOrdering &Ordering);
     bool ParseOrdering(AtomicOrdering &Ordering);

lib/AsmParser/LLToken.h

@@ -106,6 +106,7 @@ namespace lltok {
     kw_byval,
     kw_inalloca,
     kw_cold,
+    kw_dereferenceable,
     kw_inlinehint,
     kw_inreg,
     kw_jumptable,

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -588,6 +588,8 @@ static Attribute::AttrKind GetAttrFromCode(uint64_t Code) {
     return Attribute::NonLazyBind;
   case bitc::ATTR_KIND_NON_NULL:
     return Attribute::NonNull;
+  case bitc::ATTR_KIND_DEREFERENCEABLE:
+    return Attribute::Dereferenceable;
   case bitc::ATTR_KIND_NO_RED_ZONE:
     return Attribute::NoRedZone;
   case bitc::ATTR_KIND_NO_RETURN:
@@ -689,8 +691,10 @@ std::error_code BitcodeReader::ParseAttributeGroupBlock() {
             return EC;
           if (Kind == Attribute::Alignment)
             B.addAlignmentAttr(Record[++i]);
-          else
+          else if (Kind == Attribute::StackAlignment)
             B.addStackAlignmentAttr(Record[++i]);
+          else if (Kind == Attribute::Dereferenceable)
+            B.addDereferenceableAttr(Record[++i]);
         } else {                     // String attribute
           assert((Record[i] == 3 || Record[i] == 4) &&
                  "Invalid attribute group entry");

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -201,6 +201,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     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::NoRedZone:
     return bitc::ATTR_KIND_NO_RED_ZONE;
   case Attribute::NoReturn:

lib/IR/AttributeImpl.h

@@ -116,7 +116,8 @@ public:
   IntAttributeImpl(Attribute::AttrKind Kind, uint64_t Val)
       : EnumAttributeImpl(IntAttrEntry, Kind), Val(Val) {
     assert(
-        (Kind == Attribute::Alignment || Kind == Attribute::StackAlignment) &&
+        (Kind == Attribute::Alignment || Kind == Attribute::StackAlignment ||
+         Kind == Attribute::Dereferenceable) &&
         "Wrong kind for int attribute!");
   }
 
@@ -164,6 +165,7 @@ public:
 
   unsigned getAlignment() const;
   unsigned getStackAlignment() const;
+  uint64_t getDereferenceableBytes() const;
   std::string getAsString(bool InAttrGrp) const;
 
   typedef const Attribute *iterator;

lib/IR/Attributes.cpp

@@ -88,6 +88,12 @@ Attribute Attribute::getWithStackAlignment(LLVMContext &Context,
   return get(Context, StackAlignment, Align);
 }
 
+Attribute Attribute::getWithDereferenceableBytes(LLVMContext &Context,
+                                                uint64_t Bytes) {
+  assert(Bytes && "Bytes must be non-zero.");
+  return get(Context, Dereferenceable, Bytes);
+}
+
 //===----------------------------------------------------------------------===//
 // Attribute Accessor Methods
 //===----------------------------------------------------------------------===//
@@ -156,6 +162,14 @@ unsigned Attribute::getStackAlignment() const {
   return pImpl->getValueAsInt();
 }
 
+/// 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();
+}
+
 std::string Attribute::getAsString(bool InAttrGrp) const {
   if (!pImpl) return "";
 
@@ -263,6 +277,20 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
     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;
+  }
+
   // Convert target-dependent attributes to strings of the form:
   //
   //   "kind"
@@ -398,6 +426,8 @@ uint64_t AttributeImpl::getAttrMask(Attribute::AttrKind Val) {
   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");
   }
   llvm_unreachable("Unsupported attribute type");
 }
@@ -482,6 +512,13 @@ unsigned AttributeSetNode::getStackAlignment() const {
   return 0;
 }
 
+uint64_t AttributeSetNode::getDereferenceableBytes() const {
+  for (iterator I = begin(), E = end(); I != E; ++I)
+    if (I->hasAttribute(Attribute::Dereferenceable))
+      return I->getDereferenceableBytes();
+  return 0;
+}
+
 std::string AttributeSetNode::getAsString(bool InAttrGrp) const {
   std::string Str;
   for (iterator I = begin(), E = end(); I != E; ++I) {
@@ -515,6 +552,8 @@ uint64_t AttributeSetImpl::Raw(unsigned Index) const {
         Mask |= (Log2_32(ASN->getAlignment()) + 1) << 16;
       else if (Kind == Attribute::StackAlignment)
         Mask |= (Log2_32(ASN->getStackAlignment()) + 1) << 26;
+      else if (Kind == Attribute::Dereferenceable)
+        llvm_unreachable("dereferenceable not supported in bit mask");
       else
         Mask |= AttributeImpl::getAttrMask(Kind);
     }
@@ -620,6 +659,10 @@ AttributeSet AttributeSet::get(LLVMContext &C, unsigned Index,
     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
       Attrs.push_back(std::make_pair(Index, Attribute::get(C, Kind)));
   }
@@ -877,6 +920,11 @@ unsigned AttributeSet::getStackAlignment(unsigned Index) const {
   return ASN ? ASN->getStackAlignment() : 0;
 }
 
+uint64_t AttributeSet::getDereferenceableBytes(unsigned Index) const {
+  AttributeSetNode *ASN = getAttributes(Index);
+  return ASN ? ASN->getDereferenceableBytes() : 0;
+}
+
 std::string AttributeSet::getAsString(unsigned Index,
                                       bool InAttrGrp) const {
   AttributeSetNode *ASN = getAttributes(Index);
@@ -956,7 +1004,7 @@ void AttributeSet::dump() const {
 //===----------------------------------------------------------------------===//
 
 AttrBuilder::AttrBuilder(AttributeSet AS, unsigned Index)
-  : Attrs(0), Alignment(0), StackAlignment(0) {
+  : Attrs(0), Alignment(0), StackAlignment(0), DerefBytes(0) {
   AttributeSetImpl *pImpl = AS.pImpl;
   if (!pImpl) return;
 
@@ -973,13 +1021,14 @@ AttrBuilder::AttrBuilder(AttributeSet AS, unsigned Index)
 
 void AttrBuilder::clear() {
   Attrs.reset();
-  Alignment = StackAlignment = 0;
+  Alignment = StackAlignment = DerefBytes = 0;
 }
 
 AttrBuilder &AttrBuilder::addAttribute(Attribute::AttrKind Val) {
   assert((unsigned)Val < Attribute::EndAttrKinds && "Attribute out of range!");
   assert(Val != Attribute::Alignment && Val != Attribute::StackAlignment &&
-         "Adding alignment attribute without adding alignment value!");
+         Val != Attribute::Dereferenceable &&
+         "Adding integer attribute without adding a value!");
   Attrs[Val] = true;
   return *this;
 }
@@ -997,6 +1046,8 @@ AttrBuilder &AttrBuilder::addAttribute(Attribute Attr) {
     Alignment = Attr.getAlignment();
   else if (Kind == Attribute::StackAlignment)
     StackAlignment = Attr.getStackAlignment();
+  else if (Kind == Attribute::Dereferenceable)
+    DerefBytes = Attr.getDereferenceableBytes();
   return *this;
 }
 
@@ -1013,6 +1064,8 @@ AttrBuilder &AttrBuilder::removeAttribute(Attribute::AttrKind Val) {
     Alignment = 0;
   else if (Val == Attribute::StackAlignment)
     StackAlignment = 0;
+  else if (Val == Attribute::Dereferenceable)
+    DerefBytes = 0;
 
   return *this;
 }
@@ -1037,6 +1090,8 @@ AttrBuilder &AttrBuilder::removeAttributes(AttributeSet A, uint64_t Index) {
         Alignment = 0;
       else if (Kind == Attribute::StackAlignment)
         StackAlignment = 0;
+      else if (Kind == Attribute::Dereferenceable)
+        DerefBytes = 0;
     } else {
       assert(Attr.isStringAttribute() && "Invalid attribute type!");
       std::map<std::string, std::string>::iterator
@@ -1079,6 +1134,14 @@ AttrBuilder &AttrBuilder::addStackAlignmentAttr(unsigned Align) {
   return *this;
 }
 
+AttrBuilder &AttrBuilder::addDereferenceableAttr(uint64_t Bytes) {
+  if (Bytes == 0) return *this;
+
+  Attrs[Attribute::Dereferenceable] = true;
+  DerefBytes = Bytes;
+  return *this;
+}
+
 AttrBuilder &AttrBuilder::merge(const AttrBuilder &B) {
   // FIXME: What if both have alignments, but they don't match?!
   if (!Alignment)
@@ -1087,6 +1150,9 @@ AttrBuilder &AttrBuilder::merge(const AttrBuilder &B) {
   if (!StackAlignment)
     StackAlignment = B.StackAlignment;
 
+  if (!DerefBytes)
+    DerefBytes = B.DerefBytes;
+
   Attrs |= B.Attrs;
 
   for (td_const_iterator I = B.TargetDepAttrs.begin(),
@@ -1142,7 +1208,8 @@ bool AttrBuilder::operator==(const AttrBuilder &B) {
     if (B.TargetDepAttrs.find(I->first) == B.TargetDepAttrs.end())
       return false;
 
-  return Alignment == B.Alignment && StackAlignment == B.StackAlignment;
+  return Alignment == B.Alignment && StackAlignment == B.StackAlignment &&
+         DerefBytes == B.DerefBytes;
 }
 
 AttrBuilder &AttrBuilder::addRawValue(uint64_t Val) {
@@ -1151,6 +1218,8 @@ AttrBuilder &AttrBuilder::addRawValue(uint64_t Val) {
 
   for (Attribute::AttrKind I = Attribute::None; I != Attribute::EndAttrKinds;
        I = Attribute::AttrKind(I + 1)) {
+    if (I == Attribute::Dereferenceable)
+      continue;
     if (uint64_t A = (Val & AttributeImpl::getAttrMask(I))) {
       Attrs[I] = true;
  
@@ -1184,6 +1253,7 @@ AttributeSet AttributeFuncs::typeIncompatible(Type *Ty, uint64_t Index) {
       .addAttribute(Attribute::NoAlias)
       .addAttribute(Attribute::NoCapture)
       .addAttribute(Attribute::NonNull)
+      .addDereferenceableAttr(1) // the int here is ignored
       .addAttribute(Attribute::ReadNone)
       .addAttribute(Attribute::ReadOnly)
       .addAttribute(Attribute::StructRet)

lib/IR/Function.cpp

@@ -77,11 +77,17 @@ unsigned Argument::getArgNo() const {
 }
 
 /// hasNonNullAttr - Return true if this argument has the nonnull attribute on
-/// it in its containing function.
+/// it in its containing function. Also returns true if at least one byte is
+/// known to be dereferenceable and the pointer is in addrspace(0).
 bool Argument::hasNonNullAttr() const {
   if (!getType()->isPointerTy()) return false;
-  return getParent()->getAttributes().
-    hasAttribute(getArgNo()+1, Attribute::NonNull);
+  if (getParent()->getAttributes().
+        hasAttribute(getArgNo()+1, Attribute::NonNull))
+    return true;
+  else if (getDereferenceableBytes() > 0 &&
+           getType()->getPointerAddressSpace() == 0)
+    return true;
+  return false;
 }
 
 /// hasByValAttr - Return true if this argument has the byval attribute on it
@@ -113,6 +119,12 @@ unsigned Argument::getParamAlignment() const {
 
 }
 
+uint64_t Argument::getDereferenceableBytes() const {
+  assert(getType()->isPointerTy() &&
+         "Only pointers have dereferenceable bytes");
+  return getParent()->getDereferenceableBytes(getArgNo()+1);
+}
+
 /// hasNestAttr - Return true if this argument has the nest attribute on
 /// it in its containing function.
 bool Argument::hasNestAttr() const {

lib/IR/Value.cpp

@@ -15,6 +15,7 @@
 #include "LLVMContextImpl.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
@@ -504,9 +505,29 @@ static bool isDereferenceablePointer(const Value *V, const DataLayout *DL,
   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
     return !GV->hasExternalWeakLinkage();
 
-  // byval arguments are ok.
-  if (const Argument *A = dyn_cast<Argument>(V))
-    return A->hasByValAttr();
+  // byval arguments are okay. Arguments specifically marked as
+  // dereferenceable are okay too.
+  if (const Argument *A = dyn_cast<Argument>(V)) {
+    if (A->hasByValAttr())
+      return true;
+    else if (uint64_t Bytes = A->getDereferenceableBytes()) {
+      Type *Ty = V->getType()->getPointerElementType();
+      if (Ty->isSized() && DL && DL->getTypeStoreSize(Ty) <= Bytes)
+        return true;
+    }
+
+    return false;
+  }
+
+  // Return values from call sites specifically marked as dereferenceable are
+  // also okay.
+  if (ImmutableCallSite CS = V) {
+    if (uint64_t Bytes = CS.getDereferenceableBytes(0)) {
+      Type *Ty = V->getType()->getPointerElementType();
+      if (Ty->isSized() && DL && DL->getTypeStoreSize(Ty) <= Bytes)
+        return true;
+    }
+  }
 
   // For GEPs, determine if the indexing lands within the allocated object.
   if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {

test/Bitcode/attributes.ll

@@ -229,6 +229,16 @@ define void @f38() unnamed_addr jumptable {
     unreachable
 }
 
+define dereferenceable(2) i8* @f39(i8* dereferenceable(1) %a) {
+; CHECK: define dereferenceable(2) i8* @f39(i8* dereferenceable(1) %a) {
+        ret i8* %a
+}
+
+define dereferenceable(18446744073709551606) i8* @f40(i8* dereferenceable(18446744073709551615) %a) {
+; CHECK: define dereferenceable(18446744073709551606) i8* @f40(i8* dereferenceable(18446744073709551615) %a) {
+        ret i8* %a
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }

test/Transforms/InstSimplify/compare.ll

@@ -874,6 +874,21 @@ define i1 @nonnull_arg(i32* nonnull %i) {
 ; CHECK: ret i1 false
 }
 
+define i1 @nonnull_deref_arg(i32* dereferenceable(4) %i) {
+  %cmp = icmp eq i32* %i, null
+  ret i1 %cmp
+; CHECK-LABEL: @nonnull_deref_arg
+; CHECK: ret i1 false
+}
+
+define i1 @nonnull_deref_as_arg(i32 addrspace(1)* dereferenceable(4) %i) {
+  %cmp = icmp eq i32 addrspace(1)* %i, null
+  ret i1 %cmp
+; CHECK-LABEL: @nonnull_deref_as_arg
+; CHECK: icmp
+; CHECK ret
+}
+
 declare nonnull i32* @returns_nonnull_helper()
 define i1 @returns_nonnull() {
   %call = call nonnull i32* @returns_nonnull_helper()
@@ -883,6 +898,25 @@ define i1 @returns_nonnull() {
 ; CHECK: ret i1 false
 }
 
+declare dereferenceable(4) i32* @returns_nonnull_deref_helper()
+define i1 @returns_nonnull_deref() {
+  %call = call dereferenceable(4) i32* @returns_nonnull_deref_helper()
+  %cmp = icmp eq i32* %call, null
+  ret i1 %cmp
+; CHECK-LABEL: @returns_nonnull_deref
+; CHECK: ret i1 false
+}
+
+declare dereferenceable(4) i32 addrspace(1)* @returns_nonnull_deref_as_helper()
+define i1 @returns_nonnull_as_deref() {
+  %call = call dereferenceable(4) i32 addrspace(1)* @returns_nonnull_deref_as_helper()
+  %cmp = icmp eq i32 addrspace(1)* %call, null
+  ret i1 %cmp
+; CHECK-LABEL: @returns_nonnull_as_deref
+; CHECK: icmp
+; CHECK: ret
+}
+
 ; If a bit is known to be zero for A and known to be one for B,
 ; then A and B cannot be equal.
 define i1 @icmp_eq_const(i32 %a) nounwind {

test/Transforms/LICM/hoist-deref-load.ll

@@ -0,0 +1,86 @@
+; RUN: opt -S -basicaa -licm < %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; This test represents the following function:
+; void test1(int * __restrict__ a, int * __restrict__ b, int &c, int n) {
+;   for (int i = 0; i < n; ++i)
+;     if (a[i] > 0)
+;       a[i] = c*b[i];
+; }
+; and we want to hoist the load of %c out of the loop. This can be done only
+; because the dereferenceable attribute is on %c.
+
+; CHECK-LABEL: @test1
+; CHECK: load i32* %c, align 4
+; CHECK: for.body:
+
+define void @test1(i32* noalias nocapture %a, i32* noalias nocapture readonly %b, i32* nocapture readonly nonnull dereferenceable(4) %c, i32 %n) #0 {
+entry:
+  %cmp11 = icmp sgt i32 %n, 0
+  br i1 %cmp11, label %for.body, label %for.end
+
+for.body:                                         ; preds = %entry, %for.inc
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %cmp1 = icmp sgt i32 %0, 0
+  br i1 %cmp1, label %if.then, label %for.inc
+
+if.then:                                          ; preds = %for.body
+  %1 = load i32* %c, align 4
+  %arrayidx3 = getelementptr inbounds i32* %b, i64 %indvars.iv
+  %2 = load i32* %arrayidx3, align 4
+  %mul = mul nsw i32 %2, %1
+  store i32 %mul, i32* %arrayidx, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body, %if.then
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.inc, %entry
+  ret void
+}
+
+; This is the same as @test1, but without the dereferenceable attribute on %c.
+; Without this attribute, we should not hoist the load of %c.
+
+; CHECK-LABEL: @test2
+; CHECK: if.then:
+; CHECK: load i32* %c, align 4
+
+define void @test2(i32* noalias nocapture %a, i32* noalias nocapture readonly %b, i32* nocapture readonly nonnull %c, i32 %n) #0 {
+entry:
+  %cmp11 = icmp sgt i32 %n, 0
+  br i1 %cmp11, label %for.body, label %for.end
+
+for.body:                                         ; preds = %entry, %for.inc
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %cmp1 = icmp sgt i32 %0, 0
+  br i1 %cmp1, label %if.then, label %for.inc
+
+if.then:                                          ; preds = %for.body
+  %1 = load i32* %c, align 4
+  %arrayidx3 = getelementptr inbounds i32* %b, i64 %indvars.iv
+  %2 = load i32* %arrayidx3, align 4
+  %mul = mul nsw i32 %2, %1
+  store i32 %mul, i32* %arrayidx, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body, %if.then
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.inc, %entry
+  ret void
+}
+
+attributes #0 = { nounwind uwtable }
+