Extend the 'noalias' attribute to function return values. This is intended to

indicate functions that allocate, such as operator new, or list::insert. The
actual definition is slightly less strict (for now).

No changes to the bitcode reader/writer, asm printer or verifier were needed.

llvm-svn: 59934

llvm/docs/LangRef.html

@@ -890,10 +890,11 @@ declare signext i8 @returns_signed_char()
     return values. </dd>
 
     <dt><tt>noalias</tt></dt>
-    <dd>This indicates that the parameter does not alias any global or any other
-    parameter.  The caller is responsible for ensuring that this is the case,
-    usually by placing the value in a stack allocation. This is not a valid
-    attribute for return values.</dd>
+    <dd>This indicates that the pointer does not alias any global or any other
+    parameter.  The caller is responsible for ensuring that this is the
+    case. Additionally, on a function return value <tt>noalias</tt> indicates
+    that the pointer does not alias the return value from other calls of
+    itself or other noalias functions.</dd>
 
     <dt><tt>nest</tt></dt>
     <dd>This indicates that the pointer parameter can be excised using the
@@ -3155,7 +3156,7 @@ choose to align the allocation on any convenient boundary.</p>
 <h5>Semantics:</h5>
 
 <p>Memory is allocated using the system "<tt>malloc</tt>" function, and
-a pointer is returned.  The result of a zero byte allocattion is undefined.  The
+a pointer is returned.  The result of a zero byte allocation is undefined.  The
 result is null if there is insufficient memory available.</p>
 
 <h5>Example:</h5>

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -104,14 +104,24 @@ static const Value *GetGEPOperands(const Value *V,
   return V;
 }
 
+/// isNoAliasCall - Return true if this pointer is returned by a noalias
+/// function.
+static bool isNoAliasCall(const Value *V) {
+  if (isa<CallInst>(V) || isa<InvokeInst>(V))
+    return CallSite(const_cast<Instruction*>(cast<Instruction>(V)))
+      .paramHasAttr(0, Attribute::NoAlias);
+  return false;
+}
+
 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
 /// identifiable object.  This returns true for:
 ///    Global Variables and Functions
 ///    Allocas and Mallocs
 ///    ByVal and NoAlias Arguments
+///    NoAlias returns
 ///
 static bool isIdentifiedObject(const Value *V) {
-  if (isa<GlobalValue>(V) || isa<AllocationInst>(V))
+  if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V))
     return true;
   if (const Argument *A = dyn_cast<Argument>(V))
     return A->hasNoAliasAttr() || A->hasByValAttr();
@@ -138,7 +148,7 @@ static bool isKnownNonNull(const Value *V) {
 /// object that never escapes from the function.
 static bool isNonEscapingLocalObject(const Value *V) {
   // If this is a local allocation, check to see if it escapes.
-  if (isa<AllocationInst>(V))
+  if (isa<AllocationInst>(V) || isNoAliasCall(V))
     return !AddressMightEscape(V);
       
   // If this is an argument that corresponds to a byval or noalias argument,
@@ -355,8 +365,7 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
   // Are we checking for alias of the same value?
   if (V1 == V2) return MustAlias;
 
-  if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
-      V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty)
+  if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
     return NoAlias;  // Scalars cannot alias each other
 
   // Strip off cast instructions...
@@ -374,11 +383,11 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
     if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
       return NoAlias;
   
-    // Incoming argument cannot alias locally allocated object!
-    if ((isa<Argument>(O1) && isa<AllocationInst>(O2)) ||
-        (isa<Argument>(O2) && isa<AllocationInst>(O1)))
+    // Local allocations can't alias with arguments or noalias functions.
+    if ((isa<AllocationInst>(O1) && (isa<Argument>(O2) || isNoAliasCall(O2))) ||
+        (isa<AllocationInst>(O2) && (isa<Argument>(O1) || isNoAliasCall(O1))))
       return NoAlias;
-    
+
     // Most objects can't alias null.
     if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
         (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))

llvm/lib/AsmParser/llvmAsmParser.y

@@ -1284,9 +1284,10 @@ OptAttributes : /* empty */  { $$ = Attribute::None; }
               }
               ;
 
-RetAttr       : INREG    { $$ = Attribute::InReg;     }
+RetAttr       : INREG    { $$ = Attribute::InReg;    }
               | ZEROEXT  { $$ = Attribute::ZExt;     }
               | SIGNEXT  { $$ = Attribute::SExt;     }
+              | NOALIAS  { $$ = Attribute::NoAlias;  }
               ;
 
 OptRetAttrs  : /* empty */ { $$ = Attribute::None; }

llvm/test/Analysis/BasicAA/2008-11-23-NoaliasRet.ll

@@ -0,0 +1,12 @@
+; RUN: llvm-as < %s | opt -aa-eval |& grep {1 no alias response}
+
+declare noalias i32* @_Znwj(i32 %x) nounwind
+
+define i32 @foo() {
+  %A = call i32* @_Znwj(i32 4)
+  %B = call i32* @_Znwj(i32 4)
+  store i32 1, i32* %A
+  store i32 2, i32* %B
+  %C = load i32* %A
+  ret i32 %C
+}

llvm/test/Feature/noalias-ret.ll

@@ -0,0 +1,6 @@
+; RUN: llvm-as < %s
+
+define noalias i8* @_Znwj(i32 %x) nounwind {
+  %A = malloc i8, i32 %x
+  ret i8* %A
+}