If we are checking to see if the result of a call aliases a

pointer derived from a local allocation, if the local allocation
never escapes, the pointers can't alias.  This implements PR2436


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52301 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 44 insertions, 1 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 7e096cd..6ab7d94 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -317,6 +317,18 @@ static bool isKnownNonNull(const Value *V) {
   return false;
 }
 
+/// isNonEscapingLocalObject - Return true if the pointer is to a function-local
+/// object that never escapes from the function.
+static bool isNonEscapingLocalObject(const Value *V) {
+  // If this is a local allocation or byval argument, check to see if it
+  // escapes.
+  if (isa<AllocationInst>(V) ||
+      (isa<Argument>(V) && cast<Argument>(V)->hasByValAttr()))
+    return !AddressMightEscape(V);
+  return false;
+}
+
+
 /// isObjectSmallerThan - Return true if we can prove that the object specified
 /// by V is smaller than Size.
 static bool isObjectSmallerThan(const Value *V, unsigned Size,
@@ -393,7 +405,15 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
       (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
     return NoAlias;
   
-  
+  // If one pointer is the result of a call/invoke and the other is a
+  // non-escaping local object, then we know the object couldn't escape to a
+  // point where the call could return it.
+  if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
+      isNonEscapingLocalObject(O2))
+    return NoAlias;
+  if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
+      isNonEscapingLocalObject(O1))
+    return NoAlias;
   
   // If we have two gep instructions with must-alias'ing base pointers, figure
   // out if the indexes to the GEP tell us anything about the derived pointer.
diff --git a/test/Analysis/BasicAA/no-escape-call.ll b/test/Analysis/BasicAA/no-escape-call.ll
new file mode 100644
index 0000000..ab1fea7
--- /dev/null
+++ b/test/Analysis/BasicAA/no-escape-call.ll
@@ -0,0 +1,23 @@
+; RUN: llvm-as < %s | opt -basicaa -gvn -instcombine | llvm-dis | grep {ret i1 true}
+; PR2436
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
+target triple = "i386-apple-darwin8"
+
+define i1 @foo(i32 %i) nounwind  {
+entry:
+	%arr = alloca [10 x i8*]		; <[10 x i8*]*> [#uses=1]
+	%tmp2 = call i8* @getPtr( ) nounwind 		; <i8*> [#uses=2]
+	%tmp4 = getelementptr [10 x i8*]* %arr, i32 0, i32 %i		; <i8**> [#uses=2]
+	store i8* %tmp2, i8** %tmp4, align 4
+	%tmp10 = getelementptr i8* %tmp2, i32 10		; <i8*> [#uses=1]
+	store i8 42, i8* %tmp10, align 1
+	%tmp14 = load i8** %tmp4, align 4		; <i8*> [#uses=1]
+	%tmp16 = getelementptr i8* %tmp14, i32 10		; <i8*> [#uses=1]
+	%tmp17 = load i8* %tmp16, align 1		; <i8> [#uses=1]
+	%tmp19 = icmp eq i8 %tmp17, 42		; <i1> [#uses=1]
+	ret i1 %tmp19
+}
+
+declare i8* @getPtr()
+
+declare void @abort() noreturn nounwind