Don't infer whether a value is captured in the current function from the

'nocapture' attribute.

The nocapture attribute only specifies that no copies are made that
outlive the function. This isn't the same as there being no copies at all.
This fixes PR14045.

llvm-svn: 167381

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -58,12 +58,12 @@ static bool isNonEscapingLocalObject(const Value *V) {
   // then it has not escaped before entering the function.  Check if it escapes
   // inside the function.
   if (const Argument *A = dyn_cast<Argument>(V))
-    if (A->hasByValAttr() || A->hasNoAliasAttr()) {
-      // Don't bother analyzing arguments already known not to escape.
-      if (A->hasNoCaptureAttr())
-        return true;
+    if (A->hasByValAttr() || A->hasNoAliasAttr())
+      // Note even if the argument is marked nocapture we still need to check
+      // for copies made inside the function. The nocapture attribute only
+      // specifies that there are no copies made that outlive the function.
       return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
-    }
+
   return false;
 }
 

llvm/test/Analysis/BasicAA/nocapture.ll

@@ -13,3 +13,24 @@ define i32 @test2() {
        ret i32 %c
 }
 
+declare void @test3(i32** %p, i32* %q) nounwind
+
+define i32 @test4(i32* noalias nocapture %p) nounwind {
+; CHECK: call void @test3
+; CHECK: store i32 0, i32* %p
+; CHECK: store i32 1, i32* %x
+; CHECK: %y = load i32* %p
+; CHECK: ret i32 %y
+entry:
+       %q = alloca i32*
+       ; Here test3 might store %p to %q. This doesn't violate %p's nocapture
+       ; attribute since the copy doesn't outlive the function.
+       call void @test3(i32** %q, i32* %p) nounwind
+       store i32 0, i32* %p
+       %x = load i32** %q
+       ; This store might write to %p and so we can't eliminate the subsequent
+       ; load
+       store i32 1, i32* %x
+       %y = load i32* %p
+       ret i32 %y
+}