Default-addressspace null pointers don't alias anything. This allows

GVN to be more aggressive. Patch by Hans Wennborg! (with a comment added by me)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86582 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/BasicAliasAnalysis.cpp

@@ -646,6 +646,15 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
   const Value *O1 = V1->getUnderlyingObject();
   const Value *O2 = V2->getUnderlyingObject();
 
+  // Null values in the default address space don't point to any object, so they
+  // don't alias any other pointer.
+  if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
+    if (CPN->getType()->getAddressSpace() == 0)
+      return NoAlias;
+  if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
+    if (CPN->getType()->getAddressSpace() == 0)
+      return NoAlias;
+
   if (O1 != O2) {
     // If V1/V2 point to two different objects we know that we have no alias.
     if (isIdentifiedObject(O1) && isIdentifiedObject(O2))

test/Transforms/GVN/null-aliases-nothing.ll

@@ -0,0 +1,20 @@
+; RUN: opt %s -gvn -S | FileCheck %s
+
+%t = type { i32 }
+declare void @test1f(i8*)
+
+define void @test1(%t* noalias %stuff ) {
+    %p = getelementptr inbounds %t* %stuff, i32 0, i32 0
+    %before = load i32* %p
+
+    call void @test1f(i8* null)
+
+    %after = load i32* %p ; <--- This should be a dead load
+    %sum = add i32 %before, %after;
+
+    store i32 %sum, i32* %p
+    ret void
+; CHECK: load
+; CHECK-NOT: load
+; CHECK: ret void
+}