Teach BasicAA that a constant expression can't alias memory provably not

allocated until runtime (such as an alloca). Patch by Hans Wennborg!

llvm-svn: 88760

Author: nlewycky

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -659,7 +659,12 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
     // If V1/V2 point to two different objects we know that we have no alias.
     if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
       return NoAlias;
-  
+
+    // Constant pointers can't alias with non-const isIdentifiedObject objects.
+    if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
+        (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
+      return NoAlias;
+
     // Arguments can't alias with local allocations or noalias calls.
     if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
         (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))

llvm/test/Transforms/DeadStoreElimination/const-pointers.ll

@@ -0,0 +1,39 @@
+; RUN: opt %s -dse -S | FileCheck %s
+
+%t = type { i32 }
+
+@g = global i32 42;
+
+define void @test1(%t* noalias %pp) {
+  %p = getelementptr inbounds %t* %pp, i32 0, i32 0
+
+  store i32 1, i32* %p; <-- This is dead
+  %x = load i32* inttoptr (i32 12345 to i32*)
+  store i32 %x, i32* %p
+  ret void
+; CHECK define void @test1
+; CHECK: store
+; CHECK-NOT: store
+; CHECK: ret void
+}
+
+define void @test3() {
+  store i32 1, i32* @g; <-- This is dead.
+  store i32 42, i32* @g
+  ret void
+;CHECK define void @test3
+;CHECK: store
+;CHECK-NOT: store
+;CHECK: ret void
+}
+
+define void @test4(i32* %p) {
+  store i32 1, i32* %p
+  %x = load i32* @g; <-- %p and @g could alias
+  store i32 %x, i32* %p
+  ret void
+; CHECK define void @test4
+; CHECK: store
+; CHECK: store
+; CHECK: ret void
+}