llvm.noalias - isObjectSizeLessThanOrEq in InstCombine should look through them

[hfinkel]

Summary

This is part of the series started by D9375, and teaches isObjectSizeLessThanOrEq in InstCombine to look through the calls to llvm.noalias.

[reames]

Resigning as a reviewer to get a very stale review off my list of blocking tasks in phabricator. Please readd when desired.

[hfinkel]

This change should not be needed if noalias intrinsics are stripped by default.

lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

 // FIXME: This should probably live in ValueTracking (or similar).
 static bool isObjectSizeLessThanOrEq(Value *V, uint64_t MaxSize,
                                      const DataLayout &DL) {
   SmallPtrSet<Value *, 4> Visited;
   SmallVector<Value *, 4> Worklist(1, V);
 
   do {
     Value *P = Worklist.pop_back_val();
-    P = P->stripPointerCasts();
+    P = P->stripPointerCasts(/*LookThroughNoAlias*/true);
 
     if (!Visited.insert(P).second)
       continue;
 
     if (SelectInst *SI = dyn_cast<SelectInst>(P)) {
       Worklist.push_back(SI->getTrueValue());
       Worklist.push_back(SI->getFalseValue());
       continue;

test/Transforms/InstCombine/mem-gep-zidx.ll

entry:
   ret void
 
 ; CHECK-LABEL: @test2
 ; CHECK: %p = alloca i64
 ; CHECK: store i64 %v, i64* %p
 ; CHECK: ret void
 }
 
+define void @test2a(i32 signext %x, i64 %v) #0 {
+entry:
+  %p = alloca i64
+  %pa = call i64* @llvm.noalias.p0i64(i64* %p, metadata !1)
+  %idxprom = sext i32 %x to i64
+  %arrayidx = getelementptr inbounds i64, i64* %pa, i64 %idxprom
+  store i64 %v, i64* %arrayidx
+  call void @foo(i64* %pa)
+  ret void
+
+; CHECK-LABEL: @test2a
+; CHECK: %p = alloca i64
+; CHECK: store i64 %v, i64* %pa
+; CHECK: ret void
+}
+
 define signext i32 @test3(i32 signext %x, i1 %y) #0 {
 entry:
   %idxprom = sext i32 %x to i64
   %p = select i1 %y, [1 x i32]* @f.a, [1 x i32]* @f.b
   %arrayidx = getelementptr inbounds [1 x i32], [1 x i32]* %p, i64 0, i64 %idxprom
   %0 = load i32, i32* %arrayidx, align 4
   ret i32 %0
 
 ; CHECK-LABEL: @test3
 ; CHECK: getelementptr inbounds [1 x i32], [1 x i32]* %p, i64 0, i64 0
 }
 
 attributes #0 = { nounwind readnone }
 
+declare i64* @llvm.noalias.p0i64(i64*, metadata) nounwind
+
+!0 = !{!0, !"some domain"}
+!1 = !{!1, !0, !"some scope"}
+