llvm.noalias - don't block EarlyCSE

[hfinkel]

Summary

This part of the series started by D9375, and as with llvm.assume, prevents llvm.noalias from forming a generation barrier in EarlyCSE.

[reames]

LGTM

[hfinkel]

Rebased.

[majnemer]

LGTM

[hfinkel]

Rebased

[hfinkel]

Rebased.

[troyj]

LGTM too.

lib/Transforms/Scalar/EarlyCSE.cpp

if (match(Inst, m_Intrinsic<Intrinsic::assume>())) {
       if (CondI && SimpleValue::canHandle(CondI)) {
         DEBUG(dbgs() << "EarlyCSE considering assumption: " << *Inst << '\n');
         AvailableValues.insert(CondI, ConstantInt::getTrue(BB->getContext()));
       } else
         DEBUG(dbgs() << "EarlyCSE skipping assumption: " << *Inst << '\n');
       continue;
     }
 
+    // Likewise, noalias intrinsics don't actually write.
+    if (match(Inst, m_Intrinsic<Intrinsic::noalias>())) {
+      DEBUG(dbgs() << "EarlyCSE skipping intrinsic: " << *Inst << '\n');
+      continue;
+    }
+
     // Skip invariant.start intrinsics since they only read memory, and we can
     // forward values across it. Also, we dont need to consume the last store
     // since the semantics of invariant.start allow us to perform DSE of the
     // last store, if there was a store following invariant.start. Consider:
     //
     // store 30, i8* p
     // invariant.start(p)
     // store 40, i8* p

test/Transforms/EarlyCSE/basic.ll

define i32 @test2a(i32 *%P, i1 %b) {
   %V1 = load i32, i32* %P
   tail call void @llvm.assume(i1 %b)
   %V2 = load i32, i32* %P
   %Diff = sub i32 %V1, %V2
   ret i32 %Diff
   ; CHECK: ret i32 0
 }
 
+; CHECK-LABEL: @test2b(
+define i32 @test2b(i32 *%P, i1 %b) {
+  %V1 = load i32, i32* %P
+  call i8* @llvm.noalias.p0i8(i8* undef, metadata !1)
+  %V2 = load i32, i32* %P
+  %Diff = sub i32 %V1, %V2
+  ret i32 %Diff
+  ; CHECK: ret i32 0
+}
+
 ;; Cross block load value numbering.
 ; CHECK-LABEL: @test3(
 define i32 @test3(i32 *%P, i1 %Cond) {
   %V1 = load i32, i32* %P
   br i1 %Cond, label %T, label %F
 T:
   store i32 4, i32* %P
   ret i32 42

 define i32 @test6a(i32 *%P, i1 %b) {
   store i32 42, i32* %P
   tail call void @llvm.assume(i1 %b)
   %V1 = load i32, i32* %P
   ret i32 %V1
   ; CHECK: ret i32 42
 }
 
+; CHECK-LABEL: @test6b(
+define i32 @test6b(i32 *%P, i1 %b) {
+  store i32 42, i32* %P
+  call i8* @llvm.noalias.p0i8(i8* undef, metadata !1)
+  %V1 = load i32, i32* %P
+  ret i32 %V1
+  ; CHECK: ret i32 42
+}
+
 ;; Trivial dead store elimination.
 ; CHECK-LABEL: @test7(
 define void @test7(i32 *%P) {
   store i32 42, i32* %P
   store i32 45, i32* %P
   ret void
   ; CHECK-NEXT: store i32 45
   ; CHECK-NEXT: ret void

 ; CHECK: call i32 @reads_c(i32 0)
 ; CHECK: store i32 2, i32* @c, align 4
   %load = load i32, i32* @c, align 4
   store i32 0, i32* @c, align 4
   %call = call i32 @reads_c(i32 0)
   store i32 2, i32* @c, align 4
   ret void
 }
+
+declare i8* @llvm.noalias.p0i8(i8*, metadata) nounwind
+
+!0 = !{!0, !"some domain"}
+!1 = !{!1, !0, !"some scope"}