[llvm] [LoopDeletion] Unblock loop deletion with `llvm.experimental.noalias.scope.decl` (PR #108144)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Since `llvm.experimental.noalias.scope.decl` is marked as `memory(inaccessiblemem: readwrite)`, we cannot treat this annotation intrinsic as having no side effects. It will block loop deletion when this intrinsic exists inside a dead loop:
https://github.com/llvm/llvm-project/blob/3dad29b677e427bf69c035605a16efd065576829/llvm/lib/Transforms/Scalar/LoopDeletion.cpp#L103-L110

This patch marks `llvm.experimental.noalias.scope.decl` as droppable to address the issue.

Fixes https://github.com/llvm/llvm-project/issues/108052.


---
Full diff: https://github.com/llvm/llvm-project/pull/108144.diff


2 Files Affected:

- (modified) llvm/lib/IR/User.cpp (+11-1) 
- (added) llvm/test/Transforms/LoopDeletion/noalias.ll (+28) 


``````````diff
diff --git a/llvm/lib/IR/User.cpp b/llvm/lib/IR/User.cpp
index 637af7aaa24530..00dd9c72c469cb 100644
--- a/llvm/lib/IR/User.cpp
+++ b/llvm/lib/IR/User.cpp
@@ -113,7 +113,17 @@ MutableArrayRef<uint8_t> User::getDescriptor() {
 }
 
 bool User::isDroppable() const {
-  return isa<AssumeInst>(this) || isa<PseudoProbeInst>(this);
+  if (auto *II = dyn_cast<IntrinsicInst>(this)) {
+    switch (II->getIntrinsicID()) {
+    default:
+      return false;
+    case Intrinsic::assume:
+    case Intrinsic::pseudoprobe:
+    case Intrinsic::experimental_noalias_scope_decl:
+      return true;
+    }
+  }
+  return false;
 }
 
 //===----------------------------------------------------------------------===//
diff --git a/llvm/test/Transforms/LoopDeletion/noalias.ll b/llvm/test/Transforms/LoopDeletion/noalias.ll
new file mode 100644
index 00000000000000..0f3b71df942708
--- /dev/null
+++ b/llvm/test/Transforms/LoopDeletion/noalias.ll
@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=loop-deletion -S | FileCheck %s
+
+define void @pr108052(i64 %n) {
+; CHECK-LABEL: define void @pr108052(
+; CHECK-SAME: i64 [[N:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[FOR_EXIT:.*]]
+; CHECK:       [[FOR_EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.exit:
+  ret void
+
+for.body:
+  %indvar = phi i64 [ 0, %entry ], [ %inc, %for.body ]
+  call void @llvm.experimental.noalias.scope.decl(metadata !0)
+  %inc = add nuw i64 %indvar, 1
+  %exitcond.not = icmp eq i64 %inc, %n
+  br i1 %exitcond.not, label %for.exit, label %for.body
+}
+
+!0 = !{!1}
+!1 = distinct !{!1, !2, !"x: %a"}
+!2 = distinct !{!2, !"x"}

``````````

</details>


https://github.com/llvm/llvm-project/pull/108144