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

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 #108052.

Author: dtcxzyw

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;
 }
 
 //===----------------------------------------------------------------------===//

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"}