[llvm] [IPSCCP] Don't replace with constant if Value is noalias ptr or derivatives (PR #154522)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: choikwa (choikwa)

<details>
<summary>Changes</summary>

This was motivated from looking at composable kernel benchmark where IPSCCP was observed replacing noalias ptr's and their derivatives with a global alias. Doing so would lose the noalias information and target backend was more pessimistic, emitting unneeded WAITCNT instructions.

Making it a draft as it's unclear if it's beneficially moving the needle.

---
Full diff: https://github.com/llvm/llvm-project/pull/154522.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SCCPSolver.cpp (+15) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index 84485176ad4ff..3c06452a91f36 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -62,6 +62,21 @@ bool SCCPSolver::tryToReplaceWithConstant(Value *V) {
   Constant *Const = getConstantOrNull(V);
   if (!Const)
     return false;
+
+  // Don't replace noalias arg or derivatives
+  if (isa<PointerType>(V->getType())) {
+    SmallVector<const Value *, 4> Objects;
+    getUnderlyingObjects(V, Objects, nullptr);
+
+    for (const auto Obj : Objects) {
+      if (const auto *Arg = dyn_cast<Argument>(Obj)) {
+        if (isa<PointerType>(Arg->getType()) &&
+            Arg->hasNoAliasAttr())
+          return false;
+      }
+    }
+  }
+
   // Replacing `musttail` instructions with constant breaks `musttail` invariant
   // unless the call itself can be removed.
   // Calls with "clang.arc.attachedcall" implicitly use the return value and

``````````

</details>


https://github.com/llvm/llvm-project/pull/154522