[llvm] [SCEV] Add m_scev_UndefOrPoison (NFC). (PR #170740)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/170740

Add matcher for SCEVUnknown wrapping undef or poison.

>From 7742c15601ab83743ce774bb0f52120041825702 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Thu, 4 Dec 2025 20:13:45 +0000
Subject: [PATCH] [SCEV] Add m_scev_UndefOrPoison (NFC).

Add matcher for SCEVUnknown wrapping undef or poison.
---
 .../llvm/Analysis/ScalarEvolutionPatternMatch.h     | 13 +++++++++++++
 llvm/lib/Analysis/LoopAccessAnalysis.cpp            |  5 ++---
 llvm/lib/Analysis/ScalarEvolution.cpp               |  7 ++-----
 3 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/llvm/include/llvm/Analysis/ScalarEvolutionPatternMatch.h b/llvm/include/llvm/Analysis/ScalarEvolutionPatternMatch.h
index 9354eef98fe91..f285eacc4c565 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolutionPatternMatch.h
+++ b/llvm/include/llvm/Analysis/ScalarEvolutionPatternMatch.h
@@ -380,6 +380,19 @@ m_scev_AffineAddRec(const Op0_t &Op0, const Op1_t &Op1, const Loop_t &L) {
   return SCEVAffineAddRec_match<Op0_t, Op1_t, Loop_t>(Op0, Op1, L);
 }
 
+struct is_undef_or_poison {
+  bool match(const SCEV *S) const {
+    const SCEVUnknown *Unknown;
+    return SCEVPatternMatch::match(S, m_SCEVUnknown(Unknown)) &&
+           isa<UndefValue>(Unknown->getValue());
+  }
+};
+
+/// Match an SCEVUnknown wrapping undef or poison.
+inline is_undef_or_poison m_scev_UndefOrPoison() {
+  return is_undef_or_poison();
+}
+
 } // namespace SCEVPatternMatch
 } // namespace llvm
 
diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 5d88e5f54e3d6..df793de7c817b 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -2998,9 +2998,8 @@ void LoopAccessInfo::collectStridedAccess(Value *MemAccess) {
   if (!StrideExpr)
     return;
 
-  if (auto *Unknown = dyn_cast<SCEVUnknown>(StrideExpr))
-    if (isa<UndefValue>(Unknown->getValue()))
-      return;
+  if (match(StrideExpr, m_scev_UndefOrPoison()))
+    return;
 
   LLVM_DEBUG(dbgs() << "LAA: Found a strided access that is a candidate for "
                        "versioning:");
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 1d7a8b981b5ee..e1f90264be7a2 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13695,11 +13695,8 @@ SCEVAddRecExpr::getPostIncExpr(ScalarEvolution &SE) const {
 
 // Return true when S contains at least an undef value.
 bool ScalarEvolution::containsUndefs(const SCEV *S) const {
-  return SCEVExprContains(S, [](const SCEV *S) {
-    if (const auto *SU = dyn_cast<SCEVUnknown>(S))
-      return isa<UndefValue>(SU->getValue());
-    return false;
-  });
+  return SCEVExprContains(
+      S, [](const SCEV *S) { return match(S, m_scev_UndefOrPoison()); });
 }
 
 // Return true when S contains a value that is a nullptr.