[llvm] [VPlan] Introduce replaceSymbolicStrides (NFC) (PR #155842)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon created https://github.com/llvm/llvm-project/pull/155842

Introduce VPlanTransforms::replaceSymbolicStrides factoring some code from LoopVectorize.

>From 6a389c091e1ae16ca2dc4a75aa31981a7c80bd76 Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Thu, 28 Aug 2025 15:18:20 +0100
Subject: [PATCH] [VPlan] Introduce replaceSymbolicStrides (NFC)

Introduce VPlanTransforms::replaceSymbolicStrides factoring some code
from LoopVectorize.
---
 .../Transforms/Vectorize/LoopVectorize.cpp    | 38 ++---------------
 .../Transforms/Vectorize/VPlanTransforms.cpp  | 41 +++++++++++++++++++
 .../Transforms/Vectorize/VPlanTransforms.h    |  6 +++
 3 files changed, 50 insertions(+), 35 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 6317bc3c20e25..23ec073a8f4ff 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -8777,41 +8777,9 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(
                            InterleaveGroups, RecipeBuilder,
                            CM.isScalarEpilogueAllowed());
 
-  // Replace VPValues for known constant strides guaranteed by predicate scalar
-  // evolution.
-  auto CanUseVersionedStride = [&Plan](VPUser &U, unsigned) {
-    auto *R = cast<VPRecipeBase>(&U);
-    return R->getParent()->getParent() ||
-           R->getParent() ==
-               Plan->getVectorLoopRegion()->getSinglePredecessor();
-  };
-  for (auto [_, Stride] : Legal->getLAI()->getSymbolicStrides()) {
-    auto *StrideV = cast<SCEVUnknown>(Stride)->getValue();
-    auto *ScevStride = dyn_cast<SCEVConstant>(PSE.getSCEV(StrideV));
-    // Only handle constant strides for now.
-    if (!ScevStride)
-      continue;
-
-    auto *CI = Plan->getOrAddLiveIn(
-        ConstantInt::get(Stride->getType(), ScevStride->getAPInt()));
-    if (VPValue *StrideVPV = Plan->getLiveIn(StrideV))
-      StrideVPV->replaceUsesWithIf(CI, CanUseVersionedStride);
-
-    // The versioned value may not be used in the loop directly but through a
-    // sext/zext. Add new live-ins in those cases.
-    for (Value *U : StrideV->users()) {
-      if (!isa<SExtInst, ZExtInst>(U))
-        continue;
-      VPValue *StrideVPV = Plan->getLiveIn(U);
-      if (!StrideVPV)
-        continue;
-      unsigned BW = U->getType()->getScalarSizeInBits();
-      APInt C = isa<SExtInst>(U) ? ScevStride->getAPInt().sext(BW)
-                                 : ScevStride->getAPInt().zext(BW);
-      VPValue *CI = Plan->getOrAddLiveIn(ConstantInt::get(U->getType(), C));
-      StrideVPV->replaceUsesWithIf(CI, CanUseVersionedStride);
-    }
-  }
+  // Replace VPValues for known constant strides.
+  VPlanTransforms::runPass(VPlanTransforms::replaceSymbolicStrides, *Plan, PSE,
+                           Legal->getLAI()->getSymbolicStrides());
 
   auto BlockNeedsPredication = [this](BasicBlock *BB) {
     return Legal->blockNeedsPredication(BB);
diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index d32d2a9ad11f7..9a87be5fe7803 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -29,6 +29,7 @@
 #include "llvm/Analysis/IVDescriptors.h"
 #include "llvm/Analysis/InstSimplifyFolder.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolutionPatternMatch.h"
 #include "llvm/Analysis/VectorUtils.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/MDBuilder.h"
@@ -2535,6 +2536,46 @@ void VPlanTransforms::canonicalizeEVLLoops(VPlan &Plan) {
   LatchExitingBr->eraseFromParent();
 }
 
+void VPlanTransforms::replaceSymbolicStrides(
+    VPlan &Plan, PredicatedScalarEvolution &PSE,
+    const DenseMap<Value *, const SCEV *> &StridesMap) {
+  // Replace VPValues for known constant strides guaranteed by predicate scalar
+  // evolution.
+  auto CanUseVersionedStride = [&Plan](VPUser &U, unsigned) {
+    auto *R = cast<VPRecipeBase>(&U);
+    return R->getParent()->getParent() ||
+           R->getParent() == Plan.getVectorLoopRegion()->getSinglePredecessor();
+  };
+  for (const SCEV *Stride : StridesMap.values()) {
+    using namespace SCEVPatternMatch;
+    auto *StrideV = cast<SCEVUnknown>(Stride)->getValue();
+    const APInt *StrideConst;
+    if (!match(PSE.getSCEV(StrideV), m_scev_APInt(StrideConst)))
+      // Only handle constant strides for now.
+      continue;
+
+    auto *CI =
+        Plan.getOrAddLiveIn(ConstantInt::get(Stride->getType(), *StrideConst));
+    if (VPValue *StrideVPV = Plan.getLiveIn(StrideV))
+      StrideVPV->replaceUsesWithIf(CI, CanUseVersionedStride);
+
+    // The versioned value may not be used in the loop directly but through a
+    // sext/zext. Add new live-ins in those cases.
+    for (Value *U : StrideV->users()) {
+      if (!isa<SExtInst, ZExtInst>(U))
+        continue;
+      VPValue *StrideVPV = Plan.getLiveIn(U);
+      if (!StrideVPV)
+        continue;
+      unsigned BW = U->getType()->getScalarSizeInBits();
+      APInt C =
+          isa<SExtInst>(U) ? StrideConst->sext(BW) : StrideConst->zext(BW);
+      VPValue *CI = Plan.getOrAddLiveIn(ConstantInt::get(U->getType(), C));
+      StrideVPV->replaceUsesWithIf(CI, CanUseVersionedStride);
+    }
+  }
+}
+
 void VPlanTransforms::dropPoisonGeneratingRecipes(
     VPlan &Plan,
     const std::function<bool(BasicBlock *)> &BlockNeedsPredication) {
diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.h b/llvm/lib/Transforms/Vectorize/VPlanTransforms.h
index 9cf62a35ae36b..5afff8edbc511 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.h
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.h
@@ -199,6 +199,12 @@ struct VPlanTransforms {
   truncateToMinimalBitwidths(VPlan &Plan,
                              const MapVector<Instruction *, uint64_t> &MinBWs);
 
+  /// Replace the symbolic strides in \p Plan, with data from \p StridesMap,
+  /// with constants when possible.
+  static void
+  replaceSymbolicStrides(VPlan &Plan, PredicatedScalarEvolution &PSE,
+                         const DenseMap<Value *, const SCEV *> &StridesMap);
+
   /// Drop poison flags from recipes that may generate a poison value that is
   /// used after vectorization, even when their operands are not poison. Those
   /// recipes meet the following conditions: