[llvm] cba24d6 - [VPlan] Generalize folding IV increment into phi start value (NFC). (#223260)

[via llvm-commits]

Author: Florian Hahn
Date: 2026-09-13T18:32:57Z
New Revision: cba24d6a935382154b0487d09c1d91a0e2bda04b

URL: https://github.com/llvm/llvm-project/commit/cba24d6a935382154b0487d09c1d91a0e2bda04b
DIFF: https://github.com/llvm/llvm-project/commit/cba24d6a935382154b0487d09c1d91a0e2bda04b.diff

LOG: [VPlan] Generalize folding IV increment into phi start value (NFC). (#223260)

simplifyRecipe folds

  X    = phi(0, IVInc)
  IVInc = X + Step
  Def   = IVInc + Y

into a phi starting at Y, but only if Y is a live-in. Also allow values
defined in the block the phi's start value is coming from, which is
where the value conservatively has to be available.

NFC today, but preparation for modeling the full epilogue skeleton in
VPlan.

Added: 
    

Modified: 
    llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index 831eb957c8caf..dd29d99a58e16 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -1371,6 +1371,13 @@ static VPValue *simplifyRecipe(VPlan &Plan, VPSingleDefRecipe *Def) {
   return nullptr;
 }
 
+/// Returns true if \p V is available at the end of \p VPBB, i.e. it either is a
+/// live-in from the original IR or defined in \p VPBB.
+static bool isAvailableAtEndOf(VPValue *V, const VPBasicBlock *VPBB) {
+  VPRecipeBase *DefR = V->getDefiningRecipe();
+  return DefR ? DefR->getParent() == VPBB : isa<VPIRValue>(V);
+}
+
 /// Combine \p Def into a simpler recipe. May modify or create new recipes.
 static VPSingleDefRecipe *combineRecipe(VPlan &Plan, VPSingleDefRecipe *Def) {
   if (auto *V = simplifyRecipe(Plan, Def)) {
@@ -1669,20 +1676,20 @@ static VPSingleDefRecipe *combineRecipe(VPlan &Plan, VPSingleDefRecipe *Def) {
   VPValue *IVInc;
   if (match(Def, m_Add(m_VPValue(IVInc, m_Add(m_VPValue(X), m_VPValue())),
                        m_VPValue(Y))) &&
-      isa<VPIRValue>(Y) && match(X, m_VPPhi(m_ZeroInt(), m_Specific(IVInc)))) {
+      match(X, m_VPPhi(m_ZeroInt(), m_Specific(IVInc))) &&
+      IVInc->getNumUsers() == 2) {
     auto *Phi = cast<VPPhi>(X);
-    if (IVInc->getNumUsers() == 2) {
-      // If Phi has a second user (besides IVInc's defining recipe), it must
-      // be Inc = Phi + Y for the fold to apply.
-      auto *Inc = dyn_cast_or_null<VPSingleDefRecipe>(
-          findUserOf(Phi, m_Add(m_Specific(Phi), m_Specific(Y))));
-      if (Phi->getNumUsers() == 1 || (Phi->getNumUsers() == 2 && Inc)) {
-        Def->replaceAllUsesWith(IVInc);
-        if (Inc)
-          Inc->replaceAllUsesWith(Phi);
-        Phi->setOperand(0, Y);
-        return Def;
-      }
+    // If Phi has a second user (besides IVInc's defining recipe), it must be
+    // Inc = Phi + Y for the fold to apply.
+    auto *Inc = dyn_cast_if_present<VPSingleDefRecipe>(
+        findUserOf(Phi, m_Add(m_Specific(Phi), m_Specific(Y))));
+    if ((Phi->getNumUsers() == 1 || (Phi->getNumUsers() == 2 && Inc)) &&
+        isAvailableAtEndOf(Y, Phi->getIncomingBlock(0))) {
+      Def->replaceAllUsesWith(IVInc);
+      if (Inc)
+        Inc->replaceAllUsesWith(Phi);
+      Phi->setOperand(0, Y);
+      return Def;
     }
   }