[llvm] [VPlan] Enable CSE of VPWidenPHIRecipe. (PR #207573)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/207573

Add VPWidenPHIRecipe to the set of recipes CSE can handle. Besides their operands (incoming values), they also depend on their predecessors, so canHandle checks for that.

>From ef11aa7f2c4b2f571b971f9bc7fb22c5eb2ef22c Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Sun, 5 Jul 2026 08:27:26 +0100
Subject: [PATCH] [VPlan] Enable CSE of VPWidenPHIRecipe.

Add VPWidenPHIRecipe to the set of recipes CSE can handle. Besides their
operands (incoming values), they also depend on their predecessors, so
canHandle checks for that.
---
 llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp   |  7 +++++++
 .../LoopVectorize/cse-replicate-regions.ll          | 13 ++++++-------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index 39e8ae5dd3b8c..6546b83764168 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -1254,6 +1254,9 @@ getOpcodeOrIntrinsicID(const VPSingleDefRecipe *R) {
       .Case<VPInstruction, VPWidenRecipe, VPWidenCastRecipe, VPWidenGEPRecipe,
             VPReplicateRecipe>(
           [](auto *I) { return std::make_pair(false, I->getOpcode()); })
+      .Case([](const VPWidenPHIRecipe *I) {
+        return std::make_pair(true, Instruction::PHI);
+      })
       .Case([](const VPWidenIntrinsicRecipe *I) {
         return std::make_pair(true, I->getVectorIntrinsicID());
       })
@@ -2526,6 +2529,10 @@ struct VPCSEDenseMapInfo : public DenseMapInfo<VPSingleDefRecipe *> {
       if (LSIV->getInductionOpcode() !=
           cast<VPScalarIVStepsRecipe>(R)->getInductionOpcode())
         return false;
+    // Phi recipes can only be equal if they are in the same VPBB, as they
+    // implicitly depend on their predecessors.
+    if (isa<VPWidenPHIRecipe>(L) && L->getParent() != R->getParent())
+      return false;
     // Recipes in replicate regions implicitly depend on predicate. If either
     // recipe is in a replicate region, only consider them equal if both have
     // the same parent.
diff --git a/llvm/test/Transforms/LoopVectorize/cse-replicate-regions.ll b/llvm/test/Transforms/LoopVectorize/cse-replicate-regions.ll
index bcaa78386f5f4..5d52a7cdc3d31 100644
--- a/llvm/test/Transforms/LoopVectorize/cse-replicate-regions.ll
+++ b/llvm/test/Transforms/LoopVectorize/cse-replicate-regions.ll
@@ -21,19 +21,18 @@ define void @multiple_vppredinstphi_with_same_predicate(ptr %A, i32 %d) {
 ; CHECK-NEXT:    br label %[[PRED_SDIV_CONTINUE]]
 ; CHECK:       [[PRED_SDIV_CONTINUE]]:
 ; CHECK-NEXT:    [[TMP5:%.*]] = phi <2 x i32> [ poison, %[[VECTOR_BODY]] ], [ [[TMP4]], %[[PRED_SDIV_IF]] ]
-; CHECK-NEXT:    [[TMP8:%.*]] = phi <2 x i32> [ poison, %[[VECTOR_BODY]] ], [ [[TMP4]], %[[PRED_SDIV_IF]] ]
 ; CHECK-NEXT:    [[TMP6:%.*]] = extractelement <2 x i1> [[TMP1]], i64 1
 ; CHECK-NEXT:    br i1 [[TMP6]], label %[[PRED_SDIV_IF1:.*]], label %[[PRED_SDIV_CONTINUE2]]
 ; CHECK:       [[PRED_SDIV_IF1]]:
 ; CHECK-NEXT:    [[TMP7:%.*]] = sdiv i32 -10, [[D]]
-; CHECK-NEXT:    [[TMP9:%.*]] = insertelement <2 x i32> [[TMP5]], i32 [[TMP7]], i64 1
-; CHECK-NEXT:    [[TMP14:%.*]] = sdiv i32 -10, [[D]]
-; CHECK-NEXT:    [[TMP15:%.*]] = insertelement <2 x i32> [[TMP8]], i32 [[TMP14]], i64 1
+; CHECK-NEXT:    [[TMP8:%.*]] = insertelement <2 x i32> [[TMP5]], i32 [[TMP7]], i64 1
+; CHECK-NEXT:    [[TMP9:%.*]] = sdiv i32 -10, [[D]]
+; CHECK-NEXT:    [[TMP13:%.*]] = insertelement <2 x i32> [[TMP5]], i32 [[TMP9]], i64 1
 ; CHECK-NEXT:    br label %[[PRED_SDIV_CONTINUE2]]
 ; CHECK:       [[PRED_SDIV_CONTINUE2]]:
-; CHECK-NEXT:    [[TMP12:%.*]] = phi <2 x i32> [ [[TMP5]], %[[PRED_SDIV_CONTINUE]] ], [ [[TMP9]], %[[PRED_SDIV_IF1]] ]
-; CHECK-NEXT:    [[TMP13:%.*]] = phi <2 x i32> [ [[TMP8]], %[[PRED_SDIV_CONTINUE]] ], [ [[TMP15]], %[[PRED_SDIV_IF1]] ]
-; CHECK-NEXT:    [[TMP10:%.*]] = add <2 x i32> [[TMP13]], [[TMP12]]
+; CHECK-NEXT:    [[TMP14:%.*]] = phi <2 x i32> [ [[TMP5]], %[[PRED_SDIV_CONTINUE]] ], [ [[TMP8]], %[[PRED_SDIV_IF1]] ]
+; CHECK-NEXT:    [[TMP12:%.*]] = phi <2 x i32> [ [[TMP5]], %[[PRED_SDIV_CONTINUE]] ], [ [[TMP13]], %[[PRED_SDIV_IF1]] ]
+; CHECK-NEXT:    [[TMP10:%.*]] = add <2 x i32> [[TMP12]], [[TMP14]]
 ; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[TMP1]], <2 x i32> [[TMP10]], <2 x i32> zeroinitializer
 ; CHECK-NEXT:    store <2 x i32> [[PREDPHI]], ptr [[TMP0]], align 4
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 2