[llvm] [VPlan] Don't set WrapFlags for truncated IVs. (PR #188966)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/188966

The wrap flags from the IV bin-op are not guaranteed to apply to truncated inductions, which are evaluated in narrower types.

Instead of dropping them late (in expandVPWidenIntOrFpInduction), do not add them at the outset, the prevent invalid transforms based on incorrect flags in the future.

>From 57faa6b187c5a3fec2590d6f51754db460743fb0 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 27 Mar 2026 11:01:54 +0000
Subject: [PATCH] [VPlan] Don't set WrapFlags for truncated IVs.

The wrap flags from the IV bin-op are not guaranteed to apply to
truncated inductions, which are evaluated in narrower types.

Instead of dropping them late (in expandVPWidenIntOrFpInduction), do not
add them at the outset, the prevent invalid transforms based on
incorrect flags in the future.
---
 llvm/lib/Transforms/Vectorize/LoopVectorize.cpp          | 9 ++++-----
 llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp        | 2 --
 .../LoopVectorize/VPlan/vplan-iv-transforms.ll           | 2 +-
 3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 7c401becc8634..e2ff373b2bcc6 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7797,14 +7797,13 @@ VPRecipeBuilder::tryToOptimizeInductionTruncate(VPInstruction *VPI,
   VPIRValue *Start = WidenIV->getStartValue();
   const InductionDescriptor &IndDesc = WidenIV->getInductionDescriptor();
 
-  // It is always safe to copy over the NoWrap and FastMath flags. In
-  // particular, when folding tail by masking, the masked-off lanes are never
-  // used, so it is safe.
-  VPIRFlags Flags = vputils::getFlagsFromIndDesc(IndDesc);
+  // Wrap flags from the original induction do not apply to the truncated type,
+  // so do not propagate them.
   VPValue *Step =
       vputils::getOrCreateVPValueForSCEVExpr(Plan, IndDesc.getStep());
   return new VPWidenIntOrFpInductionRecipe(
-      Phi, Start, Step, &Plan.getVF(), IndDesc, I, Flags, VPI->getDebugLoc());
+      Phi, Start, Step, &Plan.getVF(), IndDesc, I,
+      VPIRFlags::WrapFlagsTy(false, false), VPI->getDebugLoc());
 }
 
 VPSingleDefRecipe *VPRecipeBuilder::tryToWidenCall(VPInstruction *VPI,
diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index 7e8496a568643..9c9ffb8b0de64 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -3794,8 +3794,6 @@ expandVPWidenIntOrFpInduction(VPWidenIntOrFpInductionRecipe *WidenIVR,
     assert(StepTy->isIntegerTy() && "Truncation requires an integer type");
     Step = Builder.createScalarCast(Instruction::Trunc, Step, Ty, DL);
     Start = Builder.createScalarCast(Instruction::Trunc, Start, Ty, DL);
-    // Truncation doesn't preserve WrapFlags.
-    Flags.dropPoisonGeneratingFlags();
     StepTy = Ty;
   }
 
diff --git a/llvm/test/Transforms/LoopVectorize/VPlan/vplan-iv-transforms.ll b/llvm/test/Transforms/LoopVectorize/VPlan/vplan-iv-transforms.ll
index b04b1f5eff8fa..70e24b8638186 100644
--- a/llvm/test/Transforms/LoopVectorize/VPlan/vplan-iv-transforms.ll
+++ b/llvm/test/Transforms/LoopVectorize/VPlan/vplan-iv-transforms.ll
@@ -160,7 +160,7 @@ define void @iv_truncated_wrap_flags(ptr %dst, i64 %n) {
 ; CHECK-NEXT:  <x1> vector loop: {
 ; CHECK-NEXT:    vector.body:
 ; CHECK-NEXT:      EMIT vp<[[VP3:%[0-9]+]]> = CANONICAL-INDUCTION ir<0>, vp<%index.next>
-; CHECK-NEXT:      ir<%iv> = WIDEN-INDUCTION nuw nsw ir<0>, ir<1>, vp<[[VP0]]> (truncated to i16)
+; CHECK-NEXT:      ir<%iv> = WIDEN-INDUCTION ir<0>, ir<1>, vp<[[VP0]]> (truncated to i16)
 ; CHECK-NEXT:      vp<[[VP4:%[0-9]+]]> = SCALAR-STEPS vp<[[VP3]]>, ir<1>, vp<[[VP0]]>
 ; CHECK-NEXT:      CLONE ir<%ptr> = getelementptr inbounds ir<%dst>, vp<[[VP4]]>
 ; CHECK-NEXT:      vp<[[VP5:%[0-9]+]]> = vector-pointer inbounds ir<%ptr>