[llvm] [VPlan] Prevent dangling references to trip count after expansion. (PR #201924)

[Florian Hahn via llvm-commits]

https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/201924

>From 0a89276dd45c17e8b96255d256b8db837d54fbb4 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 5 Jun 2026 20:37:04 +0100
Subject: [PATCH] [VPlan] Prevent dangling references to trip count after
 expansion.

Set trip count to Poison after expanding SCEVs to VPInstructions.
getTripCount should not be used after that other than printing; setting
to poison avoids accessing dangling references.
---
 llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index e0d06366592ae..fac0f9e9b7499 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -5470,8 +5470,11 @@ void VPlanTransforms::expandSCEVsToVPInstructions(VPlan &Plan,
     if (!Expanded)
       continue;
     ExpSCEV->replaceAllUsesWith(Expanded);
+    // TripCount should not be used after expansion to VPInstructions. Reset to
+    // poison to avoid dangling references.
     if (Plan.getTripCount() == ExpSCEV)
-      Plan.resetTripCount(Expanded);
+      Plan.resetTripCount(
+          Plan.getOrAddLiveIn(PoisonValue::get(ExpSCEV->getScalarType())));
     ExpSCEV->eraseFromParent();
   }
 }