[llvm] [LV] Fix crash when forced UserVF and EpilogueVF are ignored (PR #218282)

[Hassnaa Hamdi via llvm-commits]

https://github.com/hassnaaHamdi created https://github.com/llvm/llvm-project/pull/218282

`computeBestVF()` assumes/asserts that there should be only 2 vplans as
long as there is a vplan for `UserVF` and when `EpilogueVF` is forced. 
It doesn't consider the case when those VFs are ignored because of invalid costs.

This patch removes the assert and add a check for the vplan size.

>From 9ed41a85c9968f9b34f158e3b349490dcf9181d3 Mon Sep 17 00:00:00 2001
From: Hassnaa Hamdi <hassnaa.hamdi at arm.com>
Date: Sun, 23 Aug 2026 19:55:58 +0100
Subject: [PATCH] [LV] Fix crash when forced UserVF and EpilogueVF are ignored

computeBestVF() assumes/asserts that there should be 2 vplans as long
as there is a vplan for UserVF and when EpilogueVF is forced.
It doesn't consider the case when those VFs are ignored because of
invalid costs.

This patch removes the assert and add a check for the vplan size.
---
 llvm/lib/Transforms/Vectorize/LoopVectorize.cpp               | 3 +--
 llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll | 2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index bf0bb98eb75b8..c3e39f5adf175 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5820,8 +5820,7 @@ LoopVectorizationPlanner::computeBestVF() {
     return {VectorizationFactor(FirstPlan.getSingleVF(), 0, 0), &FirstPlan};
   }
 
-  if (hasPlanWithVF(UserVF) && hasForcedEpilogueVF()) {
-    assert(VPlans.size() == 2 && "Must have exactly 2 VPlans built");
+  if (hasPlanWithVF(UserVF) && hasForcedEpilogueVF() && VPlans.size() == 2) {
     assert(VPlans[0]->getSingleVF() == UserVF &&
            "expected second plan to be for the forced UserVF");
     assert(VPlans[1]->getSingleVF() == EpilogueVectorizationForceVF &&
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll b/llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll
index 73384c7d383ba..336c9f876bec9 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll
@@ -1,4 +1,4 @@
-; RUN: opt -S -passes=loop-vectorize -mattr=+sve -mtriple aarch64-unknown-linux-gnu -force-vector-width=2 -pass-remarks-analysis=loop-vectorize -pass-remarks-missed=loop-vectorize < %s 2>%t | FileCheck %s
+; RUN: opt -S -passes=loop-vectorize -mattr=+sve -mtriple aarch64-unknown-linux-gnu -force-vector-width=2 -epilogue-vectorization-force-VF=2  -pass-remarks-analysis=loop-vectorize -pass-remarks-missed=loop-vectorize < %s 2>%t | FileCheck %s
 ; RUN: FileCheck %s --check-prefix=CHECK-REMARKS < %t
 
 ; CHECK-REMARKS: UserVF ignored because of invalid costs.