[llvm] [VPlan] Collect FOR PHIs before sinking/hoisting recurrence users (PR #194671)

[via llvm-commits]

https://github.com/xiongzile created https://github.com/llvm/llvm-project/pull/194671

Avoid iterating over HeaderVPBB->phis() while potentially mutating the underlying VPBasicBlock. Collect all VPFirstOrderRecurrencePHIRecipe instances first, then process them in a separate loop.

This prevents iterator invalidation when sinking or hoisting recurrence users, and makes the transformation more robust.

Fixes: https://github.com/llvm/llvm-project/issues/194618

>From 55c4c92da9b1632a395e23c1cd0f5b295bb97d88 Mon Sep 17 00:00:00 2001
From: Zile Xiong <xiongzile99 at gmail.com>
Date: Wed, 29 Apr 2026 00:25:23 +0800
Subject: [PATCH] [VPlan] Collect FOR PHIs before sinking/hoisting recurrence
 users

Avoid iterating over HeaderVPBB->phis() while potentially mutating the
underlying VPBasicBlock. Collect all VPFirstOrderRecurrencePHIRecipe
instances first, then process them in a separate loop.

This prevents iterator invalidation when sinking or hoisting recurrence
users, and makes the transformation more robust.

Fixes: https://github.com/llvm/llvm-project/issues/194618
---
 llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp b/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
index 345bb2ef27a4f..391be24effd78 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
@@ -869,11 +869,13 @@ static bool hoistPreviousBeforeFORUsers(VPFirstOrderRecurrencePHIRecipe *FOR,
 /// fails.
 static bool tryToSinkOrHoistRecurrenceUsers(VPBasicBlock *HeaderVPBB,
                                             VPDominatorTree &VPDT) {
-  for (VPRecipeBase &R : HeaderVPBB->phis()) {
-    auto *FOR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&R);
-    if (!FOR)
-      continue;
+  SmallVector<VPFirstOrderRecurrencePHIRecipe *> FORs;
+
+  for (VPRecipeBase &R : HeaderVPBB->phis())
+    if (auto *FOR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&R))
+      FORs.push_back(FOR);
 
+  for (VPFirstOrderRecurrencePHIRecipe *FOR : FORs) {
     // Follow through FOR phi chains to find the actual Previous recipe.
     // Fixed-order recurrences do not contain cycles, so this loop is
     // guaranteed to terminate.