[llvm] [LV] Clarify nature of legacy CSE (NFC) (PR #160855)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon created https://github.com/llvm/llvm-project/pull/160855

In order to avoid conflating the legacy CSE with the VPlan-based one, rename the legacy CSE and insert a FIXME to clarify the nature of the legacy CSE.

>From 1dde0b1f7df2d695a5bcd47c51428a9ebbf94c6e Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Fri, 26 Sep 2025 12:03:49 +0100
Subject: [PATCH] [LV] Clarify nature of legacy CSE (NFC)

In order to avoid conflating the legacy CSE with the VPlan-based one,
rename the legacy CSE and insert a FIXME to clarify the nature of the
legacy CSE.
---
 llvm/lib/Transforms/Vectorize/LoopVectorize.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index a2d61d689397b..a578b088403cb 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2438,8 +2438,9 @@ struct CSEDenseMapInfo {
 
 } // end anonymous namespace
 
-///Perform cse of induction variable instructions.
-static void cse(BasicBlock *BB) {
+/// FIXME: This legacy common-subexpression-elimination routine is scheduled for
+/// removal, in favor of the VPlan-based one.
+static void legacyCSE(BasicBlock *BB) {
   // Perform simple cse.
   SmallDenseMap<Instruction *, Instruction *, 4, CSEDenseMapInfo> CSEMap;
   for (Instruction &In : llvm::make_early_inc_range(*BB)) {
@@ -2543,7 +2544,7 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State) {
   BasicBlock *HeaderBB = State.CFG.VPBB2IRBB[HeaderVPBB];
 
   // Remove redundant induction instructions.
-  cse(HeaderBB);
+  legacyCSE(HeaderBB);
 }
 
 void InnerLoopVectorizer::fixNonInductionPHIs(VPTransformState &State) {