[llvm] [SCEV] Try to re-use existing LCSSA phis when expanding SCEVAddRecExpr. (PR #147214)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/147214

If an AddRec is expanded outside a loop with a single exit block, check if any of the (lcssa) phi nodes in the exit block match the AddRec. If that's the case, simply use the existing lcssa phi.

This can reduce the number of instruction created for SCEV expansions, mainly for runtime checks generated by the loop vectorizer.

Compile-time impact should be mostly neutral
https://llvm-compile-time-tracker.com/compare.php?from=48c7a3187f9831304a38df9bdb3b4d5bf6b6b1a2&to=cf9d039a7b0db5d0d912e0e2c01b19c2a653273a&stat=instructions:u

>From 8c8c5509e948130432d0b76ce7679a837cfa735c Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 4 Jul 2025 21:45:12 +0100
Subject: [PATCH] [SCEV] Try to re-use existing LCSSA phis when expanding
 SCEVAddRecExpr.

If an AddRec is expanded outside a loop with a single exit block, check
if any of the (lcssa) phi nodes in the exit block match the AddRec. If that's
the case, simply use the existing lcssa phi.

This can reduce the number of instruction created for SCEV expansions,
mainly for runtime checks generated by the loop vectorizer.

Compile-time impact should be mostly neutral
https://llvm-compile-time-tracker.com/compare.php?from=48c7a3187f9831304a38df9bdb3b4d5bf6b6b1a2&to=cf9d039a7b0db5d0d912e0e2c01b19c2a653273a&stat=instructions:u
---
 .../Utils/ScalarEvolutionExpander.h           |  1 +
 .../Utils/ScalarEvolutionExpander.cpp         | 23 +++++++++++++++++++
 .../reuse-lcssa-phi-scev-expansion.ll         |  4 +---
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h b/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
index a101151eed7cc..39fef921a9590 100644
--- a/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
+++ b/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
@@ -530,6 +530,7 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
 
   bool isExpandedAddRecExprPHI(PHINode *PN, Instruction *IncV, const Loop *L);
 
+  Value *tryToReuseLCSSAPhi(const SCEVAddRecExpr *S);
   Value *expandAddRecExprLiterally(const SCEVAddRecExpr *);
   PHINode *getAddRecExprPHILiterally(const SCEVAddRecExpr *Normalized,
                                      const Loop *L, Type *&TruncTy,
diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 24fe08d6c3e4e..438ca6cb785be 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1223,6 +1223,24 @@ Value *SCEVExpander::expandAddRecExprLiterally(const SCEVAddRecExpr *S) {
   return Result;
 }
 
+Value *SCEVExpander::tryToReuseLCSSAPhi(const SCEVAddRecExpr *S) {
+  const Loop *L = S->getLoop();
+  BasicBlock *EB = L->getExitBlock();
+  if (!EB || !EB->getSinglePredecessor() ||
+      !SE.DT.dominates(EB, Builder.GetInsertBlock()))
+    return nullptr;
+
+  for (auto &PN : EB->phis()) {
+    if (!SE.isSCEVable(PN.getType()) || PN.getType() != S->getType())
+      continue;
+    auto *ExitV = SE.getSCEV(&PN);
+    if (S == ExitV)
+      return &PN;
+  }
+
+  return nullptr;
+}
+
 Value *SCEVExpander::visitAddRecExpr(const SCEVAddRecExpr *S) {
   // In canonical mode we compute the addrec as an expression of a canonical IV
   // using evaluateAtIteration and expand the resulting SCEV expression. This
@@ -1262,6 +1280,11 @@ Value *SCEVExpander::visitAddRecExpr(const SCEVAddRecExpr *S) {
     return V;
   }
 
+  // If there S is expanded outside the defining loop, check if there is a
+  // matching LCSSA phi node for it.
+  if (Value *V = tryToReuseLCSSAPhi(S))
+    return V;
+
   // {X,+,F} --> X + {0,+,F}
   if (!S->getStart()->isZero()) {
     if (isa<PointerType>(S->getType())) {
diff --git a/llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll b/llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll
index 2747895f06a7b..ce4270dc4b7fa 100644
--- a/llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll
+++ b/llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll
@@ -18,11 +18,9 @@ define void @reuse_lcssa_phi_for_add_rec1(ptr %head) {
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw i64 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[EC_1]], label %[[PH:.*]], label %[[LOOP_1]]
 ; CHECK:       [[PH]]:
-; CHECK-NEXT:    [[IV_2_LCSSA:%.*]] = phi i32 [ [[IV_2]], %[[LOOP_1]] ]
 ; CHECK-NEXT:    [[IV_LCSSA:%.*]] = phi i64 [ [[IV]], %[[LOOP_1]] ]
-; CHECK-NEXT:    [[IV_2_NEXT_LCSSA:%.*]] = phi i32 [ [[IV_2_NEXT]], %[[LOOP_1]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i32 [ [[IV_2_NEXT]], %[[LOOP_1]] ]
 ; CHECK-NEXT:    [[SRC_2:%.*]] = tail call noalias noundef dereferenceable_or_null(8) ptr @calloc(i64 1, i64 8)
-; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[IV_2_LCSSA]], 1
 ; CHECK-NEXT:    [[SMIN:%.*]] = call i32 @llvm.smin.i32(i32 [[TMP0]], i32 1)
 ; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 [[TMP0]], [[SMIN]]
 ; CHECK-NEXT:    [[TMP2:%.*]] = zext i32 [[TMP1]] to i64