[llvm] [SCEV] Allow adds of constants in tryToReuseLCSSAPhi. (PR #150693)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Fri Jul 25 13:13:33 PDT 2025
https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/150693
Update the logic added in
https://github.com/llvm/llvm-project/pull/147824 to also allow adds of constants. There are a number of cases where this can help remove redundant phis and replace some computation with a ptrtoint (which likely is free in the backend).
>From 3918b1241dcef507304ff8769187831178b7b60a Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 25 Jul 2025 20:58:15 +0100
Subject: [PATCH] [SCEV] Allow adds of constants in tryToReuseLCSSAPhi.
Update the logic added in
https://github.com/llvm/llvm-project/pull/147824 to also allow adds of
constants. There are a number of cases where this can help remove
redundant phis and replace some computation with a ptrtoint (which
likely is free in the backend).
---
.../lib/Transforms/Utils/ScalarEvolutionExpander.cpp | 3 ++-
.../LoopIdiom/reuse-lcssa-phi-scev-expansion.ll | 5 ++---
.../LoopVectorize/reuse-lcssa-phi-scev-expansion.ll | 12 ++++--------
3 files changed, 8 insertions(+), 12 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index ddb062b402861..21573e0244bce 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1249,7 +1249,8 @@ Value *SCEVExpander::tryToReuseLCSSAPhi(const SCEVAddRecExpr *S) {
// offset, if the offset is simpler.
const SCEV *Diff = SE.getMinusSCEV(S, ExitSCEV);
const SCEV *Op = Diff;
- match(Diff, m_scev_Mul(m_scev_AllOnes(), m_SCEV(Op)));
+ match(Diff, m_scev_Add(m_SCEVConstant(), m_SCEV(Op)));
+ match(Op, m_scev_Mul(m_scev_AllOnes(), m_SCEV(Op)));
match(Op, m_scev_PtrToInt(m_SCEV(Op)));
if (!isa<SCEVConstant, SCEVUnknown>(Op))
continue;
diff --git a/llvm/test/Transforms/LoopIdiom/reuse-lcssa-phi-scev-expansion.ll b/llvm/test/Transforms/LoopIdiom/reuse-lcssa-phi-scev-expansion.ll
index 357a7b6b7fd9a..9d840207217a6 100644
--- a/llvm/test/Transforms/LoopIdiom/reuse-lcssa-phi-scev-expansion.ll
+++ b/llvm/test/Transforms/LoopIdiom/reuse-lcssa-phi-scev-expansion.ll
@@ -33,11 +33,10 @@ define void @scev_expand_ptrtoint(i8 %x, ptr %start) {
; CHECK: [[LOOP_3_PREHEADER]]:
; CHECK-NEXT: [[INDVAR_LCSSA:%.*]] = phi i64 [ [[INDVAR]], %[[LOOP_2_HEADER]] ], [ [[INDVAR]], %[[LOOP_2_HEADER]] ]
; CHECK-NEXT: [[PTR_IV_2_LCSSA:%.*]] = phi ptr [ [[PTR_IV_2]], %[[LOOP_2_HEADER]] ], [ [[PTR_IV_2]], %[[LOOP_2_HEADER]] ]
-; CHECK-NEXT: [[TMP0:%.*]] = sub i64 0, [[START1]]
+; CHECK-NEXT: [[TMP0:%.*]] = sub i64 1, [[START1]]
; CHECK-NEXT: [[TMP1:%.*]] = ptrtoint ptr [[PTR_IV_1_LCSSA]] to i64
; CHECK-NEXT: [[TMP2:%.*]] = add i64 [[TMP1]], [[TMP0]]
-; CHECK-NEXT: [[TMP3:%.*]] = add i64 [[TMP2]], 1
-; CHECK-NEXT: [[TMP4:%.*]] = add i64 [[CMP_EXT]], [[TMP3]]
+; CHECK-NEXT: [[TMP4:%.*]] = add i64 [[CMP_EXT]], [[TMP2]]
; CHECK-NEXT: [[TMP5:%.*]] = add i64 [[INDVAR_LCSSA]], [[TMP4]]
; CHECK-NEXT: [[SCEVGEP:%.*]] = getelementptr i8, ptr [[START]], i64 [[TMP5]]
; CHECK-NEXT: [[STRLEN:%.*]] = call i64 @strlen(ptr [[SCEVGEP]])
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 fd32291123332..3eac7678bbc95 100644
--- a/llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll
+++ b/llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll
@@ -394,22 +394,18 @@ define void @scev_exp_reuse_const_add(ptr %dst, ptr %src) {
; CHECK-SAME: ptr [[DST:%.*]], ptr [[SRC:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[SRC2:%.*]] = ptrtoint ptr [[SRC]] to i64
-; CHECK-NEXT: [[DST1:%.*]] = ptrtoint ptr [[DST]] to i64
; CHECK-NEXT: br label %[[LOOP_1:.*]]
; CHECK: [[LOOP_1]]:
-; CHECK-NEXT: [[INDVAR:%.*]] = phi i64 [ [[INDVAR_NEXT:%.*]], %[[LOOP_1]] ], [ 0, %[[ENTRY]] ]
; CHECK-NEXT: [[PTR_IV_1:%.*]] = phi ptr [ [[DST]], %[[ENTRY]] ], [ [[PTR_IV_1_NEXT:%.*]], %[[LOOP_1]] ]
; CHECK-NEXT: [[PTR_IV_1_NEXT]] = getelementptr i8, ptr [[PTR_IV_1]], i64 2
; CHECK-NEXT: [[C:%.*]] = call i1 @cond()
-; CHECK-NEXT: [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
; CHECK-NEXT: br i1 [[C]], label %[[LOOP_2_PH:.*]], label %[[LOOP_1]]
; CHECK: [[LOOP_2_PH]]:
-; CHECK-NEXT: [[INDVAR_LCSSA:%.*]] = phi i64 [ [[INDVAR]], %[[LOOP_1]] ]
; CHECK-NEXT: [[PTR_IV_1_NEXT_LCSSA:%.*]] = phi ptr [ [[PTR_IV_1_NEXT]], %[[LOOP_1]] ]
; CHECK-NEXT: br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_MEMCHECK:.*]]
; CHECK: [[VECTOR_MEMCHECK]]:
-; CHECK-NEXT: [[TMP0:%.*]] = sub i64 [[DST1]], [[SRC2]]
-; CHECK-NEXT: [[TMP1:%.*]] = shl i64 [[INDVAR_LCSSA]], 1
+; CHECK-NEXT: [[TMP0:%.*]] = sub i64 -2, [[SRC2]]
+; CHECK-NEXT: [[TMP1:%.*]] = ptrtoint ptr [[PTR_IV_1_NEXT_LCSSA]] to i64
; CHECK-NEXT: [[TMP2:%.*]] = add i64 [[TMP1]], [[TMP0]]
; CHECK-NEXT: [[DIFF_CHECK:%.*]] = icmp ult i64 [[TMP2]], 4
; CHECK-NEXT: br i1 [[DIFF_CHECK]], label %[[SCALAR_PH]], label %[[VECTOR_PH:.*]]
@@ -433,11 +429,11 @@ define void @scev_exp_reuse_const_add(ptr %dst, ptr %src) {
; CHECK-NEXT: br i1 false, label %[[EXIT:.*]], label %[[SCALAR_PH]]
; CHECK: [[SCALAR_PH]]:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ 40, %[[MIDDLE_BLOCK]] ], [ 0, %[[LOOP_2_PH]] ], [ 0, %[[VECTOR_MEMCHECK]] ]
-; CHECK-NEXT: [[BC_RESUME_VAL3:%.*]] = phi ptr [ [[TMP3]], %[[MIDDLE_BLOCK]] ], [ [[PTR_IV_1_NEXT_LCSSA]], %[[LOOP_2_PH]] ], [ [[PTR_IV_1_NEXT_LCSSA]], %[[VECTOR_MEMCHECK]] ]
+; CHECK-NEXT: [[BC_RESUME_VAL2:%.*]] = phi ptr [ [[TMP3]], %[[MIDDLE_BLOCK]] ], [ [[PTR_IV_1_NEXT_LCSSA]], %[[LOOP_2_PH]] ], [ [[PTR_IV_1_NEXT_LCSSA]], %[[VECTOR_MEMCHECK]] ]
; CHECK-NEXT: br label %[[LOOP_2:.*]]
; CHECK: [[LOOP_2]]:
; CHECK-NEXT: [[IV_1:%.*]] = phi i64 [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ], [ [[IV_2_NEXT:%.*]], %[[LOOP_2]] ]
-; CHECK-NEXT: [[PTR_IV_2:%.*]] = phi ptr [ [[BC_RESUME_VAL3]], %[[SCALAR_PH]] ], [ [[PTR_IV_2_NEXT:%.*]], %[[LOOP_2]] ]
+; CHECK-NEXT: [[PTR_IV_2:%.*]] = phi ptr [ [[BC_RESUME_VAL2]], %[[SCALAR_PH]] ], [ [[PTR_IV_2_NEXT:%.*]], %[[LOOP_2]] ]
; CHECK-NEXT: [[IV_2_NEXT]] = add i64 [[IV_1]], 1
; CHECK-NEXT: [[GEP_SRC_1:%.*]] = getelementptr i16, ptr [[SRC]], i64 [[IV_2_NEXT]]
; CHECK-NEXT: [[L:%.*]] = load i16, ptr [[GEP_SRC_1]], align 2
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