[llvm] [InstCombine] Add one-use check when folding fabs over selects (PR #122270)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/122270

Fixes multi-use issue introduced by https://github.com/llvm/llvm-project/pull/86390.
It also forbids the folding of `fabs (select Cond, TrueC, FalseC)` when select has external uses. `fabs` is always cheaper than a select with constant arms.


>From 93fa070d9cbb63a54ecb40554172028765b66649 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Thu, 9 Jan 2025 20:51:40 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 .../Transforms/InstCombine/intrinsic-select.ll    | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/intrinsic-select.ll b/llvm/test/Transforms/InstCombine/intrinsic-select.ll
index 4ce2908a630785..e7b092e1f34d77 100644
--- a/llvm/test/Transforms/InstCombine/intrinsic-select.ll
+++ b/llvm/test/Transforms/InstCombine/intrinsic-select.ll
@@ -2,6 +2,7 @@
 ; RUN: opt -passes=instcombine -S < %s | FileCheck %s
 
 declare void @use(i32)
+declare void @usef32(float)
 
 declare i32 @llvm.ctlz.i32(i32, i1)
 declare <3 x i17> @llvm.ctlz.v3i17(<3 x i17>, i1)
@@ -344,3 +345,17 @@ define double @test_fabs_select_fmf2(i1 %cond, double %a) {
   %fabs = call nnan ninf nsz double @llvm.fabs.f64(double %sel1)
   ret double %fabs
 }
+
+define float @test_fabs_select_multiuse(i1 %cond, float %x) {
+; CHECK-LABEL: @test_fabs_select_multiuse(
+; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[COND:%.*]], float [[X:%.*]], float 0x7FF0000000000000
+; CHECK-NEXT:    call void @usef32(float [[SELECT]])
+; CHECK-NEXT:    [[TMP1:%.*]] = call float @llvm.fabs.f32(float [[X]])
+; CHECK-NEXT:    [[FABS:%.*]] = select i1 [[COND]], float [[TMP1]], float 0x7FF0000000000000
+; CHECK-NEXT:    ret float [[FABS]]
+;
+  %select = select i1 %cond, float %x, float 0x7FF0000000000000
+  call void @usef32(float %select)
+  %fabs = call float @llvm.fabs.f32(float %select)
+  ret float %fabs
+}

>From b3a56728885f5a09f3d062b586518336a34f35e4 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Thu, 9 Jan 2025 21:02:26 +0800
Subject: [PATCH 2/2] [InstCombine] Add one-use check when folding fabs over
 selects

---
 llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp | 2 +-
 llvm/test/Transforms/InstCombine/intrinsic-select.ll | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index c55c40c88bc845..99ff12fcb37967 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2709,7 +2709,7 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
 
     if (match(Arg, m_Select(m_Value(Cond), m_Value(TVal), m_Value(FVal)))) {
       // fabs (select Cond, TrueC, FalseC) --> select Cond, AbsT, AbsF
-      if (isa<Constant>(TVal) || isa<Constant>(FVal)) {
+      if (Arg->hasOneUse() && (isa<Constant>(TVal) || isa<Constant>(FVal))) {
         CallInst *AbsT = Builder.CreateCall(II->getCalledFunction(), {TVal});
         CallInst *AbsF = Builder.CreateCall(II->getCalledFunction(), {FVal});
         SelectInst *SI = SelectInst::Create(Cond, AbsT, AbsF);
diff --git a/llvm/test/Transforms/InstCombine/intrinsic-select.ll b/llvm/test/Transforms/InstCombine/intrinsic-select.ll
index e7b092e1f34d77..fd047e43a852e7 100644
--- a/llvm/test/Transforms/InstCombine/intrinsic-select.ll
+++ b/llvm/test/Transforms/InstCombine/intrinsic-select.ll
@@ -350,8 +350,7 @@ define float @test_fabs_select_multiuse(i1 %cond, float %x) {
 ; CHECK-LABEL: @test_fabs_select_multiuse(
 ; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[COND:%.*]], float [[X:%.*]], float 0x7FF0000000000000
 ; CHECK-NEXT:    call void @usef32(float [[SELECT]])
-; CHECK-NEXT:    [[TMP1:%.*]] = call float @llvm.fabs.f32(float [[X]])
-; CHECK-NEXT:    [[FABS:%.*]] = select i1 [[COND]], float [[TMP1]], float 0x7FF0000000000000
+; CHECK-NEXT:    [[FABS:%.*]] = call float @llvm.fabs.f32(float [[SELECT]])
 ; CHECK-NEXT:    ret float [[FABS]]
 ;
   %select = select i1 %cond, float %x, float 0x7FF0000000000000