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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

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.~~
Update: allow this to fix a regression in ocio


---
Full diff: https://github.com/llvm/llvm-project/pull/122270.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp (+2-1) 
- (modified) llvm/test/Transforms/InstCombine/intrinsic-select.ll (+27) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index c55c40c88bc845..a19eded6565997 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2709,7 +2709,8 @@ 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))
+                           : (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 4ce2908a630785..a8117ce663a6d0 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 +345,29 @@ 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:    [[FABS:%.*]] = call float @llvm.fabs.f32(float [[SELECT]])
+; 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
+}
+
+define float @test_fabs_select_multiuse_both_constant(i1 %cond, float %x) {
+; CHECK-LABEL: @test_fabs_select_multiuse_both_constant(
+; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[COND:%.*]], float -1.000000e+00, float -2.000000e+00
+; CHECK-NEXT:    call void @usef32(float [[SELECT]])
+; CHECK-NEXT:    [[FABS:%.*]] = select i1 [[COND]], float 1.000000e+00, float 2.000000e+00
+; CHECK-NEXT:    ret float [[FABS]]
+;
+  %select = select i1 %cond, float -1.0, float -2.0
+  call void @usef32(float %select)
+  %fabs = call float @llvm.fabs.f32(float %select)
+  ret float %fabs
+}

``````````

</details>


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