[llvm] [InstCombine] Fold fcmp into select (PR #86482)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

This patch simplifies `fcmp (select Cond, C1, C2), C3` patterns in ceres:
Alive2: https://alive2.llvm.org/ce/z/fWh_sD
```
define i1 @<!-- -->src(double %x) {
  %cmp1 = fcmp ord double %x, 0.000000e+00
  %sel = select i1 %cmp1, double 0xFFFFFFFFFFFFFFFF, double 0.000000e+00
  %cmp2 = fcmp oeq double %sel, 0.000000e+00
  ret i1 %cmp2
}

define i1 @<!-- -->tgt(double %x) {
  %cmp1 = fcmp uno double %x, 0.000000e+00
  ret i1 %cmp1
}

```

---
Full diff: https://github.com/llvm/llvm-project/pull/86482.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+4) 
- (modified) llvm/test/Transforms/InstCombine/fcmp-select.ll (+65) 
- (modified) llvm/test/Transforms/InstCombine/select-select.ll (+3-4) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index db302d7e526844..ba13c59e047b52 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -8020,6 +8020,10 @@ Instruction *InstCombinerImpl::visitFCmpInst(FCmpInst &I) {
       if (Instruction *NV = foldOpIntoPhi(I, cast<PHINode>(LHSI)))
         return NV;
       break;
+    case Instruction::Select:
+      if (Instruction *NV = FoldOpIntoSelect(I, cast<SelectInst>(LHSI)))
+        return NV;
+      break;
     case Instruction::SIToFP:
     case Instruction::UIToFP:
       if (Instruction *NV = foldFCmpIntToFPConst(I, LHSI, RHSC))
diff --git a/llvm/test/Transforms/InstCombine/fcmp-select.ll b/llvm/test/Transforms/InstCombine/fcmp-select.ll
index f37c586845b12d..62fad8349d22e2 100644
--- a/llvm/test/Transforms/InstCombine/fcmp-select.ll
+++ b/llvm/test/Transforms/InstCombine/fcmp-select.ll
@@ -148,3 +148,68 @@ define i1 @fcmp_ogt_select(i1 %cond, float %a, float %b) {
   %res = fcmp ogt float %lhs, %rhs
   ret i1 %res
 }
+
+define i1 @test_fcmp_select_const_const(double %x) {
+; CHECK-LABEL: @test_fcmp_select_const_const(
+; CHECK-NEXT:    [[CMP1:%.*]] = fcmp uno double [[X:%.*]], 0.000000e+00
+; CHECK-NEXT:    ret i1 [[CMP1]]
+;
+  %cmp1 = fcmp ord double %x, 0.000000e+00
+  %sel = select i1 %cmp1, double 0xFFFFFFFFFFFFFFFF, double 0.000000e+00
+  %cmp2 = fcmp oeq double %sel, 0.000000e+00
+  ret i1 %cmp2
+}
+
+define i1 @test_fcmp_select_var_const(double %x, double %y) {
+; CHECK-LABEL: @test_fcmp_select_var_const(
+; CHECK-NEXT:    [[CMP1:%.*]] = fcmp ule double [[X:%.*]], 0x3E80000000000000
+; CHECK-NEXT:    [[TMP1:%.*]] = fcmp olt double [[Y:%.*]], 0x3E80000000000000
+; CHECK-NEXT:    [[CMP2:%.*]] = select i1 [[CMP1]], i1 true, i1 [[TMP1]]
+; CHECK-NEXT:    ret i1 [[CMP2]]
+;
+  %cmp1 = fcmp ogt double %x, 0x3E80000000000000
+  %sel = select i1 %cmp1, double %y, double 0.000000e+00
+  %cmp2 = fcmp olt double %sel, 0x3E80000000000000
+  ret i1 %cmp2
+}
+
+define i1 @test_fcmp_select_var_const_fmf(double %x, double %y) {
+; CHECK-LABEL: @test_fcmp_select_var_const_fmf(
+; CHECK-NEXT:    [[CMP1:%.*]] = fcmp ule double [[X:%.*]], 0x3E80000000000000
+; CHECK-NEXT:    [[CMP2:%.*]] = fcmp nnan olt double [[SEL:%.*]], 0x3E80000000000000
+; CHECK-NEXT:    [[CMP3:%.*]] = select i1 [[CMP1]], i1 true, i1 [[CMP2]]
+; CHECK-NEXT:    ret i1 [[CMP3]]
+;
+  %cmp1 = fcmp ogt double %x, 0x3E80000000000000
+  %sel = select i1 %cmp1, double %y, double 0.000000e+00
+  %cmp2 = fcmp nnan olt double %sel, 0x3E80000000000000
+  ret i1 %cmp2
+}
+
+define <2 x i1> @test_fcmp_select_const_const_vec(<2 x double> %x) {
+; CHECK-LABEL: @test_fcmp_select_const_const_vec(
+; CHECK-NEXT:    [[CMP1:%.*]] = fcmp uno <2 x double> [[X:%.*]], zeroinitializer
+; CHECK-NEXT:    ret <2 x i1> [[CMP1]]
+;
+  %cmp1 = fcmp ord <2 x double> %x, zeroinitializer
+  %sel = select <2 x i1> %cmp1, <2 x double> <double 0xFFFFFFFFFFFFFFFF, double 0xFFFFFFFFFFFFFFFF>, <2 x double> zeroinitializer
+  %cmp2 = fcmp oeq <2 x double> %sel, zeroinitializer
+  ret <2 x i1> %cmp2
+}
+
+; Don't break clamp idioms
+
+define double @test_fcmp_select_clamp(double %x) {
+; CHECK-LABEL: @test_fcmp_select_clamp(
+; CHECK-NEXT:    [[CMP1:%.*]] = fcmp ogt double [[X:%.*]], 9.000000e-01
+; CHECK-NEXT:    [[SEL1:%.*]] = select i1 [[CMP1]], double 9.000000e-01, double [[X]]
+; CHECK-NEXT:    [[CMP2:%.*]] = fcmp olt double [[SEL1]], 5.000000e-01
+; CHECK-NEXT:    [[SEL2:%.*]] = select i1 [[CMP2]], double 5.000000e-01, double [[SEL1]]
+; CHECK-NEXT:    ret double [[SEL2]]
+;
+  %cmp1 = fcmp ogt double %x, 9.000000e-01
+  %sel1 = select i1 %cmp1, double 9.000000e-01, double %x
+  %cmp2 = fcmp olt double %sel1, 5.000000e-01
+  %sel2 = select i1 %cmp2, double 5.000000e-01, double %sel1
+  ret double %sel2
+}
diff --git a/llvm/test/Transforms/InstCombine/select-select.ll b/llvm/test/Transforms/InstCombine/select-select.ll
index 785766136fb71b..84fe973093e327 100644
--- a/llvm/test/Transforms/InstCombine/select-select.ll
+++ b/llvm/test/Transforms/InstCombine/select-select.ll
@@ -18,11 +18,10 @@ define float @foo1(float %a) {
 
 define float @foo2(float %a) {
 ; CHECK-LABEL: @foo2(
-; CHECK-NEXT:    [[B:%.*]] = fcmp ogt float [[A:%.*]], 0.000000e+00
-; CHECK-NEXT:    [[C:%.*]] = select i1 [[B]], float [[A]], float 0.000000e+00
+; CHECK-NEXT:    [[B:%.*]] = fcmp ule float [[C:%.*]], 0.000000e+00
 ; CHECK-NEXT:    [[D:%.*]] = fcmp olt float [[C]], 1.000000e+00
-; CHECK-NEXT:    [[E:%.*]] = select i1 [[B]], float [[A]], float 0.000000e+00
-; CHECK-NEXT:    [[F:%.*]] = select i1 [[D]], float [[E]], float 1.000000e+00
+; CHECK-NEXT:    [[E:%.*]] = select i1 [[D]], float [[C]], float 1.000000e+00
+; CHECK-NEXT:    [[F:%.*]] = select i1 [[B]], float 0.000000e+00, float [[E]]
 ; CHECK-NEXT:    ret float [[F]]
 ;
   %b = fcmp ogt float %a, 0.0

``````````

</details>


https://github.com/llvm/llvm-project/pull/86482