[InstCombine] Preserve the sign bit of NaN in `SimplifyDemandedUseFPClass` #137287

dtcxzyw · 2025-04-25T06:20:44Z

Alive2: https://alive2.llvm.org/ce/z/uiUzEf

Closes #137196.

Note: To avoid regression in ret_nofpclass_nopositives_copysign_nnan_flag, the second commit takes FMF into account.

…lass`

llvmbot · 2025-04-25T06:21:21Z

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

Changes

Alive2: https://alive2.llvm.org/ce/z/uiUzEf

Closes #137196.

Note: To avoid regression in ret_nofpclass_nopositives_copysign_nnan_flag, the second commit takes FMF into account.

Full diff: https://github.com/llvm/llvm-project/pull/137287.diff

2 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (+13-6)
(modified) llvm/test/Transforms/InstCombine/simplify-demanded-fpclass.ll (+8-11)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 2c8939b5a0514..a48854a191cae 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1959,9 +1959,11 @@ static Constant *getFPClassConstant(Type *Ty, FPClassTest Mask) {
   }
 }
 
-Value *InstCombinerImpl::SimplifyDemandedUseFPClass(
-    Value *V, const FPClassTest DemandedMask, KnownFPClass &Known,
-    unsigned Depth, Instruction *CxtI) {
+Value *InstCombinerImpl::SimplifyDemandedUseFPClass(Value *V,
+                                                    FPClassTest DemandedMask,
+                                                    KnownFPClass &Known,
+                                                    unsigned Depth,
+                                                    Instruction *CxtI) {
   assert(Depth <= MaxAnalysisRecursionDepth && "Limit Search Depth");
   Type *VTy = V->getType();
 
@@ -1985,7 +1987,12 @@ Value *InstCombinerImpl::SimplifyDemandedUseFPClass(
   if (!I->hasOneUse())
     return nullptr;
 
-  // TODO: Should account for nofpclass/FastMathFlags on current instruction
+  if (auto *FPOp = dyn_cast<FPMathOperator>(I)) {
+    if (FPOp->hasNoNaNs())
+      DemandedMask &= ~fcNan;
+    if (FPOp->hasNoInfs())
+      DemandedMask &= ~fcInf;
+  }
   switch (I->getOpcode()) {
   case Instruction::FNeg: {
     if (SimplifyDemandedFPClass(I, 0, llvm::fneg(DemandedMask), Known,
@@ -2013,13 +2020,13 @@ Value *InstCombinerImpl::SimplifyDemandedUseFPClass(
       if (SimplifyDemandedFPClass(I, 0, DemandedMaskAnySign, Known, Depth + 1))
         return I;
 
-      if ((DemandedMask & fcPositive) == fcNone) {
+      if ((DemandedMask & fcNegative) == DemandedMask) {
         // Roundabout way of replacing with fneg(fabs)
         I->setOperand(1, ConstantFP::get(VTy, -1.0));
         return I;
       }
 
-      if ((DemandedMask & fcNegative) == fcNone) {
+      if ((DemandedMask & fcPositive) == DemandedMask) {
         // Roundabout way of replacing with fabs
         I->setOperand(1, ConstantFP::getZero(VTy));
         return I;
diff --git a/llvm/test/Transforms/InstCombine/simplify-demanded-fpclass.ll b/llvm/test/Transforms/InstCombine/simplify-demanded-fpclass.ll
index ad88287f1d99e..b3f508f5ace33 100644
--- a/llvm/test/Transforms/InstCombine/simplify-demanded-fpclass.ll
+++ b/llvm/test/Transforms/InstCombine/simplify-demanded-fpclass.ll
@@ -682,8 +682,7 @@ define nofpclass(inf) float @ret_nofpclass_inf__copysign_negative_select_pinf_rh
 define nofpclass(pinf pnorm psub pzero) float @ret_nofpclass_nopositives_copysign(float %x, float %unknown.sign) {
 ; CHECK-LABEL: define nofpclass(pinf pzero psub pnorm) float @ret_nofpclass_nopositives_copysign
 ; CHECK-SAME: (float [[X:%.*]], float [[UNKNOWN_SIGN:%.*]]) {
-; CHECK-NEXT:    [[TMP1:%.*]] = call float @llvm.fabs.f32(float [[X]])
-; CHECK-NEXT:    [[COPYSIGN:%.*]] = fneg float [[TMP1]]
+; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.copysign.f32(float [[X]], float [[UNKNOWN_SIGN]])
 ; CHECK-NEXT:    ret float [[COPYSIGN]]
 ;
   %copysign = call float @llvm.copysign.f32(float %x, float %unknown.sign)
@@ -718,7 +717,7 @@ define nofpclass(nan pinf pnorm psub pzero) float @ret_nofpclass_nopositives_non
 define nofpclass(ninf nnorm nsub nzero) float @ret_nofpclass_nonegatives_copysign(float %x, float %unknown.sign) {
 ; CHECK-LABEL: define nofpclass(ninf nzero nsub nnorm) float @ret_nofpclass_nonegatives_copysign
 ; CHECK-SAME: (float [[X:%.*]], float [[UNKNOWN_SIGN:%.*]]) {
-; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.fabs.f32(float [[X]])
+; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.copysign.f32(float [[X]], float [[UNKNOWN_SIGN]])
 ; CHECK-NEXT:    ret float [[COPYSIGN]]
 ;
   %copysign = call float @llvm.copysign.f32(float %x, float %unknown.sign)
@@ -763,7 +762,7 @@ define nofpclass(pinf pnorm psub pzero) float @ret_nofpclass_nopositives__copysi
 define nofpclass(inf nnorm nsub nzero) float @ret_nofpclass_no_negatives_noinf__copysign_unknown_select_pinf_rhs(i1 %cond, float %x, float %unknown.sign) {
 ; CHECK-LABEL: define nofpclass(inf nzero nsub nnorm) float @ret_nofpclass_no_negatives_noinf__copysign_unknown_select_pinf_rhs
 ; CHECK-SAME: (i1 [[COND:%.*]], float [[X:%.*]], float [[UNKNOWN_SIGN:%.*]]) {
-; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.fabs.f32(float [[X]])
+; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.copysign.f32(float [[X]], float [[UNKNOWN_SIGN]])
 ; CHECK-NEXT:    ret float [[COPYSIGN]]
 ;
   %select = select i1 %cond, float %x, float 0x7FF0000000000000
@@ -775,8 +774,7 @@ define nofpclass(inf nnorm nsub nzero) float @ret_nofpclass_no_negatives_noinf__
 define nofpclass(inf pnorm psub pzero) float @ret_nofpclass_no_positives_noinf__copysign_unknown_select_pinf_rhs(i1 %cond, float %x, float %unknown.sign) {
 ; CHECK-LABEL: define nofpclass(inf pzero psub pnorm) float @ret_nofpclass_no_positives_noinf__copysign_unknown_select_pinf_rhs
 ; CHECK-SAME: (i1 [[COND:%.*]], float [[X:%.*]], float [[UNKNOWN_SIGN:%.*]]) {
-; CHECK-NEXT:    [[TMP1:%.*]] = call float @llvm.fabs.f32(float [[X]])
-; CHECK-NEXT:    [[COPYSIGN:%.*]] = fneg float [[TMP1]]
+; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.copysign.f32(float [[X]], float [[UNKNOWN_SIGN]])
 ; CHECK-NEXT:    ret float [[COPYSIGN]]
 ;
   %select = select i1 %cond, float %x, float 0x7FF0000000000000
@@ -788,8 +786,8 @@ define nofpclass(inf pnorm psub pzero) float @ret_nofpclass_no_positives_noinf__
 define nofpclass(ninf nnorm nsub nzero) float @ret_nofpclass_no_negatives__copysign_unknown_select_pinf_rhs(i1 %cond, float %x, float %unknown.sign) {
 ; CHECK-LABEL: define nofpclass(ninf nzero nsub nnorm) float @ret_nofpclass_no_negatives__copysign_unknown_select_pinf_rhs
 ; CHECK-SAME: (i1 [[COND:%.*]], float [[X:%.*]], float [[UNKNOWN_SIGN:%.*]]) {
-; CHECK-NEXT:    [[TMP1:%.*]] = call float @llvm.fabs.f32(float [[X]])
-; CHECK-NEXT:    [[COPYSIGN:%.*]] = select i1 [[COND]], float [[TMP1]], float 0x7FF0000000000000
+; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[COND]], float [[X]], float 0x7FF0000000000000
+; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.copysign.f32(float [[SELECT]], float [[UNKNOWN_SIGN]])
 ; CHECK-NEXT:    ret float [[COPYSIGN]]
 ;
   %select = select i1 %cond, float %x, float 0x7FF0000000000000
@@ -801,9 +799,8 @@ define nofpclass(ninf nnorm nsub nzero) float @ret_nofpclass_no_negatives__copys
 define nofpclass(pinf pnorm psub pzero) float @ret_nofpclass_no_positives__copysign_unknown_select_pinf_rhs(i1 %cond, float %x, float %unknown.sign) {
 ; CHECK-LABEL: define nofpclass(pinf pzero psub pnorm) float @ret_nofpclass_no_positives__copysign_unknown_select_pinf_rhs
 ; CHECK-SAME: (i1 [[COND:%.*]], float [[X:%.*]], float [[UNKNOWN_SIGN:%.*]]) {
-; CHECK-NEXT:    [[TMP1:%.*]] = call float @llvm.fabs.f32(float [[X]])
-; CHECK-NEXT:    [[DOTNEG:%.*]] = fneg float [[TMP1]]
-; CHECK-NEXT:    [[COPYSIGN:%.*]] = select i1 [[COND]], float [[DOTNEG]], float 0xFFF0000000000000
+; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[COND]], float [[X]], float 0x7FF0000000000000
+; CHECK-NEXT:    [[COPYSIGN:%.*]] = call float @llvm.copysign.f32(float [[SELECT]], float [[UNKNOWN_SIGN]])
 ; CHECK-NEXT:    ret float [[COPYSIGN]]
 ;
   %select = select i1 %cond, float %x, float 0x7FF0000000000000

nikic

Looks fine, but I don't think there is enough coverage for the new FMF handling? ninf is untested.

arsenm · 2025-04-25T08:36:48Z

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

-Value *InstCombinerImpl::SimplifyDemandedUseFPClass(
-    Value *V, const FPClassTest DemandedMask, KnownFPClass &Known,
-    unsigned Depth, Instruction *CxtI) {
+Value *InstCombinerImpl::SimplifyDemandedUseFPClass(Value *V,


Not sure why this changed, the signature didn't?

It is needed by DemandedMask &= ~fcNan;.

arsenm · 2025-04-25T08:39:31Z

Looks fine, but I don't think there is enough coverage for the new FMF handling? ninf is untested.

I think it is, just not standalone without nnan

…lass` (llvm#137287) Alive2: https://alive2.llvm.org/ce/z/uiUzEf Closes llvm#137196. Note: To avoid regression in `ret_nofpclass_nopositives_copysign_nnan_flag`, the second commit takes FMF into account.

dtcxzyw added 2 commits April 25, 2025 13:55

[InstCombine] Preserve the sign bit of NaN in `SimplifyDemandedUseFPC…

e94b6c3

…lass`

[InstCombine] Handle FMF in SimplifyDemandedUseFPClass

6c3097d

dtcxzyw requested a review from arsenm April 25, 2025 06:20

dtcxzyw requested a review from nikic as a code owner April 25, 2025 06:20

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Apr 25, 2025

nikic reviewed Apr 25, 2025

View reviewed changes

Add more tests. NFC.

81c9a92

arsenm approved these changes Apr 25, 2025

View reviewed changes

dtcxzyw merged commit 1f69d63 into llvm:main Apr 28, 2025
11 checks passed

dtcxzyw deleted the fix-137196 branch April 28, 2025 09:01

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[InstCombine] Preserve the sign bit of NaN in `SimplifyDemandedUseFPClass` #137287

[InstCombine] Preserve the sign bit of NaN in `SimplifyDemandedUseFPClass` #137287