[llvm] [InstCombine] Preserve signbit semantics of NaN with fold to fabs (PR #136648)
Yingwei Zheng via llvm-commits
llvm-commits at lists.llvm.org
Mon Apr 21 20:24:18 PDT 2025
https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/136648
As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is preserved if there is no floating-point operation being performed.
See also https://github.com/llvm/llvm-project/commit/862e35e25a68502433da0a8d0819448ff5745339 for reference.
Alive2: https://alive2.llvm.org/ce/z/4mBcJx
Closes https://github.com/llvm/llvm-project/issues/136646
>From 98992d305f493d40da23cfe8841fbffb1a3ce267 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Tue, 22 Apr 2025 11:18:43 +0800
Subject: [PATCH] [InstCombine] Preserve signbit semantics of NaN with fold to
fabs
---
.../InstCombine/InstCombineSelect.cpp | 6 +++-
llvm/test/Transforms/InstCombine/fabs.ll | 35 +++++++++++++------
2 files changed, 29 insertions(+), 12 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 4bba2f406b4c1..06792e32de3e1 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2793,7 +2793,11 @@ static Instruction *foldSelectWithFCmpToFabs(SelectInst &SI,
// fold (X <= +/-0.0) ? (0.0 - X) : X to fabs(X), when 'Swap' is false
// fold (X > +/-0.0) ? X : (0.0 - X) to fabs(X), when 'Swap' is true
- if (match(TrueVal, m_FSub(m_PosZeroFP(), m_Specific(X)))) {
+ // Note: We require "nnan" for this fold because fcmp ignores the signbit
+ // of NAN, but IEEE-754 specifies the signbit of NAN values with
+ // fneg/fabs operations.
+ if (cast<FPMathOperator>(CondVal)->hasNoNaNs() &&
+ match(TrueVal, m_FSub(m_PosZeroFP(), m_Specific(X)))) {
if (!Swap && (Pred == FCmpInst::FCMP_OLE || Pred == FCmpInst::FCMP_ULE)) {
Value *Fabs = IC.Builder.CreateUnaryIntrinsic(Intrinsic::fabs, X, &SI);
return IC.replaceInstUsesWith(SI, Fabs);
diff --git a/llvm/test/Transforms/InstCombine/fabs.ll b/llvm/test/Transforms/InstCombine/fabs.ll
index 7b9a672f188ca..827315b184085 100644
--- a/llvm/test/Transforms/InstCombine/fabs.ll
+++ b/llvm/test/Transforms/InstCombine/fabs.ll
@@ -256,6 +256,19 @@ define double @select_fcmp_ole_zero(double %x) {
; CHECK-LABEL: @select_fcmp_ole_zero(
; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
+;
+ %lezero = fcmp nnan ole double %x, 0.0
+ %negx = fsub double 0.0, %x
+ %fabs = select i1 %lezero, double %negx, double %x
+ ret double %fabs
+}
+
+define double @select_fcmp_ole_zero_no_nnan(double %x) {
+; CHECK-LABEL: @select_fcmp_ole_zero_no_nnan(
+; CHECK-NEXT: [[LEZERO:%.*]] = fcmp ole double [[X:%.*]], 0.000000e+00
+; CHECK-NEXT: [[NEGX:%.*]] = fsub double 0.000000e+00, [[X]]
+; CHECK-NEXT: [[FABS:%.*]] = select i1 [[LEZERO]], double [[NEGX]], double [[X]]
+; CHECK-NEXT: ret double [[FABS]]
;
%lezero = fcmp ole double %x, 0.0
%negx = fsub double 0.0, %x
@@ -268,7 +281,7 @@ define double @select_fcmp_nnan_ole_zero(double %x) {
; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
- %lezero = fcmp ole double %x, 0.0
+ %lezero = fcmp nnan ole double %x, 0.0
%negx = fsub nnan double 0.0, %x
%fabs = select i1 %lezero, double %negx, double %x
ret double %fabs
@@ -279,7 +292,7 @@ define double @select_nnan_fcmp_nnan_ole_zero(double %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
- %lezero = fcmp ole double %x, 0.0
+ %lezero = fcmp nnan ole double %x, 0.0
%negx = fsub nnan double 0.0, %x
%fabs = select nnan i1 %lezero, double %negx, double %x
ret double %fabs
@@ -292,7 +305,7 @@ define double @select_fcmp_nnan_ule_zero(double %x) {
; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
- %lezero = fcmp ule double %x, 0.0
+ %lezero = fcmp nnan ule double %x, 0.0
%negx = fsub nnan double 0.0, %x
%fabs = select i1 %lezero, double %negx, double %x
ret double %fabs
@@ -320,7 +333,7 @@ define <2 x float> @select_fcmp_nnan_ole_negzero(<2 x float> %x) {
; CHECK-NEXT: [[FABS:%.*]] = call <2 x float> @llvm.fabs.v2f32(<2 x float> [[X:%.*]])
; CHECK-NEXT: ret <2 x float> [[FABS]]
;
- %lezero = fcmp ole <2 x float> %x, <float -0.0, float -0.0>
+ %lezero = fcmp nnan ole <2 x float> %x, <float -0.0, float -0.0>
%negx = fsub nnan <2 x float> <float 0.0, float poison>, %x
%fabs = select <2 x i1> %lezero, <2 x float> %negx, <2 x float> %x
ret <2 x float> %fabs
@@ -331,7 +344,7 @@ define <2 x float> @select_nnan_fcmp_nnan_ole_negzero(<2 x float> %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan <2 x float> @llvm.fabs.v2f32(<2 x float> [[X:%.*]])
; CHECK-NEXT: ret <2 x float> [[FABS]]
;
- %lezero = fcmp ole <2 x float> %x, <float -0.0, float -0.0>
+ %lezero = fcmp nnan ole <2 x float> %x, <float -0.0, float -0.0>
%negx = fsub nnan <2 x float> <float 0.0, float poison>, %x
%fabs = select nnan <2 x i1> %lezero, <2 x float> %negx, <2 x float> %x
ret <2 x float> %fabs
@@ -344,7 +357,7 @@ define fp128 @select_fcmp_ogt_zero(fp128 %x) {
; CHECK-NEXT: [[FABS:%.*]] = call fp128 @llvm.fabs.f128(fp128 [[X:%.*]])
; CHECK-NEXT: ret fp128 [[FABS]]
;
- %gtzero = fcmp ogt fp128 %x, zeroinitializer
+ %gtzero = fcmp nnan ogt fp128 %x, zeroinitializer
%negx = fsub fp128 zeroinitializer, %x
%fabs = select i1 %gtzero, fp128 %x, fp128 %negx
ret fp128 %fabs
@@ -382,7 +395,7 @@ define fp128 @select_fcmp_nnan_ogt_zero(fp128 %x) {
; CHECK-NEXT: [[FABS:%.*]] = call fp128 @llvm.fabs.f128(fp128 [[X:%.*]])
; CHECK-NEXT: ret fp128 [[FABS]]
;
- %gtzero = fcmp ogt fp128 %x, zeroinitializer
+ %gtzero = fcmp nnan ogt fp128 %x, zeroinitializer
%negx = fsub nnan fp128 zeroinitializer, %x
%fabs = select i1 %gtzero, fp128 %x, fp128 %negx
ret fp128 %fabs
@@ -393,7 +406,7 @@ define fp128 @select_nnan_fcmp_nnan_ogt_zero(fp128 %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan fp128 @llvm.fabs.f128(fp128 [[X:%.*]])
; CHECK-NEXT: ret fp128 [[FABS]]
;
- %gtzero = fcmp ogt fp128 %x, zeroinitializer
+ %gtzero = fcmp nnan ogt fp128 %x, zeroinitializer
%negx = fsub nnan fp128 zeroinitializer, %x
%fabs = select nnan i1 %gtzero, fp128 %x, fp128 %negx
ret fp128 %fabs
@@ -406,7 +419,7 @@ define half @select_fcmp_nnan_ogt_negzero(half %x) {
; CHECK-NEXT: [[FABS:%.*]] = call half @llvm.fabs.f16(half [[X:%.*]])
; CHECK-NEXT: ret half [[FABS]]
;
- %gtzero = fcmp ogt half %x, -0.0
+ %gtzero = fcmp nnan ogt half %x, -0.0
%negx = fsub nnan half 0.0, %x
%fabs = select i1 %gtzero, half %x, half %negx
ret half %fabs
@@ -417,7 +430,7 @@ define half @select_nnan_fcmp_nnan_ogt_negzero(half %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan half @llvm.fabs.f16(half [[X:%.*]])
; CHECK-NEXT: ret half [[FABS]]
;
- %gtzero = fcmp ogt half %x, -0.0
+ %gtzero = fcmp nnan ogt half %x, -0.0
%negx = fsub nnan half 0.0, %x
%fabs = select nnan i1 %gtzero, half %x, half %negx
ret half %fabs
@@ -430,7 +443,7 @@ define half @select_fcmp_nnan_ugt_negzero(half %x) {
; CHECK-NEXT: [[FABS:%.*]] = call half @llvm.fabs.f16(half [[X:%.*]])
; CHECK-NEXT: ret half [[FABS]]
;
- %gtzero = fcmp ugt half %x, -0.0
+ %gtzero = fcmp nnan ugt half %x, -0.0
%negx = fsub nnan half 0.0, %x
%fabs = select i1 %gtzero, half %x, half %negx
ret half %fabs
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