release/20.x: [InstCombine] Preserve signbit semantics of NaN with fold to fabs (#136648) #137608
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@arsenm What do you think about merging this PR to the release branch? |
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@llvm/pr-subscribers-llvm-transforms Author: None (llvmbot) ChangesBackport 3e1e406 Requested by: @dtcxzyw Full diff: https://github.com/llvm/llvm-project/pull/137608.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 29c5cef84ccdb..9cd234dd3babf 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2838,7 +2838,14 @@ 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 (match(TrueVal, m_FSub(m_PosZeroFP(), m_Specific(X))) &&
+ (cast<FPMathOperator>(CondVal)->hasNoNaNs() || SI.hasNoNaNs() ||
+ isKnownNeverNaN(X, /*Depth=*/0,
+ IC.getSimplifyQuery().getWithInstruction(
+ cast<Instruction>(CondVal))))) {
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..f449d4b8e6b37 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
@@ -263,12 +276,34 @@ define double @select_fcmp_ole_zero(double %x) {
ret double %fabs
}
+define double @select_fcmp_ole_zero_no_nnan_input_nofpclass_nan(double nofpclass(nan) %x) {
+; CHECK-LABEL: @select_fcmp_ole_zero_no_nnan_input_nofpclass_nan(
+; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
+; CHECK-NEXT: ret double [[FABS]]
+;
+ %lezero = fcmp 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_select_nnan(double %x) {
+; CHECK-LABEL: @select_fcmp_ole_zero_select_nnan(
+; CHECK-NEXT: [[FABS:%.*]] = call nnan double @llvm.fabs.f64(double [[X:%.*]])
+; CHECK-NEXT: ret double [[FABS]]
+;
+ %lezero = fcmp ole double %x, 0.0
+ %negx = fsub double 0.0, %x
+ %fabs = select nnan i1 %lezero, double %negx, double %x
+ ret double %fabs
+}
+
define double @select_fcmp_nnan_ole_zero(double %x) {
; CHECK-LABEL: @select_fcmp_nnan_ole_zero(
; 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 +314,7 @@ 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 +327,7 @@ 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 +355,7 @@ 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 +366,7 @@ 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 +379,7 @@ 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 +417,7 @@ 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 +428,7 @@ 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 +441,7 @@ 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 +452,7 @@ 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 +465,7 @@ 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
|
arsenm
approved these changes
Apr 29, 2025
github-project-automation
bot
moved this from Needs Triage
to Needs Merge
in LLVM Release Status
…vm#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 llvm@862e35e for reference. Alive2: https://alive2.llvm.org/ce/z/QYtEGj Closes llvm#136646 (cherry picked from commit 3e1e406)
github-project-automation
bot
moved this from Needs Merge
to Done
in LLVM Release Status
|
@dtcxzyw (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR. |
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Backport 3e1e406
Requested by: @dtcxzyw