https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/137608
Backport 3e1e4062e1e95031c32c0ed9786647ef1a4141aa Requested by: @dtcxzyw >From fbe3db8268cbb4f2a3e479520637a5bfd4e6a302 Mon Sep 17 00:00:00 2001 From: Yingwei Zheng <dtcxzyw2...@gmail.com> Date: Sat, 26 Apr 2025 14:03:12 +0800 Subject: [PATCH] [InstCombine] Preserve signbit semantics of NaN with fold to fabs (#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/QYtEGj Closes https://github.com/llvm/llvm-project/issues/136646 (cherry picked from commit 3e1e4062e1e95031c32c0ed9786647ef1a4141aa) --- .../InstCombine/InstCombineSelect.cpp | 9 ++- llvm/test/Transforms/InstCombine/fabs.ll | 57 +++++++++++++++---- 2 files changed, 54 insertions(+), 12 deletions(-) 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 _______________________________________________ llvm-branch-commits mailing list llvm-branch-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
