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[HLSL] [DirectX] translate llvm fast math flags to llvm 3.7 fast math flags #122025

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Merged
merged 1 commit into from
Jan 8, 2025
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[HLSL] [DirectX] translate llvm fast math flags to llvm 3.7 fast math flags #122025

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8 changes: 2 additions & 6 deletions llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -749,8 +749,8 @@ uint64_t DXILBitcodeWriter::getOptimizationFlags(const Value *V) {
if (PEO->isExact())
Flags |= 1 << bitc::PEO_EXACT;
} else if (const auto *FPMO = dyn_cast<FPMathOperator>(V)) {
if (FPMO->hasAllowReassoc())
Flags |= bitc::AllowReassoc;
if (FPMO->hasAllowReassoc() || FPMO->hasAllowContract())
Flags |= bitc::UnsafeAlgebra;
if (FPMO->hasNoNaNs())
Flags |= bitc::NoNaNs;
if (FPMO->hasNoInfs())
Expand All @@ -759,10 +759,6 @@ uint64_t DXILBitcodeWriter::getOptimizationFlags(const Value *V) {
Flags |= bitc::NoSignedZeros;
if (FPMO->hasAllowReciprocal())
Flags |= bitc::AllowReciprocal;
if (FPMO->hasAllowContract())
Flags |= bitc::AllowContract;
if (FPMO->hasApproxFunc())
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Why is "approx func" removed?

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https://releases.llvm.org/3.7.1/docs/LangRef.html#fast-math-flags
Not listed in fast math flags for 3.7, is this an error?

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Ah, good point... I don't want to stand in the way of this getting in, but neither is "AllowContract". Should we be asserting on these instead? (it seems "UnsafeAlgebra" was just renamed to "AllowReassoc")
https://github.com/microsoft/DirectXShaderCompiler/blob/070d0d5a2beacef9eeb51037a9b04665716fd6f3/include/llvm/IR/Operator.h#L171

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@llvm-beanz thoughts on this?

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ApproxFunc has no equivalent in LLVM 3.7 so there is no bitcode representation of it. We have no choice but to drop it.

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Also, in LLVM 3.7 UnsafeAlgebra was a magic catch all flag for "all things fast", it was separated out into granular flags in this commit - 629c41153876b.

It had the impact in the LLVM 3.7 optimizer of controlling both contraction and reassociation (to the extent that LLVM 3.7 had optimizations).

Flags |= bitc::ApproxFunc;
}

return Flags;
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23 changes: 23 additions & 0 deletions llvm/test/tools/dxil-dis/fastmath.ll
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@@ -0,0 +1,23 @@
; RUN: llc %s --filetype=obj -o - | dxil-dis -o - | FileCheck %s
target triple = "dxil-unknown-shadermodel6.7-library"

define float @fma(float %0, float %1, float %2) #0 {
; verify reassoc and contract are converted to fast
; CHECK: %4 = fmul fast float %0, %1
%4 = fmul reassoc float %0, %1
; CHECK-NEXT: %5 = fadd fast float %4, %2
%5 = fadd contract float %4, %2
; verify these are converted to a single fast flag
; CHECK-NEXT: %6 = fmul fast float %0, %1
%6 = fmul reassoc contract float %0, %1
; verify these flags are maintained
; CHECK-NEXT: %7 = fadd nnan ninf nsz arcp float %0, %1
%7 = fadd nnan ninf nsz arcp float %0, %1
; verify that afn is removed
; CHECK-NEXT: %8 = fmul float %0, %1
%8 = fmul afn float %0, %1
ret float %5
}

attributes #0 = { norecurse nounwind readnone willreturn "disable-tail-calls"="false" "waveops-include-helper-lanes" "fp32-denorm-mode"="any" "hlsl.export" }

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