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Fix constant folding of frac for negative values #8980

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Fix constant folding of frac for negative values #8980

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3 changes: 3 additions & 0 deletions docs/ReleaseNotes.md
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Expand Up @@ -67,6 +67,9 @@ line upon naming the release. Refer to previous for appropriate section names.

- Fixed an optimizer crash when scalarizing an out-of-bounds vector access
[#8940](https://github.com/microsoft/DirectXShaderCompiler/issues/8940).
- Fixed constant folding of `frac` returning `abs(x - trunc(x))` instead of
`x - floor(x)` for negative values, such as matrix arguments
[#8937](https://github.com/microsoft/DirectXShaderCompiler/issues/8937).

### Upcoming Preview Release

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7 changes: 3 additions & 4 deletions lib/Analysis/DxilConstantFolding.cpp
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Expand Up @@ -192,10 +192,9 @@ static Constant *ConstantFoldUnaryFPIntrinsic(OP::OpCode opcode, Type *Ty,
case OP::OpCode::Exp:
return DxilConstantFoldFP(exp2, Op, Ty);
case OP::OpCode::Frc: {
NativeFPUnaryOp f = [](double x) {
double unused;
return fabs(modf(x, &unused));
};
// frac(x) = x - floor(x), so negative inputs also give a result in
// [+0, 1), the same as the front end's folding of the frac intrinsic
NativeFPUnaryOp f = [](double x) { return x - floor(x); };
return DxilConstantFoldFP(f, Op, Ty);
}
case OP::OpCode::Log:
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7 changes: 6 additions & 1 deletion tools/clang/test/HLSLFileCheck/hlsl/intrinsics/basic/frac_literal.hlsl
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Expand Up @@ -56,8 +56,13 @@ void main() {
f4buf[2] = float4(fmat_splat[0][0], fmat_splat[0][1], fmat_splat[1][0], fmat_splat[1][1]) ;

// matrix non-splat
// CHECK: float 0x3F2A400000000000, float 0x3F53A92A40000000, float 0x3FCD710000000000, float 0.000000e+00
// CHECK: float 0x3FEFFE5C00000000, float 0x3FEFF62B60000000, float 0x3FCD710000000000, float 0.000000e+00
float2x2 fmat = { -21.0002, -0.0012, 421.23, 1 };
fmat = frac(fmat);
f4buf[3] = float4(fmat[0][0], fmat[0][1], fmat[1][0], fmat[1][1]) ;

// matrix with negative values: frac(x) = x - floor(x)
// CHECK: float 2.500000e-01, float 7.500000e-01, float 7.500000e-01, float 2.500000e-01
float2x2 fmat_neg = frac(float2x2(-1.75, -0.25, 0.75, 1.25));
f4buf[4] = float4(fmat_neg[0][0], fmat_neg[0][1], fmat_neg[1][0], fmat_neg[1][1]);
}
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