[SPIR-V] HLSL round intrinsic is translated to "Round" instead of to "RoundEven" which is incorrect based on the HLSL docs (works with the DXIL backend and FXC)

[Dredhog]

Title

[SPIR-V] HLSL round intrinsic is translated to "Round" instead of to "RoundEven" which is incorrect based on the HLSL docs (works with the DXIL backend and FXC)

Functional impact

Functional parity when using the HLSL round intrinsic is lost between the DXIL and SPIR-V backends for floating-point arguments exactly in between integers.

Based on the HLSL docs for round: Halfway cases are rounded to the nearest even.
and indeed FXC and the DXIL backend generate round_ne (which means "round to nearest even")

Minimal repro steps

1. Compile the following shader with dxc.exe -T ps_6_0 -E PSMain -spirv repro_shader.txt:

float4 PSMain(float4 color : COLOR) : SV_TARGET
{
	return round(color);
}

Expected result

The generated SPIR-V should use the RoundEven instruction from the GLSL.std.450 extended instruction set

Actual result

The generated SPIR-V generates the following instructions:

 %1 = OpExtInstImport "GLSL.std.450"
...
%13 = OpExtInst %v4float %1 Round %12

Further technical details

Used DXC binary from the latest official release v1.6.2112

[sudonatalie]

Thanks for identifying this issue @Dredhog ! A fix is currently in review.