MSL: no matching function for call to faceforward on macOS 10.14

[jiradeto]

On MacOS 10.14, when using the variable identifier as the parameter of the built-in function faceforward, the MSL compiler gives the following error:

result.msl:9:15: error: no matching function for call to 'faceforward'
    float b = faceforward(1.0, a, 1.0);
              ^~~~~~~~~~~

The GLSL fragment shader:

void main()
{
    float a = 1.0;
    float b = faceforward(1.0, a, 1.0);
}

MSL obtained by SPIRV-Cross:

fragment void main0()
{
    float a = 1.0;
    float b = faceforward(1.0, a, 1.0);
}

Versions:

• SPIRV-Cross: git-33d2bbcf69c15680919c1f6ffeafb2b744460c66 (Most recent version of this writing)
• glslangValidator: Glslang Version: 7.12.3226

Steps to reproduce:

• Convert the GLSL shader to SPIR-V using glslangValidator with the command glslangValidator -V sample.frag -o sample.spv
• Validate the generated SPIR-V with the command spirv-val sample.spv
• Convert the SPIR-V to MSL using SPIRV-Cross with the command spirv-cross --msl sample.spv --output result.msl
• Build a Metal library with the command xcrun --sdk macosx10.14 metal -x metal -std=macos-metal1.2 result.msl

This Archive.zip contains the original fragment shader, the associated SPIR-V, and MSL shader.

Issue found using GraphicsFuzz.

[cdavis5e]

This is yet another function from <metal_graphics> that doesn't have a scalar overload. At this point, I think we should just add scalar versions of all those functions.

[HansKristian-Work]

Ye ... I was doing a pass over the builtin functions a few days ago, but somehow forgot to check if this function also had this weird scalar-only restriction. Guess it's time to go over all functions one by one and see what works and what doesn't.