MSL: using unpackUnorm4x8 as an argument to outerProduct built-ins fails on macOS 10.14

[jiradeto]

On MacOS 10.14, I tried to convert SPIR-V to MSL and found an issue that when using the unpackUnorm4x8 as the argument of the outerProduct function, the MSL compiler gives the following error:

result.msl:14:16: error: use of undeclared identifier 'outerProduct'
    float4 a = outerProduct(unpack_unorm4x8_to_float(80045u), float2(1.0))[1];
               ^

The GLSL fragment shader:

void main()
{
 vec4 a = outerProduct(unpackUnorm4x8(80045u), vec2(1.0))[1]; 
 _GLF_color = vec4(1.0);
}

MSL obtained by SPIRV-Cross:

fragment main0_out main0()
{
    main0_out out = {};
    float4 a = outerProduct(unpack_unorm4x8_to_float(80045u), float2(1.0))[1];
    out._GLF_color = float4(1.0);
    return out;
}

Versions:

• SPIRV-Cross: git-8ee8e60f70f937c72379ab1fc404a1c36d660a31 (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
• 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

Note that before running SPIRV-Cross, I also used spirv-val to check that the SPIR-V produced by glslangValidator is valid.

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

Issue found using GraphicsFuzz.

[cdavis5e]

The problem is that OpOuterProduct itself is not supported by the MSL backend. It winds up delegating back to the GLSL backend, which emits a call to outerProduct(), which doesn't exist in MSL. I don't think there's anything in the Metal standard library that corresponds directly to this, which is part of why it's unimplemented. The other part is, I suspect that this operation is fairly rare, so it hasn't come up before.

[HansKristian-Work]

Right, I don't think it's implemented at all in MSL yet, I thought it was, but I guess we didn't have any test coverage for it. Should be easy to add though. I'll get to it on Monday.