MSL: using findLSB() as an argument to clamp built-ins fails on macOS 10.14

[jiradeto]

On MacOS 10.14, I tried to convert SPIR-V to MSL and found that when using the findLSB() as the argument of the clamp function, the MSL compiler gives the following error:

result.msl:17:9: error: no matching function for call to 'clamp'
    if (clamp(47910, findLSB(40365u), 1) > 0)
        ^~~~~

The GLSL fragment shader:

void main()
{
 if(clamp(47910, findLSB(40365u), 1) > 0)
  {
   discard;
  }
}

MSL obtained by SPIRV-Cross:

fragment void main0()
{
    if (clamp(47910, findLSB(40365u), 1) > 0)
    {
        discard_fragment();
    }
}

Versions:

• SPIRV-Cross: git-44f688bf0b77d4f1a2c87de85980be0696974881 (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.

[HansKristian-Work]

Seems like sign mismatch where findMSB expects to return an int, but it's not, and overload resolution fails.