OpUConvert for specialization constant should not generate spirv-opt error.

[castano]

When I compile the following program:

#version 450
#extension GL_EXT_shader_explicit_arithmetic_types_int16 : require

layout (constant_id = 0) const uint local_size_x = 8;
layout (constant_id = 1) const uint local_size_y = 8;
layout (local_size_x_id = 0, local_size_y_id = 1, local_size_z = 1) in;

void main() {
    u16vec2 tid = u16vec2(gl_LocalInvocationID.xy);
    uint16_t idx = tid.y * uint16_t(gl_WorkGroupSize.x) + tid.x;
}

As follows:

glslangValidator.exe -V100 -e main repro.comp.glsl -o spirv/repro.comp.spv
spirv-opt.exe -O spirv/repro.comp.spv -o spirv/repro.comp.opt.spv

spirv-opt produces the following error:

error: line 38: Prior to SPIR-V 1.4, specialization constant operation UConvert requires Kernel capability or extension SPV_AMD_gpu_shader_int16
  %28 = OpSpecConstantOp %ushort UConvert %27

According to KhronosGroup/glslang#848 the spec has been updated to include OpUConvert in the list of permitted opcodes. Is this a spirv-opt error?

I'm using the most recent builds that are publicly available:

$ glslangValidator.exe --version
Glslang Version: 11:14.0.0
ESSL Version: OpenGL ES GLSL 3.20 glslang Khronos. 14.0.0
GLSL Version: 4.60 glslang Khronos. 14.0.0
SPIR-V Version 0x00010600, Revision 1
GLSL.std.450 Version 100, Revision 1
Khronos Tool ID 8
SPIR-V Generator Version 11
GL_KHR_vulkan_glsl version 100
ARB_GL_gl_spirv version 100

$ spirv-opt.exe --version
SPIRV-Tools v2023.6 v2023.6.rc1-0-gf0cc85ef

[cassiebeckley]

The SPIR-V spec does state that OpUConvert for OpSpecConstantOp is missing before version 1.4: https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpSpecConstantOp.

It looks like you can tell glslangValidator to generate SPIR-V 1.4 using the --target-env argument:

glslangValidator.exe -V100 --target-env spirv1.4 -e main repro.comp.glsl -o spirv/repro.comp.spv
spirv-opt.exe -O spirv/repro.comp.spv -o spirv/repro.comp.opt.spv

As mentioned in KhronosGroup/glslang#3506, this likely is a glslangValidator bug, since it is generating invalid code for SPIR-V 1.0.

[castano]

Thanks for the clarification. I don't think we can target spir-v versions greater than 1.3 yet, so it would be great to have this fixed in the glslang compiler.