When targeting SpirV, Slang will misscompile global variables bound to a semantic when they are used in the same expression as WorkgroupSize().

[gan74]

Hi !

Slang misscompiles this code:

RWStructuredBuffer<uint> out_buffer;

in uint3 gid : SV_GroupID;

[shader("compute")]
[numthreads(8, 8)]
void comp_main() {
    uint expr = WorkgroupSize().x + gid.x;
    out_buffer[0] = expr;
}

When compiled with slangc -target spirv test.slang -o test.spv then dumped into glsl using spirv-cross test.spv, I get this:

#version 450
layout(local_size_x = 8, local_size_y = 8, local_size_z = 1) in;

layout(binding = 0, std430) buffer RWStructuredBuffer
{
    uint _m0[];
} out_buffer;

void main()
{
    uvec3 gid;
    out_buffer._m0[0] = uint(8 + int(gid.x));
}

Here gid isn't initialized, which leads to problems (first detected as a device lost due to an incorrect index computation leading to an out of bound access).

Weirdly this doesn't happen if I use gid directly without mixing it with WorkgroupSize() or if I take SV_GroupID as main argument.
It isn't specific to SV_GroupID, other semantics are also affected. It appears to be specific to WorkgroupSize() though.

This is a regression, it didn't happen before v2024.1.24. A quick bisection seems to point to commit 0b51902

[ArielG-NV]

Note: It looks like only GLSL & SPIR-V experience invalid behavior (both do not translate gid into a system value). I do not think the single change inside 0b51902 is the cause of the bug (from what I see when testing).