nonuniform descriptor indexing: is OpDecorate NonUniform intentionally unsupported?

[abbator]

The Vulkan spec requires OpDecorate NonUniform on non-uniform descriptor array indices (VUID-RuntimeSpirv-NonUniform-06274).

descriptor_set(0) descriptor_binding(0)
uniform_constant storageImage2D images[4096];

u32 idx = buffer[tid];
storageImage2D img = images[idx];
native_vec4 val = imageLoad(img, coord);

Compiling this with Vcc, the generated SPIR-V has no NonUniform decoration, and I didn't find any handling of it in the shady source.

Is this an intentional omission, or just something that hasn't come up yet? If the latter, would a PR be welcome that adds something like an asm("shady::nonuniform") intrinsic that propagates as a decoration to the SPIR-V backend?

[Hugobros3]

The IR tracks scope/frequency of every SSA value so these kinds of decorations can be applied automatically. I'm unsure what the status is for this in the SPIR-V backend, but the idea is to use the information the compiler has about the shape of variable (the "qualifier" in "qualified type" in the grammar) to generate those without user intervention.

See also: #54

[abbator]

Understood. I had a look to see if the backend actually handles this currently, and the answer appears to be no. As is it still triggers VUID-RuntimeSpirv-NonUniform-06274.

I took a stab at fixing it. In emit_spv_value.c, where the access chain gets built, I check the index: if it's per-thread and it's indexing into a descriptor, I tag it with OpDecorate NonUniform and also add the ShaderNonUniform capability and SPV_EXT_descriptor_indexing.

This appears to work. I can make a PR if you like.

[Hugobros3]

By appear to work you mean it silences the validation layers? The real test is whether it works on platforms that actually require the decoration, I recall the spec being quite loose on where the decorations are needed, and I've heard of other spirv producers adding the decoration aggressively to defs that consume the access chain.

[abbator]

Appears to work meaning that looking at the generated SPIR-V under spirv-val, the NonUniform decoration appears on the index into the descriptor array, which appeases VUID-RuntimeSpirv-NonUniform-06274.

Unfortunately the only hardware I have to test it on is NVidia.

[Hugobros3]

NVidia hardware doesn't actually need the decoration so it's not a good oracle