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non-literal workgroup size #299
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I don't remember if const generics support attributes, but if they do, that might be a solution for CPU code with
WorkgroupSize. With current Rust nightly it'll look something like this:const fn shared_size(workgroup_size: u32) -> usize { workgroup_size as usize * 2 } pub fn directional_shadows<#[spirv(workgroup_size)] const WORKGROUP_SIZE: u32>( #[spirv(workgroup)] shared: &[f32; shared_size(WORKGROUP_SIZE)], ) where [(); shared_size(WORKGROUP_SIZE)]: { ... }
This requires experimental nightly features, but it is really powerful even with current limitations.
Reacted by Alexander MeißnerI don't believe they do and don't immediately see a nightly feature to flip on.
What about not requiring it to be
UVec3? We are in a const context and therefore have access to the values at compile time and can support other types as long as they are in the proper range (we'd still support primitive parts of course, but those too don't need to be u32)#[cfg(feature = "something")] const WORKGROUP_SIZE: u32works on nightly, meaning it is a valid syntax that macro could ingest and strip for non-SPIR-V target and replace with something else for SPIR-V target.Reacted by Alexander Meißner@Firestar99 we could only do
LocalSizeIdfor vuklan1.2+, right? We'd need anotehr strategy for lower targets. OpenCL apparently needs WorkGroupSize built-in as well, not sure how well those targets are supported though.Issue #756: Deprecated the use of BuiltIn to decorate a constant to set its value and removed the deprecation of the WorkgroupSize built-in. That is, WorkgroupSize is kept but no longer marked as deprecated (it is still required by OpenCL). The use of BuiltIn to decorate a constant to set its value was only for WorkgroupSize, which has been superseded by the LocalSizeId execution mode.
How would spec constants look UX-wise?
#[spirv(compute(threads(x_spec, y_spec, 1)] fn main( #[spirv(spec_constant(id = 1))] x_spec: u32, #[spirv(spec_constant(id = 9000, default = 4))] y_spec: u32, ) {}
Yes
LocalSizeIdis vulkan1.2+, but that should be 99% of our user base. So I actually don't mind the idea of requiring vulkan1.2 to be able to use constants instead of literals, and if only literals are in use, defer to the existing system.I would not actually worry about spec constants for workgroup size until someone actually needs it. You could probably get away with a bit of
macro_rules!copy-paste for the few use-cases i can imagine.Found and fixed an issue in
rspirvwithLocalSizeId: gfx-rs/rspirv#263Reacted by Firestar99- added a commit that references this issue
on Feb 22, 2026
Original discussion in #298
Currently, workgroup size must be a number literal and does not accept const expr. So to have a const if your workgroup size, you must copy-paste the literal in two / three locations:
I'd much rather have the proc macro accept a const expr instead of a literal, so we can do this:
Alternative: WorkgroupSize builtin
The WorkgroupSize builtin as in #298 is not sufficient for these use-cases:
Potential Implementation Path
I wonder if we actually need to parse out the actual value within that macro....
Currently we're emitting this, for which we clearly need to parse the literals:
But with Vulkan1.2 we get the new fancy
LocalSizeIdinstead ofLocalSize, so we can do this:Note how at the point we're writing the
OpExecutionModewe don't actually write any literaly, just references to constants. Why couldn't they be references to actual constants in rust code? And if we have number literals, we can still parse and emit them as constants. It would also open up a path towards workgroup size being a spec constant, see%5.