64 bit and 128 bit integers are not supported at all

[hacatu]

Expected Behaviour

I want to port some code using fixed::FixedI128 to the gpu, so I tried using rust-gpu to avoid needing to re-implement the wheel.

Well, this failed epically.

Figuring out how to use rust-gpu from the docs and examples is impossible, so I looked at rust-gpu-chimera, and I was able to figure out how to use it.

Of course, even though fixed is no-std and does not allocate, it does not compile, so I started to try to clone it locally and monkeypatch any issues until it would build.

But after the errors just never ended, I thought it more prudent to LITERALLY manually dig through the thousands of lines of macro-generated code for fixed and inline all the code I was using.

Of course, this failed because rust-gpu does not ACTUALLY support 128 bit integers.

Of course, the errors don't explicitly say this, instead they say "unsupported type for constant i128" or something else, and never explicitly admit to not supporting 128 bit integers.

Ok, that's really disappointing, but sure I'll rewrite all these functions to work on 64 bit limbs explicitly instead.

Well, actually, rust-gpu does not support 64 bit integers either, so that was a waste of time.

Now the error does sort of explicitly say this: "error: u64 type used without OpCapability Int64".

This is pretty surprising because on 32 bit CPU targets we can use 128 bit ints with reckless abandon and the compiler will just figure it out, but some part of this when translating from rust to llvm ir to spriv is not putting the square peg into the square hole.

Example & Steps To Reproduce

This kernel stub should have the same error, even with none of the implementation actually present:

#[cfg(target_arch = "spirv")]
#[spirv(compute(threads(1024)))]
pub fn bqa_kernel(
    #[spirv(global_invocation_id)] gid: UVec3,
    #[spirv(storage_buffer, descriptor_set = 0, binding = 0)] data: &mut [((u64, i64), i64, i64)],
    #[spirv(push_constant)] param: &BqaParams,
) {
    let thread_id = ThreadId::new(gid.x);
    bqa_helper(thread_id, data, (param.r0, param.r1), param.b_min, param.b_max);
}

we don't even need to look into bqa_helper or any other implementation details, the error will be something like

     Compiling kernel v0.1.0 (/home/hac/projects/rust-gpu-chimera/kernel)
  error: `u64` type used without `OpCapability Int64`
      |
      = note: used by `core::option::Option<i64>`
      = note: used by `((u64, i64), i64, i64)`
      = note: used by unnamed type
      = note: used by unnamed type
  note: used from within GLCompute entry-point `bqa_kernel`
     --> kernel/src/lib.rs:171:5
      |
  171 |     #[spirv(storage_buffer, descriptor_set = 0, binding = 0)] data: &mut [((u64, i64), i64, i64)],
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  error: `i64` type used without `OpCapability Int64`

System Info

Rustc version is pinned to nightly-2025-06-23 by rust-gpu-chimera
SPIRV-Tools v2025.3 v2025.3.rc1-0-g33e025681
OS: linux 6.15.8-zen (arch linux)
GPU: Intel Arc B580
I'm using the webgpu backend following rust-gpu-chimera

[LegNeato]

Thank you for the detailed report! Actually just dealt with similar stuff on rust-cuda (Rust-GPU/Rust-CUDA#245). Agreed this is painful.

[LegNeato]

Related: #307

[hacatu]

Yes this is mostly just the same as #307, my bad.

I got my code working by just using OpenCL since I'm a lot more familiar with that, so I can confirm that it's only 2x faster on my GPU than my CPU (probably a combination of low end GPU, high end CPU, and GPUs not liking 64 bit int arithmetic).

But in addition to 307, I might have found another issue:

According to this website, 64 bit int support is implemented in vulkan driver versions >= 101.6252 on my GPU (Intel Arc B580), so if I have that driver version or newer, I would expect rust-gpu to support 64 bit ints even without the polyfills.

I'm not sure what my version of the driver is though, the vulkan-intel package on my system is version 1:25.1.6-1, which looks like it's just a totally different format since 25 is so much smaller than 101

So hopefully someone with a B580 who knows for sure that they have this driver version can check if rust-gpu supports 64 bit ints

[LegNeato]

You have to explicitly opt-in with OpCapability Int64, as rust-gpu has no way to know which devices you will target (you can take the spirv and go run it somewhere else).

[LegNeato]

(if you are using spirv_builder:

rust-gpu/crates/spirv-builder/src/lib.rs

Line 577 in df1628a

pub fn capability(mut self, capability: Capability) -> Self {

)