Using pointers with OpPhi requires capability VariablePointers or VariablePointersStorageBuffer

[nazar-pc]

Code:

use spirv_std::glam::UVec4;
use spirv_std::spirv;

#[derive(Copy, Clone)]
#[repr(C, align(16))]
pub struct ChaCha8Block(pub [UVec4; 4]);

#[spirv(compute(threads(64, 1, 1)))]
pub fn chacha8_keystream_block(
    #[spirv(uniform, descriptor_set = 1, binding = 0)] initial_state: &ChaCha8Block,
) {
    ChaCha8State::from_repr(*initial_state);
}

#[derive(Copy, Clone)]
pub(crate) struct ChaCha8State {
    data: [u32; 16],
}

impl ChaCha8State {
    pub(crate) fn from_repr(data: ChaCha8Block) -> Self {
        Self {
            data: [
                data.0[0][0], data.0[0][1], data.0[0][2], data.0[0][3],
                data.0[1][0], data.0[1][1], data.0[1][2], data.0[1][3],
                data.0[2][0], data.0[2][1], data.0[2][2], data.0[2][3],
                data.0[3][0], data.0[3][1], data.0[3][2], data.0[3][3],
            ],
        }
    }
}

Results in this:

  error: Using pointers with OpPhi requires capability VariablePointers or VariablePointersStorageBuffer
    |
    = note: module `.../spirv-unknown-vulkan1.1/release/deps/ab_proof_of_space_gpu.spv`

  warning: an unknown error occurred
    |
    = note: spirv-opt failed, leaving as unoptimized
    = note: module `.../spirv-unknown-vulkan1.1/release/deps/ab_proof_of_space_gpu.spv`

  error: error:0:0 - Using pointers with OpPhi requires capability VariablePointers or VariablePointersStorageBuffer
           %58 = OpPhi %_ptr_Function_uint %54 %50 %55 %51 %56 %52 %57 %53
    |
    = note: spirv-val failed
    = note: module `.../spirv-unknown-vulkan1.1/release/deps/ab_proof_of_space_gpu.spv`

It looks like almost anything involving references, de-referencing, etc. results in this error. I'm a bit puzzled how to write anything useful at all if this doesn't work.

Using current main: e6d017d

[nazar-pc]

This happens to work, but only with storage_buffer:

#[spirv(storage_buffer, descriptor_set = 1, binding = 0)] initial_state: &[u32; 16],

Changing to uniform or [UVec4; 4] prevents it from compiling.

[LegNeato]

Have you tried requiring the capability?
If you are using cargo-gpu, it is in Cargo.toml:

[package.metadata.rust-gpu.build]
capabilities = ["VariablePointers"]

Or you can pass them as RUSTFLAGS or using spirv-builder https://rust-gpu.github.io/rust-gpu/api/spirv_builder/enum.Capability.html

We have a test that does similar stuff but requires the capability:

https://github.com/Rust-GPU/rust-gpu/blob/b19a8586e13677ae9437ece3337aee2c76a0c76d/tests/compiletests/ui/lang/consts/nested-ref.rs

[nazar-pc]

Yep, apparently this is one of Vulkan-isms, SpirvBuilder::capability(builder, Capability::VariablePointers) worked, thanks!

[LegNeato]

Yeah, #[spirv(storage_buffer)] works because Vulkan 1.2 includes VariablePointersStorageBuffer.

#[spirv(uniform)] fails because it doesn’t use VariablePointersStorageBuffer, and VariablePointers is optional and not part of any Vulkan core version. But it looks like it is pretty supported

[LegNeato]

@nazar-pc Depending on what you are doing, you may want to look into https://rust-gpu.github.io/rust-gpu/api/spirv_std/byte_addressable_buffer/struct.ByteAddressableBuffer.html as well.

[nazar-pc]

I ended up rewriting the code in a way that doesn't require this capability anymore. The only thing I'd like to see is a wrapper type that ensures proper alignment due to std140 requirement for #[spirv(uniform)], though I just use #[spirv(storage_buffer)] for now.

GPU programming is very new to me, but at least I'm getting some errors instead of UB (for now). Well, at least in some cases, without #98 reading from output arguments, for example, can quietly result in UB.