RenderPass.setPushConstant broken on Dx12

[fyellin]

set_push_constant seems to be broken on Dx12. I have a small test that runs fine on GL, Vulkan, and Metal, but breaks on Dx12.

The test is simple. My push constants are an array of sixteen i32s, (the squares from 0 to 255). In either the vertex shader or the fragment shader I copy the constants to a buffer. I then read the contents of the buffer and compare with the values that were originally pushed.

assert_eq(result, expected_result);

Panic: assertion `left == right` failed
      left: [0, 16, 64, 144, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
     right: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225]

It seems strange that I'm seeing every fourth constant. And I'm only seeing this on Dx12.

bug_report.zip contains a single file bug_report.rs, which is a wgpu test.

bug_report.zip

[teoxoy]

I think this is because HLSL doesn't support arrays for root constants.

See: https://learn.microsoft.com/en-us/windows/win32/direct3d12/using-constants-directly-in-the-root-signature

I would have thought we'd get an error but it seems the shader is being miscompiled and we are indexing the internal vec4s and getting the first element?

[fyellin]

I changed the code so that `let count = 8, and modified the shaders to be:

    struct PushConstants {
        value1: vec4i,
        value2: vec4i,
    }

and

    fn write(ix: u32) {
        let value = select(push_constants.value1, push_constants.value2, ix >= 4u);
        data[ix] = value[ix % 4];
    }

Everything worked fine on all platforms.

I'm not sure what to do with this bug. Is it a validator bug, and arrays shouldn't be allowed in push constants? Can we have platform-specific validator bugs, where arrays are just not allowed in DX12 push constants?

In any case, it's good to have a better understanding of what's going on.

[teoxoy]

I think this is actually a bug in our HLSL backend, we should work around the HLSL limitation.

[cwfitzgerald]

Aha! This is an FXC bug. I was wondering how our CI wasn't broken and how WARP wasn't broken, but locally it was and that is one major differences between the environments.

Setting WGPU_DX12_COMPILER='static-dxc' and running makes it work.

[teoxoy]

Arrays are not permitted in constant buffers that get mapped onto root constants since dynamic indexing in the root signature space is not supported.

from Microsoft docs in previous comment #6558 (comment)

Maybe this was just talking about an FXC limitation then?