[WGSL - Naga] Storage buffer nested array of matrices Struct member values are garbage when not accessed directly.

[martinlaxenaire]

I've ran into a really weird issue when using a nested array of matrices Struct member where if you don't access it directly in your WGSL (i.e. if you're assigning it to a variable first), then the values become complete garbage.

  // 'color' member is originally this Float32Array:
  // [1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
  struct Storages {
    color : array<mat4x4f, 1>,
  }

  struct NestedStorages {
    elements: array<Storages>,
  }

  @binding(0) @group(0) var<storage, read> storages : NestedStorages;

  @fragment
  fn main() -> @location(0) vec4f {
    // THIS WORKS (displays a red triangle)
    // return storages.elements[0].color[0][0];

    // HOWEVER THIS DOES NOT (displays a white triangle)
    let storageElement = storages.elements[0];
    return storageElement.color[0][0];
  }

Here is a minimal reproduction of the issue: https://jsfiddle.net/9jLv5rzq/5/

The triangle drawn should always be red, however on load it is white on Firefox on Windows.

Note that if you use an array of vec4f instead for example, it works again example:

  // THIS WORKS
  struct Storages {
    color : array<vec4f, 4>,
  }

Reproduced on both of my Windows computers:
Windows 10 (version 22H2), GPU: AMD Radeon Vega 8, Firefox 142.0.1
Windows 11 (version 23H2), GPU: Intel UHD 620, Firefox 142.0.1

[cwfitzgerald]

Could you run this through the naga clinic repo and have it generate hlsl? That would be quite helpful.

[martinlaxenaire]

Hey, so this is what naga-cli generated.

1. Working WGSL to HLSL

WGSL input:

struct Storages {
    color: array<mat4x4f, 1>,
}

struct NestedStorages {
    elements: array<Storages>,
}

@binding(0) @group(0) var<storage, read> storages : NestedStorages;

@fragment
fn main() -> @location(0) vec4f {
    return storages.elements[0].color[0][0];
}

HLSL output:

struct Storages {
    row_major float4x4 color[1];
};

ByteAddressBuffer storages : register(t0);

float4 main() : SV_Target0
{
    float4 _e6 = asfloat(storages.Load4(0+0+0+0+0));
    return _e6;
}

2. Buggy WGSL to HLSL

WGSL input:

struct Storages {
    color: array<mat4x4f, 1>,
}

struct NestedStorages {
    elements: array<Storages>,
}

@binding(0) @group(0) var<storage, read> storages : NestedStorages;

@fragment
fn main() -> @location(0) vec4f {
    let storageElement = storages.elements[0];
    return storageElement.color[0][0];
}

HLSL output:

struct Storages {
    row_major float4x4 color[1];
};

ByteAddressBuffer storages : register(t0);

typedef float4x4 ret_Constructarray1_float4x4_[1];
ret_Constructarray1_float4x4_ Constructarray1_float4x4_(float4x4 arg0) {
    float4x4 ret[1] = { arg0 };
    return ret;
}

Storages ConstructStorages(float4x4 arg0[1]) {
    Storages ret = (Storages)0;
    ret.color = arg0;
    return ret;
}

float4 main() : SV_Target0
{
    Storages storageElement = ConstructStorages(Constructarray1_float4x4_(float4x4(asfloat(storages.Load4(0+0+0+0+0)), asfloat(storages.Load4(0+0+0+0+16)), asfloat(storages.Load4(0+0+0+0+32)), asfloat(storages.Load4(0+0+0+0+48)))));
    return storageElement.color[0][0];
}

Hope that helps,