Is there any way to enforce a 16 byte alignment for a uniform buffer in GLSL?

Question

I'm targeting WebGL via wgpu and am running into an issue with uniform buffer alignment.

I am trying to use this uniform:

layout(set=0, binding=2, std140)
uniform TexSize {
    ivec2 dimensions;
};

And I get an error BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED .

Checking with the maintainers of wgpu, I was informed this was because of the flavor of GLSL used by WebGPU, and that the uniform buffer in my shader must be 16-byte-aligned.

I can solve this by padding the struct out to have a 16 byte alignment:

layout(set=0, binding=2, std140)
uniform TexSize {
    ivec2 dimensions;
    ivec2 padding;
};

But this seems rather inelegant. Is there any way to set the alignment of TexSize without just adding other members to pad it out?

Answer 1

No. uniforms require 16 byte (4 float) spacing. you need to add padding.

Address Space Layout Constraints of wgsl specification: https://www.w3.org/TR/WGSL/#address-space-layout-constraints

GPUs need to do allot of 3 or 4 Dimensional Matrix arithmetic on 16Byte floats. This is why GPUs are heavily optimised for this type of Calculations. And this is why The Spacing of uniforms is 4 X 16Byte Float.