Do push constants have alignment requirements?

[junglie85]

I am using a push constant to pass the model transform to my shader. This is a glam::Mat4 as an array, which is uploaded as a [f32; 16] and has a size of 64. This works absolutely fine.

However, when I try uploading a SpriteInstance struct that is bytemuck::Pod and bytemuck::Zeroable, the screen is blank. This is only a size of 36 but AFAIK it meets the requirements for push constant data size (multiple of 4).

#[repr(C)]
#[derive(Debug, Clone, Copy)]
struct SpriteInstance {
    translation: [f32; 2],
    rotation: f32,
    scale: [f32; 2],
    tint: [f32; 4],
}

unsafe impl Pod for SpriteInstance {}
unsafe impl Zeroable for SpriteInstance {}

...

let sprite_push_constant_range = PushConstantRange {
        stages: ShaderStages::VERTEX,
        range: 0..size_of::<SpriteInstance>() as u32,
    };

...

 let sprite_pipeline_layout = device.create_pipeline_layout(&PipelineLayoutDescriptor {
        bind_group_layouts: &[
            &view_transforms_bind_group_layout,
            &sprite_bind_group_layout,
        ],
        push_constant_ranges: &[sprite_push_constant_range],
        label: Some("pipeline_layout::sprite"),
    });

...

rpass.set_push_constants(ShaderStages::VERTEX, 0, cast_slice(&[instance]));

And the shader:

struct SpriteInstance {
    translation: vec2<f32>,
    rotation: f32,
    scale: vec2<f32>,
    tint: vec4<f32>,
};

var<push_constant> instance: SpriteInstance;

FWIW, I also tried uploading each item separately, e.g.:

rpass.set_push_constants(ShaderStages::VERTEX, 0, cast_slice(&[instance.translation])); // offset 0, size 8
rpass.set_push_constants(ShaderStages::VERTEX, size_of::<[f32; 2]>() as u32, cast_slice(&[instance.rotation])); // offset 8, size 4
rpass.set_push_constants(ShaderStages::VERTEX, size_of::<[f32; 3]>() as u32, cast_slice(&[instance.scale])); // offset 12, size 8
rpass.set_push_constants(ShaderStages::VERTEX, size_of::<[f32; 5]>() as u32, cast_slice(&[instance.tint])); // offset 20, size 16

This neither worked with the push constant expecting the full struct nor with individual push constants.

I'm calculating the model matrix in the shader, but that works just fine (I use it and the SpriteInstance struct as instance data in another pipeline just fine). So I'm assuming it's to do with the push constants.

What's going on with the push constant that's preventing this from working?

-- Edit --

Having played a bit more and hardcoding some values in the shader, I've narrowed the issue down to the scale. Although the data size meets the requirements, so I ned to take alignment into account somehow?

[cwfitzgerald]

Push constants still follow std430 rules (i.e. wgsl block's rules for storage buffers).

• translation is at offset 0, size 8, alignment 8
• rotation is at offset 8, size 8, alignment 4
• scale is at offset 16, size 8, alignment 8
• tint is at offset 32, size 16, alignment 16

[junglie85]

Makes sense, thanks for confirming.