Push Constants

[MDeiml]

What problem does this solve or what need does it fill?

It would be nice to support using push constants. Even though the initial wgsl spec will not have push constants (gpuweb/gpuweb#612) they are supported in naga (https://github.com/gfx-rs/naga/blob/master/tests/in/push-constants.wgsl) as well as wgpu.

But it seems that there was a decision not to include them. Specifically you cannot create pipelines with layouts that contain push constants. This is because the concept of a RenderPipelineLayout in wgpu was not translated to the bevy API and instead became just &[BindGroupLayout]. To create a RenderPipelineLayout in wgpu you specify bind group layouts and push constant ranges, so this seems like a weird design to me.

I'm talking about this code, which always sets the push constant ranges to be empty:

bevy/crates/bevy_render/src/render_resource/pipeline_cache.rs

Lines 236 to 239 in fed93a0

	render_device.create_pipeline_layout(&PipelineLayoutDescriptor {
	bind_group_layouts: &bind_group_layouts,
	..default()
	})

What solution would you like?

Change RenderPipelineDescriptor or it's layout field so it's possible to specify push constant ranges.

What alternative(s) have you considered?

Create a pipeline without using pipeline cache or don't use push constants.

Additional Context

In order to use push constants in shaders we would also need #4824

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I'd be happy to implement this, but it would require minor breaking changes to RenderPipelineDescriptor and it seems to me that maybe there already was a decision to not support them, so I wanted to ask first.

[alice-i-cecile]

Previously discussed in #1733 and #1369.

For internal adoption, my only major concern is "web compatibility". I don't want to maintain two render paths yet (one for web and one for "native"). Imo we should have a real "motivating" use case before merging. If it can't be internal (for now), then I think it needs to be "allowing users to adopt push constants". And if thats true, we should probably have an example.

The rework directly exposes the ability to enable push constants via gpu (because it uses wgpu directly), so I do think we can close this out.

• @cart in Adding push constants support #1369.

I don't have the expertise to evaluate the matter further, but I figured I can surface the most relevant bits of the conversation.

[MDeiml]

Thanks for the context. Assuming the mentioned rework is already done, this is still relevant. Push constants are still not exposed in RenderPipelineDescriptor.

Even if push constants will not be used with the "built-in" shaders, it would still make sense to make them available to people who want to write their own shaders / render pipelines.

EDIT: I was wrong, TrackedRenderPass already supports push constants. Does that mean they are supposed to be supported?

[cart]

I would like to support push constants. Things have changed a lot since #1733 and #1369. I think the only missing piece at this point is updating the RenderPipelineDescriptor to include the push constant layout and adapting LayoutCache to accept push constant layouts (and take them into account for layout identity ... they must be a part of the key used to identify the layout).

[james7132]

@cart would you be opposed to having platform specific optimizations using push constants? The push constant size limitations are basically just big enough to fit two Affine3s and a few extra bits, which coincidentally is exactly the right size for a MeshUniform. It'd preclude users from using them for mesh rendering, but it'd likely be a sizable speed up opportunity on platforms that support it.

[cart]

If this can be done without adding extra complexity to user-facing shader apis, I think I'm open to it. Hard to say before looking at the impl + perf tradeoffs first, but its an area I think we should explore.

[MDeiml]

I would like to support push constants. Things have changed a lot since #1733 and #1369. I think the only missing piece at this point is updating the RenderPipelineDescriptor to include the push constant layout and adapting LayoutCache to accept push constant layouts (and take them into account for layout identity ... they must be a part of the key used to identify the layout).

That and #4824. I got push constants working in my fork. It's not a lot of changes, but RenderPipelineDescriptor does have to change I think.

[alice-i-cecile]

@MDeiml if you're comfortable, a link to that branch or a PR would be great. It doesn't need to be perfect to start, but having something tangible will help focus the discussion.