Buffer types + arrayLength() issues

[JMS55]

Bevy version

acbed60

Problem

StorageBuffer/UniformBuffer/etc will reuse its GPU buffer if it's big enough.

Say one frame you want a buffer of size N*2, and the next, N.

Because StorageBuffer/etc use as_entire_buffer_binding(), on the second frame, you'll end up with a buffer of size N*2, and not N.

If the buffer stores an unsized array, any shaders using arrayLength() in the shader will return N*2, but again we'd be expecting N here as that's how many items we wrote this frame.

Affected shaders

• mesh_preprocess.wgsl
• get_visibility_range_dither_level()
• Some meshlet stuff (I fixed this on a branch)

Solution

The buffer types should probably not use as_entire_buffer_binding(), and instead should bind only the section of the buffer actually written to in the last write_buffer() call.

[tychedelia]

I think the fix here is easy (use offset + len instead of as_entire_binding), but do we need to do this for 0.15 or is the ad hoc fix for meshlets sufficient.

[JMS55]

I'd rather we do it for 0.15

[alice-i-cecile]

I'm having trouble fully understanding the context here. What are the consequences here, how much code does this affect and why was this marked critical?

[JMS55]

GPU-driven rendering and visibility ranges will break if the amount of meshes decreases between frames. Probably also 3rd party crates using our buffer abstractions + arrayLength().

[alice-i-cecile]

Okay, that's definitely very bad. Thanks!

[djeedai]

I'm having a hard time understanding why as_entire_buffer_binding() would return N/2 if the buffer was reallocated. By definition as_entire_buffer_binding() will return the entire buffer. So if it returns N/2 that means your buffer is of size N/2, and no amount of manually specifying a buffer size will change that. In fact manually specifying N when the buffer is only of size N/2 would be a bug. The only way I see that would be an issue would be if the binding returned by as_entire_buffer_binding() was cached, but that seems very unlikely given it has a lifetime bound.