PushConstant not being OpTypeStruct is invalid on Vulkan hosts

[hrydgard]

Quoth Baldur (hope he doesn't mind I post it here):

BTW I also noticed some invalid SPIR-V in your shader which may also affect this, you have a PushConstant storage class variable defined that is not an OpTypeStruct. All uniforms and push constant variables must be struct pointers with explicitly laid out structs.

[XAMPPRocky]

Thank you for your issue! While I'm not doubting that is the case, I cannot find any reference in the SPIR-V spec that lays out that requirement. Perhaps they could expand further? @baldurk

The closest thing I could find is the following:

Composite objects in the UniformConstant, Uniform, and PushConstant Storage Classes must be explicitly laid out. The following apply to all the aggregate and matrix types describing such an object, recursively through their nested types

Where composite is defined as "An aggregate, a matrix, or a vector.", which would imply to me that this is not a requirement for those storage classes containing scalars.

[msiglreith]

@XAMPPRocky That's probably referred to using Vulkan as host which comes with its own set of requirements and constraints:
https://vulkan.lunarg.com/doc/view/1.2.131.1/mac/chunked_spec/chap14.html#interfaces-resources-pushconst

[hrydgard]

That's a good catch @msiglreith !

Given how we want to use rust-gpu, having an option to, or at least initially, always following Vulkan's extra rules seems like the way to go.

[khyperia]

I believe this was fixed by two things:

1. ABI rewriting shenanigans of rustc taking a push constant struct and replacing it with a scalar - fixed by abi: readjust FnAbis to remove unsupported PassModes, via query hooks. #766
2. Push constants being used directly instead of structs (i.e. #[spirv(push_constant)] asdf: &f32) - fixed by wrapping everything in a struct Deprecate #[spirv(block)] and auto-wrap in "interface blocks" instead. #576