Explicit interface of entry points

[kvark]

SPIR-V lists all the inputs and outputs for each entry point. The question is whether it's a good idea to represent this somehow in the IR. See gpuweb/gpuweb#592 for WGSL discussion.

Pros for specifying:

• match SPIR-V
• easier to translate from IR to anything, since we know the interface

Cons for specifying:

• more verbose and redundant
• incomplete - stuff like uniform or storage buffers are also technically the interface, so we still need to go and scan the uses of the globals

Another thought I had is that it doesn't make sense to specify that interface per entry point. It seems like it would make more sense to have for the function.

[kvark]

Saying that, it sounds like having the interface in the IR would be useful (i.e. to avoid deriving that over and over each time a module is used). We just need to tweak it a bit. First, by moving it into the Function. Second, instead of listing inputs and outputs, we'd have a Map<Token<GlobalVariable>, GlobalUse>, where:

enum GlobalUse {
  Input,
  Output,
  Constant,
  Storage,
}

[Napokue]

I think this is a good idea. Because now, we have to iterate the Globals for far more than necessary. If I understand your proposal correctly, that means that we in theory have to iterate once to know what Globals are used for what purpose. I think that is a good idea.