Add type punning rules to vk::BufferPointer

[greg-lunarg]

Fixes #43

[evaspitslines]

Is there currently already a PR that implements these functions? Or is it being worked on? 👀

[greg-lunarg]

Is there currently already a PR that implements these functions? Or is it being worked on?

There is a PR implementing something similar, but it was rejected because it did not follow the process being followed here.

An implementation of this proposal will likely be based on the previous PR.

[s-perron]

LGTM

[llvm-beanz]

I'm not sure this wording is sufficient.

Take this case:

Buffer<Int> I;

union Pain {
  vk::BufferPointer<SomeStruct> Ptr;
  uint64_t Int;
  double FP;
};

void Init(inout Pain P) {
  P.Int = I[0];
}

SomeStruct Load(Pain P) {
  return P.Ptr.Get();
}

void Fn() {
  Pain P;
  Init(P);
  SomeStruct S = Load(P);
}

The union P in Fn is initialized in Init to a uint64_t), then used as a vk::BufferPointer. What is the expected behavior here?

It can't be a compile error because Init, Load and Fn don't need to be defined in the same translation unit so we can't analyze that usage pattern. Is it undefined?

[greg-lunarg]

Yes, my intention is that this would be undefined behavior.

In general, type punning across members of a union is considered undefined behavior in C++. I don't see anything in the HLSL union proposal that changes this. So it seems that this should be true no matter the types of the members, including buffer pointers.

I will augment the text of this section to be more explicit about this.

[llvm-beanz]

I'm going to call this good enough. I don't think this is the right language or describes the behavior we want, but I think we can defer getting it correct until the Unions proposal actually defines how type punning works.