Add type punning rules to vk::BufferPointer#55
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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. |
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| ### Buffer Pointers and Type Punning Through Unions | ||
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| While buffer pointer types are allowed in unions, type punning with buffer pointer types is disallowed. |
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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?
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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
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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.
This adds a resource chapter that documents typed and raw buffers including their methods and compatible operators along with short descriptions of their functionality. It also adds a description of binding annotations for all types. Stub sections are included for constant buffers and samplers, but no information is provided as yet. Fixes microsoft#55 Fixes microsoft#56
* Add type punning rules to vk::BufferPointer Fixes microsoft#43 * Clarify that type punning with buffer pointers leads to undefined behavior.
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Fixes #43