[PATCH] D88345: [CUDA] Allow local `static const {__constant__, __device__}` variables.

[Artem Belevich via Phabricator via cfe-commits]

tra added a comment.

In D88345#2298688 <https://reviews.llvm.org/D88345#2298688>, @jlebar wrote:

> OK, backing up, what are the semantics of `static` on `__constant__`, `__device__`, and `__shared__`?
>
> - My understanding is that `__shared__` behaves the same whether or not it's static.  It's not equivalent to `namespace a { __shared__ int c = 4; }`, because that's illegal.

Yes. `__shared__` is an odd duck. It is implicitly static, so whether we explicitly specify `static` makes no difference.
We're not changing anything about how it's implemented.

> - Does `__constant__` behave the same whether or not it's static?  A static `__constant__` is equivalent to `namespace a { __constant__ int c = 4; }`, and a non-static `__constant__` is *also* equivalent to that?

No. `__constant__` is not allowed on non-static local variables as it can't be allocated on stack.

> - And `__device__` does not behave the same whether or not it's static?

Correct.

> In function scope `__device__ int x;` is a variable local to the thread.

Correct. `__device__` in a device function is effectively a no-op and can be placed on stack as a regular local variable.

> Whereas in global scope `__device__ int x;` is a global variable that lives in device memory (?).

Correct.

> In function scope `static __device__ int x;` is equivalent to `static int x;` which is equivalent to `int x;` in namespace scope?

Yes, assuming you mean a `__device__` function and `__device__ int x;` in the namespace scope.

> Should we mandate that you initialize `static __constant__` variables in function scope?
> That is, if you write `static __constant__ int x;` in a function, then x is always uninitialized (right)?  You should do `static __constant__ int x = 42;`?

No. Accoring to PTX spec:  `Variables in .const and .global state spaces are initialized to zero by default.`
Those are the address spaces `__constant__` and `__device__` variables map to.
https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#state-spaces


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