[SYCL][ESIMD] Implement compile-time getNextPowerOf2 w/o recursion.

[kbobrovs]

Device code is restricted not to allow recursion.

Signed-off-by: Konstantin S Bobrovsky [email protected]

[kbobrovs]

Some more info on the problem is here #2104. Simply allowing recursion for constexpr functions does not work as they can be called from non-constexpr context.

[keryell]

I do not understand want you are doing here.
You had a recursion before, you have a recursion after...
Are you looking at something like https://github.com/triSYCL/triSYCL/blob/master/include/triSYCL/pipe.hpp#L55 ?

[kbobrovs]

You had a recursion before, you have a recursion after...

I had a real recursion before, where compiler would leave the recursive function in the IR if is called from non-constexpr context.

In the version "after" the expansion is guaranteed to happen at compile-time, and IR is guaranteed not to have recursive function.

[againull]

If I understand it correctly, log2 also have real recursion and should be rewritten too?
Will it be done separately?

[kbobrovs]

If I understand it correctly, log2 also have real recursion and should be rewritten too?
Will it be done separately?

good catch. It is not used now, hence does not cause compilation errors. Should be removed for now - will do.

[kbobrovs]

actually it is used in esimd_reduce, so will re-implement

[kbobrovs]

Had to force-push to provide good commit message. Hope this is OK, since the patch is small.

[kbobrovs]

@keryell, are you OK with the patch?

[keryell]

This is a good trade-off while waiting for simplifying all of this with C++20 some day.