cgtblas: sycl backend does not handle nullptr case

[bd4]

Existing use of gpublas_set_stream in GENE passes nullptr to reset to default stream behavior. The stream refactor broke this for SYCL backend, where that has to be special cased to handle. For CUDA and HIP backends, the underlying type is a pointer and nullptr is the low level value that means resetting.

The easiest fix would be to add the special casing in SYCL specialization, but it's still a fragile design for the C interface, exposing this low level thing that just happens to work without special casing on some backends. We could have a gpublas_set_stream_default() as part of the interface. Also, GENE is not currently benefiting from using a separate stream for the solve, so we could remove these while we decide on a design.

[germasch]

So I think there's been some subtlety in the type used for sycl, anyway, in that a reference type (queue&) has been used, if I even understood things correctly. It seems to me that sycl could be made to behave more cuda/hip like by having some

struct sycl_stream_t
{
   sycl::queue *q;
}

And that could hopefully be used to eliminate those subtle differences at the interface level?

[bd4]

The queue is the gateway to all operations in SYCL, so it can't be null - something needs to intercept the attempt to set a null queue and replace it with the default for that device, or the interface needs to change so setting a null "stream" is not alowed, and resetting to the default is done another way.

[germasch]

Right, the queue can't be null, but a pointer to it can be. So if that's what we're passing from Fortran, we can still use C_NULL_PTR. Currently, we directly use queue&, which never can be officially NULL (though in practice if one passes is from Fortran, with Fortran thinking it's a pointer, it will be null. On the C side, one basically has to go, somewhere,

if (queue_pointer) return *queue; else return default queue;