[Starpu-devel] Bug report: STARPU_SPMD does not handle buffers without explicit backing memory

["Jan Polák" <456647@mail.muni.cz>]

Hello, I have encountered a strange error which I have managed to reproduce in
the following code: https://pastebin.com/Rqjwtkpp (in the latest StarPU,
1.3.2).

The gist of the problem is that any buffers without explicit backing memory
(home node -1, null backing memory) are not passed through to the codelet
functions when running on combined workers through STARPU_SPMD. (And maybe
STARPU_FORKJOIN as well, but I've been unable to make "peager" planner run the
task on multiple workers for that case.)

Note that the whole interface in `buffers[1]` is NULL, not just the pointer to
data. This also causes crashes inside StarPU when tracing with FxT.

I have been unable to find a workaround yet.

The reproduction code demonstrates this by creating two buffer handles, one
with explicit memory and one without. The codelet then prints the memory
contents of the buffers. `buffer[0]` points to uninitialized (for simplicity),
but valid memory. `buffer[1]` is always NULL, unless the parallelism type is
changed.

I have also had a version of the code which wrote into the buffers in an
OpenCL kernel before passing them to the parallel CPU kernel. The writing has
worked, but the issue was still present.

Thank you and have a nice day,
Jan Polák