public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug libgomp/97291] New: [SIMT] Move SIMT_XCHG_* out of non-uniform execution region
@ 2020-10-05 11:51 vries at gcc dot gnu.org
  2020-10-05 14:19 ` [Bug libgomp/97291] " amonakov at gcc dot gnu.org
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: vries at gcc dot gnu.org @ 2020-10-05 11:51 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97291

            Bug ID: 97291
           Summary: [SIMT] Move SIMT_XCHG_* out of non-uniform execution
                    region
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

We have:
...
/* Allocate per-lane storage and begin non-uniform execution region.  */

static void
expand_GOMP_SIMT_ENTER_ALLOC (internal_fn, gcall *stmt)
...
and:
...
/* Deallocate per-lane storage and leave non-uniform execution region.  */

static void
expand_GOMP_SIMT_EXIT (internal_fn, gcall *stmt)
...

So, if the SIMT_ENTER_ALLOC and the SIMT_EXIT mark the start and end of a
region of non-uniform execution, it's strange that such a region can contain
SIMT_XCHG_*, which on nvptx requires uniform execution.

Moving SIMT_VOTE_ANY/SIMT_LAST_LANE/SIMT_XCHG_* as a whole after SIMT_EXIT is
not possible given that VOTE_ANY may have data dependencies to storage that is
deallocated by SIMT_EXIT (as Alexander mentioned here:
https://gcc.gnu.org/pipermail/gcc-patches/2020-October/555475.html).

A possible solution would be to split the SIMT_EXIT into separate bits for
exiting non-uniform execution and deallocation, and have:
- SIMT_ENTER_ALLOC
- SIMT_EXIT_UNI
- SIMT_VOTE_ANY/SIMT_LAST_LANE/SIMT_XCHG_*
- SIMT_EXIT_DEALLOC

Also I've wondered if we could do:
- SIMT_ENTER_ALLOC
- SIMT_VOTE_ANY
- SIMT_EXIT
- SIMT_LAST_LANE/SIMT_XCHG_*
but perhaps there are again data dependency problems.

^ permalink raw reply	[flat|nested] 5+ messages in thread
end of thread, other threads:[~2020-10-16 10:55 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-10-05 11:51 [Bug libgomp/97291] New: [SIMT] Move SIMT_XCHG_* out of non-uniform execution region vries at gcc dot gnu.org
2020-10-05 14:19 ` [Bug libgomp/97291] " amonakov at gcc dot gnu.org
2020-10-16 10:41 ` cvs-commit at gcc dot gnu.org
2020-10-16 10:47 ` cvs-commit at gcc dot gnu.org
2020-10-16 10:55 ` sripar01 at gcc dot gnu.org

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).