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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/95654] nvptx offloading: FAIL:
 libgomp.fortran/pr66199-5.f90   -O3 -fomit-frame-pointer -funroll-loops
 -fpeel-loops -ftracer -finline-functions  execution test
Date: Thu, 17 Sep 2020 10:20:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95654

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #11 from Tom de Vries <vries at gcc dot gnu.org> ---
So, at this point we know that duplicating the BB containing VOTE_ANY causes
problems in executing.  But AFAIU, we do not know why.

Is VOTE_ANY not supposed to be duplicated by design? If so, is there any
documentation of that design, that explains that?

At the nvptx level, VOTE_ANY translates to vote.ballot.b32, which does
cross-lane communication, but has defined behaviour in divergent mode AFAIC=
T.
>>From that perspective at least, there's no problem with duplicating VOTE_AN=
Y.

My guess at this point, is that duplicating the block with VOTE_ANY has the
effect that the JIT compiler doesn't recognize control flow divergence befo=
re
XCHG_IDX, and fails to insert the proper barrier.

And XCHG_IDX translates to shfl.idx.b32, which has undefined behaviour in
divergent mode.=