[Bug fortran/95654] nvptx offloading: FAIL: libgomp.fortran/pr66199-5.f90 -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions execution test

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95654

--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tom de Vries <vr...@gcc.gnu.org>:

https://gcc.gnu.org/g:ab3f4b27abe8abc947e84ef84bfc9a18797c5868

commit r11-3648-gab3f4b27abe8abc947e84ef84bfc9a18797c5868
Author: Tom de Vries <tdevr...@suse.de>
Date:   Tue Sep 22 16:38:07 2020 +0200

    [omp, ftracer] Don't duplicate blocks in SIMT region

    When running the libgomp testsuite on x86_64-linux with nvptx accelerator
on
    the test-case included in this patch, we run into:
    ...
    FAIL: libgomp.fortran/pr95654.f90 -O3 -fomit-frame-pointer -funroll-loops \
      -fpeel-loops -ftracer -finline-functions  execution test
    ...

    The test-case is a minimal version of this FAIL:
    ...
    FAIL: libgomp.fortran/pr66199-5.f90 -O3 -fomit-frame-pointer -funroll-loops
\
      -fpeel-loops -ftracer -finline-functions  execution test
    ...
    but that one has stopped failing at commit c2ebf4f10de "openmp: Add support
    for non-rect simd and improve collapsed simd support".

    The problem is that ftracer duplicates a block containing
GOMP_SIMT_VOTE_ANY.

    That is, before ftracer we have (dropping the GOMP_SIMT_ prefix):
    ...
    bb4(ENTER_ALLOC)
    *----------+
    |           \
    |            \
    |             v
    |             *
    v             bb8
    *<------------*
    bb5(VOTE_ANY)
    *-------------+
    |             |
    |             |
    |             |
    |             |
    |             v
    |             *
    v             bb7(XCHG_IDX)
    *<------------*
    bb6(EXIT)
    ...

    The XCHG_IDX internal-fn does inter-SIMT-lane communication, which for
nvptx
    maps onto shfl, an operator which has the requirement that the warp
executing
    the operator is convergent.  The warp diverges at bb4, and
    reconverges at bb5, and does not diverge by going to bb7, so the shfl is
    indeed executed by a convergent warp.

    After ftracer, we have:
    ...
    bb4(ENTER_ALLOC)
    *----------+
    |           \
    |            \
    |             \
    |              \
    v               v
    *               *
    bb5(VOTE_ANY)   bb8(VOTE_ANY)
    *               *
    |\             /|
    | \  +--------+ |
    |  \/           |
    |  /\           |
    | /  +----------v
    |/              *
    v               bb7(XCHG_IDX)
    *<--------------*
    bb6(EXIT)
    ...

    The warp diverges again at bb5, but does not reconverge again before bb6,
so
    the shfl is executed by a divergent warp, which causes the FAIL.

    Fix this by making ftracer ignore blocks containing ENTER_ALLOC, VOTE_ANY
and
    EXIT, effectively treating the SIMT region conservatively.

    An argument can be made that the test needs to be added in a more
    generic place, like gimple_can_duplicate_bb_p or some such, and that
ftracer
    then needs to use the generic test.  But that's a discussion with a much
    broader scope, so I'm leaving that for another patch.

    Bootstrapped and reg-tested on x86_64-linux.

    Build on x86_64-linux with nvptx accelerator, tested with libgomp.

    gcc/ChangeLog:

            PR fortran/95654
            * tracer.c (ignore_bb_p): Ignore GOMP_SIMT_ENTER_ALLOC,
            GOMP_SIMT_VOTE_ANY and GOMP_SIMT_EXIT.

    libgomp/ChangeLog:

    2020-10-05  Tom de Vries  <tdevr...@suse.de>

            PR fortran/95654
            * testsuite/libgomp.fortran/pr95654.f90: New test.