[Bug target/104005] New: Regression on arm+sve with -O2 -fPIC

["gilles.gouaillardet at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104005

            Bug ID: 104005
           Summary: Regression on arm+sve with -O2 -fPIC
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gilles.gouaillardet at gmail dot com
  Target Milestone: ---

The GROMACS (2021.3) non regression test suite now hangs with the latest trunk
when shared libraries are used from -O2 when two or more thread-MPI are used. I
only tried 512 bit SVE vectors.

I ran a git bisect and it pointed to the following commit:

commit 526e1639aa76b0a8496b0dc3a3ff2c450229544e (refs/bisect/bad)
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Fri Nov 12 17:33:00 2021 +0000

    aarch64: Detect more consecutive MEMs


When investigating this issue, I found out that rebuilding the
src/gromacs/domdec/redistribute.cpp file **without** -fPIC is enough to avoid
the issue (a crash or a hang depending on the test cases).

FWIW, thread-MPI is a "fake" MPI implementation that used threads instead
processes. In this case, that means two threads. Then each thread enter its own
#pragma omp parallel section (and each thread-MPI thread has a unique OpenMP
thread)


The attached tarball includes preprocessed source (redistribute.cpp.i) and
assembly with -fPIC (redistribute.cpp.PIC.s) and without -fPIC
(redistribute.cpp.noPIC.s) for the commit mentioned above and the commit right
before.


Please let me know if this is enough for you to make sense of this bug,
otherwise I will provide instructions on how to build GROMACS from sources and
manually workaround the issue by "removing" the -fPIC flags for the
redistribute.cpp file.