[Bug c/80640] New: Missing memory side effect

[nicolas@morey-chaisemartin.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80640

            Bug ID: 80640
           Summary: Missing memory side effect
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nicolas@morey-chaisemartin.com
  Target Milestone: ---

I found what seems to be a GCC issue while building/running the OpenMPI 2.1.0
testsuite using GCC 6.3.1 on i586 (Suse Tumbleweed)

I attached the tarball with the preprocessed source file and the minimum
libraries.
It compiles with:
--------
 gcc  -o opal_fifo opal_fifo.c  libopen-pal.so.20 -ldl libsupport.a -lrt -lm
-lutil -pthread -Wl,-rpath -Wl,$(pwd) 
--------

When compiling witrh O0, or O1 the test works. It stalls with O2 and O3.

From what I could find, the program get stuck here:
--------
            while (&fifo->opal_fifo_ghost == item->opal_list_next) {
                opal_atomic_rmb ();
            }
--------

opal_atomic_rmb is defined like this
--------
static inline void opal_atomic_rmb(void)
{
    __atomic_thread_fence (2);
}
--------

The assembly for this loop looks like this:
--------
=> 0x080495a5 <+325>:   cmp    %edi,%eax
   0x080495a7 <+327>:   je     0x80495a5 <thread_test+325>
--------

I'm a little rusty on x86 assembly but for me it means GCC cached the value to
compare and never reloads them from memory.

I would expect the __atomic_thread_fence atomic to have some kind of memory
side effect that means these values should be reloaded.

Adding a simple
--------
    __asm__ __volatile__("": : :"memory");
--------
to opal_atomic_rmb seems to fix the issue.