[Bug c/64923] New: [s390] Generated code uses struct padding instead of just the bitfield

[siddhesh at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64923

            Bug ID: 64923
           Summary: [s390] Generated code uses struct padding instead of
                    just the bitfield
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: siddhesh at redhat dot com

Reproducer:

struct obstack
{
  unsigned use_extra_arg:1;
  unsigned extra_bitfield:1;
};

void
myfree (void *f)
{
}

void (*freefun) (void *) = myfree;

void
obstack_free (struct obstack *h, void *obj)
{
  if (h->use_extra_arg)
    freefun ((void *) 0x42);
  else
    freefun ((void *) 0x0);
}

int
main (int argc, char **argv)
{
  struct obstack ob;

  ob.use_extra_arg = 0;

  obstack_free (&ob, (void *) 0);
  return 0;
}

Build with:

gcc -g -O2 -o obs{,.c}

$ valgrind ./obs
==20292== Memcheck, a memory error detector
==20292== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==20292== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==20292== Command: ./obs
==20292== 
==20292== Conditional jump or move depends on uninitialised value(s)
==20292==    at 0x800005B4: obstack_free (obs.min.c:17)
==20292==    by 0x80000455: main (obs.min.c:30)
==20292== 
==20292== 
==20292== HEAP SUMMARY:
==20292==     in use at exit: 0 bytes in 0 blocks
==20292==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==20292== 
==20292== All heap blocks were freed -- no leaks are possible
==20292== 
==20292== For counts of detected and suppressed errors, rerun with: -v
==20292== Use --track-origins=yes to see where uninitialised values come from
==20292== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

This works correctly on x86 since the generated code explicitly selects the
relevant bit in the bitfield to test. s390x on the other hand doesn't when
built with -O2.  It looks like the instruction combiner pass removes the AND
operation that selects the bit and instead tests the entire word.