Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref

[Eric Botcazou <botcazou@adacore.com>]

> The test was only failing for me with -m32 (and not -m64), so I didn't
> notice until now. That probably means we should make the test fail more
> reliably if the padding isn't being cleared.

The tests fail randomly for me on SPARC64/Linux:

FAIL: 29_atomics/atomic/compare_exchange_padding.cc execution test
FAIL: 29_atomics/atomic_ref/compare_exchange_padding.cc execution test

/home/ebotcazou/src/libstdc++-v3/testsuite/29_atomics/atomic_ref/
compare_exchange_padding.cc:34: int main(): Assertion 'compare_struct(ts, es)' 
failed.
FAIL: 29_atomics/atomic_ref/compare_exchange_padding.cc execution test

  std::atomic<S> as{ s };
  auto ts = as.load();
  VERIFY( !compare_struct(s, ts) ); // padding cleared on construction
  as.exchange(s);
  auto es = as.load();
  VERIFY( compare_struct(ts, es) ); // padding cleared on exchange

How is it supposed to pass exactly?  AFAICS you have no control on the padding 
bits of ts or es and, indeed, at -O2 the loads are scalarized:

  __buf$c_81 = MEM[(struct S *)&__buf].c;
  __buf$s_59 = MEM[(struct S *)&__buf].s;
  __buf ={v} {CLOBBER(eol)};
  ts.c = __buf$c_81;
  ts.s = __buf$s_59;
[...]
  __buf$c_100 = MEM[(struct S *)&__buf].c;
  __buf$s_35 = MEM[(struct S *)&__buf].s;
  __buf ={v} {CLOBBER(eol)};
  es.c = __buf$c_100;
  es.s = __buf$s_35;
  _66 = MEM <unsigned int> [(char * {ref-all})&ts];
  _101 = MEM <unsigned int> [(char * {ref-all})&es];
  if (_66 != _101)
    goto <bb 5>; [0.04%]
  else
    goto <bb 6>; [99.96%]

so the result of the 4-byte comparison is random.

-- 
Eric Botcazou