> 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
