Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
> This should be fixed now. I rewrote the test to check the padding byte > directly, instead of inspecting a copy of it which might not preserve > the padding bits. Great, thanks! -- Eric Botcazou
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
On Mon, 31 Oct 2022 at 15:34, Eric Botcazou wrote:
>
> > 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 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 [(char * {ref-all})];
> _101 = MEM [(char * {ref-all})];
> if (_66 != _101)
> goto ; [0.04%]
> else
> goto ; [99.96%]
>
> so the result of the 4-byte comparison is random.
This should be fixed now. I rewrote the test to check the padding byte
directly, instead of inspecting a copy of it which might not preserve
the padding bits.
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
> Do those loads still get scalarized at -O0? Presumably not at the GIMPLE level, but possibly at the RTL level. -- Eric Botcazou
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
On Mon, 31 Oct 2022 at 17:05, Jonathan Wakely wrote: > > On Mon, 31 Oct 2022 at 17:03, Eric Botcazou wrote: > > > > > I suppose we could use memcmp on the as variable itself, to inspect > > > the actual stored padding rather than the returned copy of it. > > > > Yes, that's probably the only safe stance when optimization is enabled. > > > Strictly speaking, it's not safe, because it's undefined to use memcmp > on an object of a non-trivial type. But it should work. Do those loads still get scalarized at -O0?
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
On Mon, 31 Oct 2022 at 17:03, Eric Botcazou wrote: > > > I suppose we could use memcmp on the as variable itself, to inspect > > the actual stored padding rather than the returned copy of it. > > Yes, that's probably the only safe stance when optimization is enabled. Strictly speaking, it's not safe, because it's undefined to use memcmp on an object of a non-trivial type. But it should work.
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
> I suppose we could use memcmp on the as variable itself, to inspect > the actual stored padding rather than the returned copy of it. Yes, that's probably the only safe stance when optimization is enabled. -- Eric Botcazou
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
On Mon, 31 Oct 2022 at 15:34, Eric Botcazou wrote:
>
> > 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 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 [(char * {ref-all})];
> _101 = MEM [(char * {ref-all})];
> if (_66 != _101)
> goto ; [0.04%]
> else
> goto ; [99.96%]
>
> so the result of the 4-byte comparison is random.
I suppose we could use memcmp on the as variable itself, to inspect
the actual stored padding rather than the returned copy of it.
Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
> 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 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 [(char * {ref-all})];
_101 = MEM [(char * {ref-all})];
if (_66 != _101)
goto ; [0.04%]
else
goto ; [99.96%]
so the result of the 4-byte comparison is random.
--
Eric Botcazou
[committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
Tested x86_64-linux, pushed to trunk.
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.
-- >8 --
This test was written assuming that std::atomic_ref clears its target's
padding on construction, but that could introduce data races. Change the
test to store a value after construction and check that its padding is
cleared by the store.
libstdc++-v3/ChangeLog:
* testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc:
Store value with non-zero padding bits after construction.
---
.../29_atomics/atomic_ref/compare_exchange_padding.cc | 11 ++-
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git
a/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
index 1b1a12a..e9f8a4bdf2a 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
@@ -20,14 +20,15 @@ int
main ()
{
S s;
- fill_struct(s);
- s.c = 'a';
- s.s = 42;
-
S ss{ s };
+ fill_struct(ss);
+ ss.c = 'a';
+ ss.s = 42;
+
std::atomic_ref as{ s };
+ as.store(ss);
auto ts = as.load();
- VERIFY( !compare_struct(ss, ts) ); // padding cleared on construction
+ VERIFY( !compare_struct(ss, ts) ); // padding cleared on store
as.exchange(ss);
auto es = as.load();
VERIFY( compare_struct(ts, es) ); // padding cleared on exchange
--
2.37.3
