From: Eric Botcazou <botcazou@adacore.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: [committed] libstdc++: Fix compare_exchange_padding.cc test for std::atomic_ref
Date: Mon, 31 Oct 2022 16:34:27 +0100 [thread overview]
Message-ID: <5772422.MhkbZ0Pkbq@fomalhaut> (raw)
In-Reply-To: <20220916202127.579816-1-jwakely@redhat.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
next prev parent reply other threads:[~2022-10-31 15:34 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-16 20:21 Jonathan Wakely
2022-10-31 15:34 ` Eric Botcazou [this message]
2022-10-31 15:37 ` Jonathan Wakely
2022-10-31 17:03 ` Eric Botcazou
2022-10-31 17:05 ` Jonathan Wakely
2022-11-01 13:53 ` Jonathan Wakely
2022-11-01 15:26 ` Eric Botcazou
2023-09-01 15:21 ` Jonathan Wakely
2023-09-01 18:14 ` Eric Botcazou
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5772422.MhkbZ0Pkbq@fomalhaut \
--to=botcazou@adacore.com \
--cc=gcc-patches@gcc.gnu.org \
--cc=jwakely@redhat.com \
--cc=libstdc++@gcc.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).