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From: "algrant at acm dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/59448] Code generation doesn't respect C11 address-dependency Date: Thu, 12 Dec 2013 18:07:00 -0000 [thread overview] Message-ID: <bug-59448-4-cBwa1v8q53@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-59448-4@http.gcc.gnu.org/bugzilla/> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448 --- Comment #4 from algrant at acm dot org --- So using g++, #include <atomic> int f1(std::atomic<int> const *p, std::atomic<int> const *q) { int flag = p->load(std::memory_order_consume); return flag ? (q + flag - flag)->load(std::memory_order_relaxed) : 0; } demonstrates the same lack of ordering. You suggest that this might be a problem with the atomic built-ins - and yes, if this had been a load-acquire, it would be a problem with the built-in not introducing a barrier or using a load-acquire instruction. But for a load-consume on this architecture, no barrier is necessary to separate the load-consume from a load that is address-dependent on it. The programmer wrote a dependency but the compiler lost track of it. It's not necessary to demonstrate failure - there's an architectural race condition here. Even if it doesn't fail now there's no guarantee it will never fail on future more aggressively reordering cores.
next prev parent reply other threads:[~2013-12-12 18:07 UTC|newest] Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-12-10 12:07 [Bug target/59448] New: ARM code " algrant at acm dot org 2013-12-10 14:19 ` [Bug c/59448] Code " rearnsha at gcc dot gnu.org 2013-12-10 16:43 ` algrant at acm dot org 2013-12-10 17:47 ` joseph at codesourcery dot com 2013-12-12 18:07 ` algrant at acm dot org [this message] 2013-12-16 14:38 ` [Bug middle-end/59448] " joseph at codesourcery dot com 2014-01-20 8:52 ` algrant at acm dot org 2014-01-20 9:02 ` pinskia at gcc dot gnu.org 2014-01-20 10:13 ` algrant at acm dot org 2014-01-20 14:21 ` joseph at codesourcery dot com 2014-01-23 22:28 ` torvald at gcc dot gnu.org 2014-02-17 10:26 ` algrant at acm dot org 2014-02-17 21:03 ` torvald at gcc dot gnu.org 2014-02-17 22:22 ` algrant at acm dot org 2014-10-28 10:56 ` ramana at gcc dot gnu.org 2014-10-28 12:48 ` amacleod at redhat dot com 2014-10-28 13:43 ` joseph at codesourcery dot com 2014-10-28 17:37 ` t.p.northover at gmail dot com 2014-10-29 1:48 ` joseph at codesourcery dot com 2014-10-29 9:23 ` torvald at gcc dot gnu.org 2014-10-30 21:08 ` torvald at gcc dot gnu.org 2014-10-30 22:16 ` filter-gcc at preshing dot com 2014-11-24 12:13 ` filter-gcc at preshing dot com 2015-01-14 13:59 ` amacleod at redhat dot com 2022-01-15 1:47 ` pinskia at gcc dot gnu.org 2022-01-15 1:53 ` pinskia at gcc dot gnu.org
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