From: Jonathan Wakely <jwakely@redhat.com>
To: Iain Sandoe <iain@sandoe.co.uk>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org,
Thomas Rodgers <trodgers@redhat.com>
Subject: Re: [RFA] choosing __platform_wait_t on targets without lock-free 64 atomics
Date: Fri, 6 Jan 2023 00:22:39 +0000 [thread overview]
Message-ID: <20230106002239.102638-1-jwakely@redhat.com> (raw)
In-Reply-To: <CAMmuTO9F_RuHaP6cot5=b59uhH+-C8N7TdoZJBapSHsmvZqXdw@mail.gmail.com>
How about this?
I don't think we should worry about targets without atomic int, so don't
bother using types smaller than int.
-- >8 --
For non-futex targets the __platform_wait_t type is currently uint64_t,
but that requires a lock in libatomic for some 32-bit targets. We don't
really need a 64-bit type, so use unsigned long if that is lock-free,
and int otherwise. This should mean it's lock-free on a wider set of
targets.
libstdc++-v3/ChangeLog:
* include/bits/atomic_wait.h (__detail::__platform_wait_t):
Define as unsigned long if always lock-free, and unsigned int
otherwise.
---
libstdc++-v3/include/bits/atomic_wait.h | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index bd1ed56d157..46f39f10cbc 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -64,7 +64,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// and __platform_notify() if there is a more efficient primitive supported
// by the platform (e.g. __ulock_wait()/__ulock_wake()) which is better than
// a mutex/condvar based wait.
- using __platform_wait_t = uint64_t;
+# if ATOMIC_LONG_LOCK_FREE == 2
+ using __platform_wait_t = unsigned long;
+# else
+ using __platform_wait_t = unsigned int;
+# endif
inline constexpr size_t __platform_wait_alignment
= __alignof__(__platform_wait_t);
#endif
--
2.39.0
next prev parent reply other threads:[~2023-01-06 0:22 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-29 11:28 Iain Sandoe
2022-12-29 12:09 ` Jonathan Wakely
2022-12-29 15:30 ` Iain Sandoe
2022-12-29 15:44 ` Jonathan Wakely
2022-12-29 15:56 ` Iain Sandoe
2022-12-29 17:02 ` Jonathan Wakely
2022-12-30 10:51 ` Iain Sandoe
2023-01-02 0:53 ` Thomas Rodgers
2023-01-02 7:47 ` Iain Sandoe
2023-01-03 1:13 ` Thomas Rodgers
2023-01-06 0:22 ` Jonathan Wakely [this message]
2023-01-12 1:27 ` Thomas Rodgers
2023-01-12 11:01 ` Jonathan Wakely
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