From: Iain Sandoe <iain@sandoe.co.uk>
To: libstdc++ <libstdc++@gcc.gnu.org>
Subject: [RFA] choosing __platform_wait_t on targets without lock-free 64 atomics
Date: Thu, 29 Dec 2022 11:28:45 +0000 [thread overview]
Message-ID: <6640F26B-F267-40E0-9223-2F0F45462176@sandoe.co.uk> (raw)
Hi,
The recent addition of the tz handling has pulled in a dependency on </bits/atomic_wait.h>
This currently specifies __platform_wait_t as a 64bit quatity on platforms without _GLIBCXX_HAVE_LINUX_FUTEX.
PowerPC does not have a 64b atomic without library support - so that this causes a bootstrap
fail on powerpc-darwin (and I guess any other 32b powerpc non-futex target).
Rather than contrive to build and add libatomic (which is not at present available at the point
that libstdc++ is built), I wonder if there is any specific reason that __platform_wait_t needs
to be 64 bits on these platforms? (Especially since the futex case uses an int.)
Advice on the right way to fix this welcome — as a work-around to allow bootstrap to complete
I applied the patch below - but that seems unlikely to be the right thing generically .
thanks
Iain
----
diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index bd1ed56..2f67180 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.
+#if __LP64__
using __platform_wait_t = uint64_t;
+#else
+ using __platform_wait_t = uint32_t;
+#endif
inline constexpr size_t __platform_wait_alignment
= __alignof__(__platform_wait_t);
next reply other threads:[~2022-12-29 11:28 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-29 11:28 Iain Sandoe [this message]
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
2023-01-12 1:27 ` Thomas Rodgers
2023-01-12 11:01 ` Jonathan Wakely
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