From: Richard Henderson <rth@twiddle.net>
To: Torvald Riegel <triegel@redhat.com>, Florian Weimer <fweimer@redhat.com>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: __HAVE_64B_ATOMICS and alignment
Date: Mon, 17 Apr 2017 07:37:00 -0000 [thread overview]
Message-ID: <54996b83-b556-2a31-8a7a-a0700b83cf26@twiddle.net> (raw)
In-Reply-To: <1492103812.5374.218.camel@redhat.com>
On 04/13/2017 10:16 AM, Torvald Riegel wrote:
> On Thu, 2017-04-13 at 07:10 +0200, Florian Weimer wrote:
>> On 11/28/2016 12:51 PM, Torvald Riegel wrote:
>>>> This means that for an LP32 architecture such as i686 which could
>>>> conceivable provide 64-bit atomics, we might try to perform an atomic
>>>> operation on a potentially misaligned uint64_t value.
>>> i686 has no 64b atomic loads or stores.
>>
>> double loads and stores via the FPU are atomic, and they preserve all
>> bit patterns.
>
> I believe that the precise status on this is that we believe that they
> are atomic, but it's not explicitly guaranteed.
No, it is guaranteed. See Vol 3, Section 8.1.1:
# The Pentium processor (and newer processors since) guarantees that
# the following additional memory operations will always be carried
# out atomically:
# • Reading or writing a quadword aligned on a 64-bit boundary
r~
next prev parent reply other threads:[~2017-04-17 7:37 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-26 18:49 Florian Weimer
2016-11-26 20:52 ` Andreas Schwab
2016-11-27 11:33 ` Florian Weimer
2016-11-27 12:36 ` Andreas Schwab
2016-11-28 11:51 ` Torvald Riegel
2017-04-13 5:11 ` Florian Weimer
2017-04-13 17:16 ` Torvald Riegel
2017-04-17 7:37 ` Richard Henderson [this message]
2017-04-18 22:13 ` Joseph Myers
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