From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <676c3c4f0512090859n72ec5bd1rfa7742b43faf526b@mail.gmail.com>
Date: Fri,  9 Dec 2005 11:59:01 -0500
From: Richard Bilson <rcbilson@gmail.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] const
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On 12/9/05, David Leimbach <leimy2k@gmail.com> wrote:
>
> Volatile is useful for telling compilers not to stick  data in registers,
> which is helpful for multi-threaded situations with shared memory ...
> sometimes :)
>
> If I were to write a semaphore or another synchronization method using
> shared memory on unix I'd probably type volatile and then pray :).
>
> It often feels more of a faith issue than a knowledge issue though :)
>

It is a matter of faith, and usually misplaced at that, since volatile isn'=
t
usually defined to place any constraints on the order in which operations
become visible to other processors. Certainly not in the C, C++, or POSIX
standards, although some platform ABIs do specify this to some degree.

Both POSIX and C++ are trying to deal with this, but it's an uphill battle
since processors differ widely on the sorts of constraints that they can
support efficiently.

In the meantime, people sprinkle volatile around and pray. All the world's
an x86, right?

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On 12/9/05, <b class=3D"gmail_sendername">David Leimbach</b> &lt;<a href=3D=
"mailto:leimy2k@gmail.com">leimy2k@gmail.com</a>&gt; wrote:<div><span class=
=3D"gmail_quote"></span><blockquote class=3D"gmail_quote" style=3D"border-l=
eft: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left:=
 1ex;">
<div><div>Volatile is useful for telling
compilers not to stick&nbsp; data in registers, which is helpful for
multi-threaded situations with shared memory ... sometimes :)<br><br>If
I were to write a semaphore or another synchronization method using
shared memory on unix I'd probably type volatile and then pray :).
<br><br>It often feels more of a faith issue than a knowledge issue though =
:)</div></div></blockquote><div><br>
</div></div>It is a matter of faith, and usually misplaced at that,
since volatile isn't usually defined to place any constraints on the
order in which operations become visible to other processors. Certainly
not in the C, C++, or POSIX standards, although some platform ABIs do
specify this to some degree.<br>
<br>
Both POSIX and C++ are trying to deal with this, but it's an uphill
battle since processors differ widely on the sorts of constraints that
they can support efficiently.<br>
<br>
In the meantime, people sprinkle volatile around and pray. All the world's =
an x86, right?<br>

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