Re: threads, signals, and atomicity of basic types

From	Nobody <nobody@nowhere.com>
Subject	Re: threads, signals, and atomicity of basic types
Date	2011-06-07 22:48 +0100
Message-Id	<pan.2011.06.07.21.48.22.453000@nowhere.com>
Newsgroups	comp.unix.programmer
References	(4 earlier) <a2202888-0292-42b4-a374-1f0223bb29d0@s16g2000prf.googlegroups.com> <87oc2cs2x6.fsf@sapphire.mobileactivedefense.com> <e2e8d17b-03a8-4f86-a3a7-ff78b75430e2@35g2000prp.googlegroups.com> <pan.2011.06.07.05.50.49.375000@nowhere.com> <6ec30746-62a1-4ae5-96c1-9fe0e0043364@h12g2000pro.googlegroups.com>
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On Tue, 07 Jun 2011 06:04:20 -0700, David Schwartz wrote:

>> The only cases where the standard really says anything specific about
>> "volatile" are with regard to setjmp/longjmp and signals. The rest boils
>> down to the compiler doing the "sane" thing. In much the same way that the
>> standard doesn't actually specify what it means to "read" or "write" a
>> "file"; it just assumes that these concepts are meaningful.
> 
> Unfortunately, the compiler doing the sane thing doesn't help you one
> bit. Any optimization the compiler can make can, at least in theory,
> be made by other components of the system. Because the standard
> doesn't specify where accesses are observed, it's not clear what
> that's supposed to mean. For example, should 'volatile' bypass cache
> and force access to/from memory directly? It doesn't on any platform I
> know of. But arguably cache optimizes away 'volatile' accesses,
> modifying the observable behavior of the system. The compiler could
> bypass the cache for these accesses.

"Sane" is a judgment call, not something which can formally be specified.

In practice, it usually means that the compiler will emit load/store
instructions which correspond directly to the source code (i.e. if you
read an lvalue within a loop, the compiler won't move the load instruction
outside the loop).

It usually doesn't involve emitting specialised instructions (e.g. memory
barriers), accessing specialised registers, manipulating hardware (e.g.
the MMU), etc.

"volatile" was added to ANSI C to deal with the specific case that many
generally-useful optimisations can be problematic in a very few, very
specialised cases (e.g. accessing memory-mapped hardware).

I've always interpreted it as "don't optimise this", something which
should only be necessary if optimisations are enabled in the first place.
As such, I wouldn't expect its presence to cause the compiler to emit
anything which it would never emit in its absence.

IOW, it stops the compiler performing its own optimisations. It doesn't
cause the compiler to add workarounds for the hardware's optimisations.

Thread

threads, signals, and atomicity of basic types Urs Thuermann <urs@isnogud.escape.de> - 2011-05-31 21:07 +0200
  Re: threads, signals, and atomicity of basic types Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-05-31 22:44 +0200
    Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-01 11:58 +0100
      Re: threads, signals, and atomicity of basic types Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-06-01 14:15 +0200
        Re: threads, signals, and atomicity of basic types Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-06-02 13:49 +0200
          Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 13:40 +0100
  Re: threads, signals, and atomicity of basic types Richard Kettlewell <rjk@greenend.org.uk> - 2011-05-31 22:50 +0100
  Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-31 21:32 -0400
    Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-01 12:35 +0100
      Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-01 23:01 -0400
        Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 11:22 +0100
          Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 07:52 -0400
            Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 13:38 +0100
              Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 21:27 -0400
  Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-01 12:55 -0700
    Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-01 22:54 +0100
      Re: threads, signals, and atomicity of basic types Alex Fraser <me@privacy.net> - 2011-06-02 21:00 +0100
        Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-04 21:53 -0700
          Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-05 16:39 +0100
            Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-06 18:48 -0700
              Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-07 06:50 +0100
                Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-07 06:04 -0700
                Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-07 22:48 +0100
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 23:18 +0100
                Re: threads, signals, and atomicity of basic types gordonb.lhpq6@burditt.org (Gordon Burditt) - 2011-06-07 18:28 -0500
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-08 11:09 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-08 14:32 -0700
                Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-09 18:10 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-10 15:26 +0100
                Re: threads, signals, and atomicity of basic types James Waldby <not@valid.invalid> - 2011-06-10 16:49 +0000
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-10 19:07 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-10 13:13 -0700
              Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 12:55 +0100
                Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-07 06:02 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 16:05 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-07 13:37 -0700
                Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-14 21:43 +0000
                Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-15 03:07 +0100
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 10:22 +0100
                Re: threads, signals, and atomicity of basic types Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2011-06-15 09:40 +0000
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 13:20 +0100
                Re: threads, signals, and atomicity of basic types Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2011-06-15 13:19 +0000
                Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-15 17:41 +0100
                Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-15 20:20 -0700
                Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-15 17:31 +0100
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 18:31 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 13:32 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 22:19 +0100
                Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-16 05:02 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-14 23:23 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 10:12 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 02:53 -0700
                Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-08 10:19 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-10 16:50 +0100
                Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-15 20:24 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-16 22:11 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-17 04:43 -0700
                Re: threads, signals, and atomicity of basic types Michael Press <rubrum@pacbell.net> - 2011-06-19 01:43 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-19 13:35 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-19 17:06 -0700
                Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-20 16:22 +0000
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-20 19:16 +0100
                Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-21 17:30 +0000
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-21 13:02 -0700
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-20 14:32 -0700
                Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-21 17:05 +0000
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-19 17:01 -0700
                Re: threads, signals, and atomicity of basic types Michael Press <rubrum@pacbell.net> - 2011-06-22 01:55 -0700
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-22 03:36 -0700
      Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-02 21:53 -0700
      Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-03 04:24 -0700
      Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-03 07:52 -0400
  Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-02 00:23 +0200
    Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-01 20:11 -0700
      Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-02 22:54 +0200
        Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 14:03 -0700
          Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-03 02:49 +0200
            Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 17:54 -0700
              Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 21:36 -0400
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-03 16:25 +0100
            Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-03 03:12 +0200
  Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-02 00:45 +0200
    Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 11:26 +0100
      Re: threads, signals, and atomicity of basic types Alex Fraser <me@privacy.net> - 2011-06-02 21:05 +0100
  Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-01 20:21 -0700
    Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 11:31 +0100
      Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 09:12 -0700
        Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 17:54 +0100
          Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 14:00 -0700
            Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-03 15:19 +0100
              Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-03 15:50 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-05 16:46 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-05 15:28 -0700
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-06 13:35 +0100
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-06 09:19 -0700
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-06 09:26 -0700
                Re: threads, signals, and atomicity of basic types Ian Collins <ian-news@hotmail.com> - 2011-06-07 09:43 +1200
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 13:29 +0100
                Re: threads, signals, and atomicity of basic types Ian Collins <ian-news@hotmail.com> - 2011-06-08 09:04 +1200
                Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-08 12:34 +0100
                Re: threads, signals, and atomicity of basic types Ian Collins <ian-news@hotmail.com> - 2011-06-09 07:37 +1200
                Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-07 14:55 -0700

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