Re: Sequence points and commit to memory

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Sequence points and commit to memory
Date	2015-05-19 12:05 -0400
Organization	Self
Message-ID	<555B5F39.1050807@verizon.net> (permalink)
References	(8 earlier) <mj6pca$5ab$1@dont-email.me> <mj8t7s$2rm$1@dont-email.me> <mjdoqe$gdj$1@dont-email.me> <555A6DE5.2010303@verizon.net> <mjenju$353$1@dont-email.me>

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On 05/19/2015 03:10 AM, David Brown wrote:
> On 19/05/15 00:55, James Kuyper wrote:
>> On 05/18/2015 06:25 PM, David Brown wrote:
>> ...
>>> You are missing the "as-if" rule.  Yes, there are sequence points at the 
>>> end of every statement.  But the compiler only has to honour them to the 
>>> extent that the externally visible effects of the code are "as if" it 
>>> had honoured them slavishly.
>>
>> So, why do you care? If you can write strictly conforming code whose
>> behavior would otherwise depend upon whether or not the compiler
>> performed a given optimization, then the as-if rule can not be used to
>> justify the optimization.
>>
> 
> The problem is that James had written code where the correctness of the
> algorithm (i.e., the programmer's idea of correctness and order) depends
> on an ordering that is not guaranteed by the rules of C.  Your comment
> about sequence points makes it look like sequence points fully determine
> ordering of statements in C, and that is simply wrong - they determine
> only the "as-if" ordering of the virtual machine, not the real-world
> ordering as run by the processor.  And James' code requires certain
> ordering on the processor.  To specify that, he needs something stronger
> than sequence points - and in C, that "something" is "volatile".

Sequence points don't fully determine the ordering of statements in C, I
never suggested otherwise. They don't even fully determine the "as-if"
ordering, as you call it: in a()+b(), the two function calls can be
evaluated in either order - a conforming implementation must produce
results "as-if" one of those two orders were followed, but it's free to
choose either one. Also, the standard has always allowed for signal
handlers, and C2011 adds support for multi-threaded code, so a fully
conforming implementation of C can choose from a large number of
different "as-if" orders.

However, the subject of this thread has, from the beginning, been the
guarantees provided by the standard about two expressions separated by a
sequence point where the standard does guarantee the order of the two
sub-expressions. Using the new terms introduced in C2011, that means
that one expression is "sequenced after" (5.1.2.3p3) the other.

Now, you're right that it's only the as-if ordering that is guaranteed
by the standard. But, when one expression is, per the standard,
"sequenced after" another expression, the as-if ordering should be all
that matters, as far as strictly-conforming code is concerned, because
the "as-if" ordering is effectively the only ordering that can be seen
by such code, even if the actual ordering is quite different. If you
insert debugging fprintf()s in various locations in the code, and send
them to an unbuffered stream, the data received by that stream must
match the "as-if" order, even if the actual order in which the relevant
expressions were executed is quite different.

Thread

Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-14 11:43 +0100
  Re: Sequence points and commit to memory Richard Heathfield <rjh@cpax.org.uk> - 2015-05-14 12:02 +0100
    Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-14 13:39 +0200
      Re: Sequence points and commit to memory Richard Heathfield <rjh@cpax.org.uk> - 2015-05-14 13:18 +0100
        Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-14 23:11 +0200
  Re: Sequence points and commit to memory Stephen Sprunk <stephen@sprunk.org> - 2015-05-14 09:06 -0500
  Re: Sequence points and commit to memory Siri Cruz <chine.bleu@yahoo.com> - 2015-05-14 08:35 -0700
    Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-14 10:23 -0700
      Re: Sequence points and commit to memory Siri Cruz <chine.bleu@yahoo.com> - 2015-05-14 10:44 -0700
        Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-14 11:09 -0700
          Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-14 12:58 -0700
          Re: Sequence points and commit to memory Morris Dovey <mrdovey@iedu.com> - 2015-05-14 16:31 -0500
            Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-14 17:24 -0700
              Re: Sequence points and commit to memory Morris Dovey <mrdovey@iedu.com> - 2015-05-14 20:18 -0500
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-15 09:34 +0200
                Re: Sequence points and commit to memory Morris Dovey <mrdovey@iedu.com> - 2015-05-15 10:18 -0500
                Re: 'register' attribute (was: Sequence points and commit to memory) Morris Dovey <mrdovey@iedu.com> - 2015-05-16 01:26 -0500
                Re: 'register' attribute David Brown <david.brown@hesbynett.no> - 2015-05-17 00:02 +0200
                Re: 'register' attribute Morris Dovey <mrdovey@iedu.com> - 2015-05-16 19:29 -0500
            Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-15 09:52 -0700
              Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-15 13:27 -0400
                Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-15 10:50 -0700
              Re: Sequence points and commit to memory Morris Dovey <mrdovey@iedu.com> - 2015-05-15 12:32 -0500
      Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-14 23:23 +0200
        Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-15 09:42 -0700
          Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-15 13:13 -0400
            Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-16 08:32 -0700
          Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-15 10:35 -0700
          Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-15 11:08 -0700
            Re: Sequence points and commit to memory Robert Wessel <robertwessel2@yahoo.com> - 2015-05-15 16:40 -0500
            Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-16 08:48 -0700
          Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-15 21:46 +0100
            Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-16 08:56 -0700
              Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-16 19:35 +0100
                Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-18 09:49 -0700
                Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-19 09:04 +0100
                Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-19 08:42 -0700
    Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-14 22:00 +0100
      Re: Sequence points and commit to memory Siri Cruz <chine.bleu@yahoo.com> - 2015-05-14 14:26 -0700
        Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-14 22:33 +0100
      Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-14 23:38 +0200
        Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-14 23:04 +0100
          Re: Sequence points and commit to memory Stephen Sprunk <stephen@sprunk.org> - 2015-05-15 13:09 -0500
            Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-15 21:56 +0100
              Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-15 17:30 -0400
                Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-16 07:51 +0100
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-16 22:10 -0400
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 00:25 +0200
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-18 18:55 -0400
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 09:10 +0200
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-19 12:05 -0400
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 18:42 +0200
              Re: Sequence points and commit to memory Stephen Sprunk <stephen@sprunk.org> - 2015-05-16 16:33 -0500
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 00:27 +0200
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-18 19:05 -0400
                Re: Sequence points and commit to memory glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-05-19 00:09 +0000
                Re: Sequence points and commit to memory Richard Heathfield <rjh@cpax.org.uk> - 2015-05-19 01:11 +0100
                Re: Sequence points and commit to memory Tim Rentsch <txr@alumni.caltech.edu> - 2015-05-23 07:02 -0700
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-18 23:21 -0400
                Re: Sequence points and commit to memory glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-05-19 06:25 +0000
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 08:58 +0200
                Re: Sequence points and commit to memory glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-05-19 23:20 +0000
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-20 12:31 +0200
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-19 12:10 -0400
                Re: Sequence points and commit to memory glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-05-19 23:24 +0000
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 09:05 +0200
                Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-19 08:37 -0700
                Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-19 12:14 -0400
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 19:02 +0200
            Re: Sequence points and commit to memory Robert Wessel <robertwessel2@yahoo.com> - 2015-05-15 16:46 -0500
          Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-17 00:20 +0200
            Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-17 18:19 +0100
              Re: Sequence points and commit to memory Philip Lantz <prl@canterey.us> - 2015-05-17 16:11 -0700
                Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-18 08:30 +0100
                Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-18 10:31 -0700
                Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-18 11:11 -0700
                Re: Sequence points and commit to memory David Kleinecke <dkleinecke@gmail.com> - 2015-05-19 09:37 -0700
                Re: Sequence points and commit to memory Ian Collins <ian-news@hotmail.com> - 2015-05-19 17:40 +1200
                Re: Sequence points and commit to memory Morris Dovey <mrdovey@iedu.com> - 2015-05-19 01:31 -0500
                Re: Sequence points and commit to memory Ian Collins <ian-news@hotmail.com> - 2015-05-19 20:38 +1200
                Re: Sequence points and commit to memory Morris Dovey <mrdovey@iedu.com> - 2015-05-19 10:50 -0500
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 00:46 +0200
              Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 00:35 +0200
      Re: Sequence points and commit to memory Robert Wessel <robertwessel2@yahoo.com> - 2015-05-15 16:42 -0500
        Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-16 07:58 +0100
          Re: Sequence points and commit to memory Robert Wessel <robertwessel2@yahoo.com> - 2015-05-16 02:18 -0500
            Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-16 08:36 +0100
              Re: Sequence points and commit to memory Robert Wessel <robertwessel2@yahoo.com> - 2015-05-17 04:31 -0500
                Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-17 18:24 +0100
                Re: Sequence points and commit to memory David Brown <david.brown@hesbynett.no> - 2015-05-19 00:55 +0200
  Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-14 11:26 -0700
    Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-14 22:07 +0100
      Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-14 14:22 -0700
        Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-14 22:36 +0100
    Re: Sequence points and commit to memory Tim Rentsch <txr@alumni.caltech.edu> - 2015-05-22 08:16 -0700
      Re: Sequence points and commit to memory Keith Thompson <kst-u@mib.org> - 2015-05-22 11:31 -0700
  Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-15 00:05 -0400
    Re: Sequence points and commit to memory "James Harris" <james.harris.1@gmail.com> - 2015-05-15 22:25 +0100
      Re: Sequence points and commit to memory James Kuyper <jameskuyper@verizon.net> - 2015-05-15 17:56 -0400
  Re: Sequence points and commit to memory Nobody <nobody@nowhere.invalid> - 2015-05-17 10:04 +0100
  Re: Sequence points and commit to memory Tim Rentsch <txr@alumni.caltech.edu> - 2015-05-24 08:11 -0700

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