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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Unspecified behavior in C |
| Date | 2015-07-31 18:28 -0700 |
| Organization | None to speak of |
| Message-ID | <ln8u9vlrun.fsf@kst-u.example.com> (permalink) |
| References | (16 earlier) <1622f824-7bca-4214-8768-86abf1c5d867@googlegroups.com> <82f9a602-1177-4a53-bfe6-3be2fa7ac76d@googlegroups.com> <4938af2d-22ef-4887-8538-129ee573557e@googlegroups.com> <mpev80$mfg$1@dont-email.me> <8d3d7a31-90d4-4e81-bdd3-0e82fa5f970a@googlegroups.com> |
David Kleinecke <dkleinecke@gmail.com> writes:
> On Thursday, July 30, 2015 at 9:59:20 PM UTC-7, James Kuyper wrote:
> You don't comprehend what I mean by documenting so I suggest you stop
> trying to disprove the strawman you set up instead.
>
>> > I assume there is a natural order in expression evaluation.
>>
>> That's a faulty assumption that sheds new light on many of your
>> previously bizarre comments. The sub-expressions of most C expressions
>> have no natural order, which is why the C standard specifies the order
>> only for a few of them.
>
> I don't understand this at all. What, for example, do you mean by
> sub-expression.
A sub-expression is an expression that's part of another expression.
> Perhaps you mean things like
> a + b + c
> where there are three possible orders of evaluation (all leading to
> the same result by a well known theorem. Nevertheless there is a
> natural order
> (a + b) + c
> You may not have encountered this use of the word "natural" before. It
> means naively obvious.
In C, the expressions
a + b + c
and
(a + b) + c
have exactly the same meaning, regardless of what you might think is
"natural". The subexpressions a, b, and c may be evaluated in any
of the 6 possible orders; if they have side effects, those side
effects may occur in any of those 6 possible orders. The standard
expresses no preference whatsoever for which of the 6 orders should
be chosen.
In the abstract machine, the first "+" is evaluated after the
results of evaluating a and b are determined, and the second "+"
is evaluated after the results of a, b, c, and the first "+"
are determined. A compiler may generate code that does these
evaluations in a different order if and only if the change doesn't
alter the observable behavior. (There's no such restriction on
variant orderings of evaluating a, b, and c.)
This is all essentially the same in C90, C99 and C11.
That's (part of) how expression evaluation is defined in C. If you
want to discuss it (how you think it *should* work, what you think
should be documented, etc.), you should at least understand how
it's currently defined.
> Not everything has a natural representation - finite groups, for example,
> do not. But things like a set of integers do.
>
> Natural does not mean "required". An optimizer can juggle the order
> freely (within the obvious limitation).
>
> I admit there are other ways of doing the arithmetic. They just aren't
> the natural way. But they may be more efficient.
You haven't defined what you mean by "natural".
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-07 14:44 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-07 15:34 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 08:47 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-08 09:11 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 10:24 -0700
Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-08 10:46 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 13:54 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-08 11:12 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-09 09:15 -0700
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-09 17:33 +0100
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 09:24 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-10 12:54 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-09 10:58 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 09:33 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-10 12:11 -0700
Re: Unspecified behavior in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-10 21:05 +0000
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-10 17:14 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-11 09:32 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-11 12:59 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-11 17:21 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-11 09:23 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-11 13:03 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-11 17:15 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-12 09:52 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-12 14:13 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-13 09:02 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-13 13:18 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-13 11:09 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-13 15:06 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 08:14 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 12:02 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-13 12:03 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 07:54 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 11:16 -0400
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-14 16:17 +0100
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 09:16 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 12:29 -0400
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 14:57 -0400
Re: Unspecified behavior in C Öö Tiib <ootiib@hot.ee> - 2015-07-14 13:12 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 16:30 -0400
Re: Unspecified behavior in C Öö Tiib <ootiib@hot.ee> - 2015-07-14 15:54 -0700
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-14 21:41 -0500
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-14 21:51 -0500
Re: Unspecified behavior in C Öö Tiib <ootiib@hot.ee> - 2015-07-15 12:42 -0700
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-15 16:13 -0500
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-16 08:49 -0500
Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 10:10 -0700
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-20 13:24 -0500
Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-23 02:56 -0700
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-23 11:26 -0500
Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-26 11:24 -0700
Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-23 19:03 -0500
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-15 21:19 -0700
Re: Unspecified behavior in C Martin Shobe <martin.shobe@yahoo.com> - 2015-07-15 23:45 -0500
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 01:37 -0400
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-16 09:42 +0100
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-16 17:05 -0700
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-17 01:32 +0100
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 21:07 -0400
We're good - don't you worry 'bout a thing, babe (Was: Unspecified behavior in C) gazelle@shell.xmission.com (Kenny McCormack) - 2015-07-17 07:21 +0000
Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-19 20:59 +0000
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-19 22:02 +0100
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 17:59 -0400
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 23:09 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-17 18:48 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-17 23:09 -0400
Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-19 20:57 +0000
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-19 17:22 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-19 18:57 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-19 22:16 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-19 19:30 -0700
Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-30 14:20 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-30 15:30 -0700
Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-30 15:53 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-30 16:46 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-31 00:59 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-31 17:15 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-31 18:28 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-31 23:28 -0400
Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-04 04:07 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-04 18:02 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 21:27 -0400
Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-24 09:33 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-24 17:11 -0700
Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 12:44 -0700
Re: Unspecified behavior in C Ken Brody <kenbrody@spamcop.net> - 2015-08-03 12:09 -0400
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-03 13:26 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-03 17:53 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-08-03 18:44 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-04 17:59 -0700
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-08-05 02:23 +0100
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 21:39 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-08-04 19:27 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 00:25 -0400
Re: Unspecified behavior in C Ken Brody <kenbrody@spamcop.net> - 2015-08-04 11:55 -0400
Re: Unspecified behavior in C Philip Lantz <prl@canterey.us> - 2015-08-04 10:31 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 13:32 -0400
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 22:46 -0400
Re: Unspecified behavior in C Ken Brody <kenbrody@spamcop.net> - 2015-08-10 12:16 -0400
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-10 13:26 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-03 17:46 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-05 12:34 -0400
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-14 17:57 +0100
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 10:31 -0700
Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-14 18:50 +0100
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-15 20:59 -0700
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-14 11:34 -0700
Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-19 20:46 +0000
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-12 12:07 -0700
Re: Unspecified behavior in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-12 19:53 +0000
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-12 16:12 -0400
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-09 14:20 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-09 12:18 -0700
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 09:47 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-10 13:36 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-11 09:19 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-11 12:56 -0400
Re: Unspecified behavior in C gordonb.j19di@burditt.org (Gordon Burditt) - 2015-07-11 20:36 -0500
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-12 10:10 -0700
Re: Unspecified behavior in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-09 20:13 +0100
Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-09 11:17 +0000
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 10:37 -0700
Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-08 10:51 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 14:14 -0400
Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-08 12:19 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 12:14 -0400
Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 10:40 -0700
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 14:29 -0400
Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-09 11:20 +0000
Re: Unspecified behavior in C Robert Wessel <robertwessel2@yahoo.com> - 2015-07-09 02:30 -0500
Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-07 22:30 -0400
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