Re: Infinity + Infinity (or NegInfinity - NegInfinity)

From	Keith Thompson <kst-u@mib.org>
Newsgroups	alt.comp.lang.borland-delphi, alt.math, comp.arch, comp.graphics.algorithms, comp.lang.c
Subject	Re: Infinity + Infinity (or NegInfinity - NegInfinity)
Followup-To	comp.lang.c
Date	2011-10-12 12:34 -0700
Organization	None to speak of
Message-ID	<lnbotmt34t.fsf@nuthaus.mib.org> (permalink)
References	(1 earlier) <j6qte6$pd$1@dont-email.me> <j6rnsc$km1$1@gosset.csi.cam.ac.uk> <88cbbc8b-1fab-4dde-96b0-4dd0cd7c183d@b6g2000vbz.googlegroups.com> <j74805$d1b$1@gosset.csi.cam.ac.uk> <4E95BBEE.3000005@verizon.net>

Cross-posted to 5 groups.

Followups directed to: comp.lang.c

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James Kuyper <jameskuyper@verizon.net> writes:
> On 10/12/2011 10:25 AM, nmm1@cam.ac.uk wrote:
> ...
>> There is no such statement in the C standard, and I can tell you
>> (from personal recollection) that requiring it was NOT WG14's intent
>> during the standardisation of C90.  The BNF was intended to specify
>> ONLY the precedence of operators and NOT the evaluation order; that
>> was specified by the side-effect rules.
>> 
>> If you had looked up the section of the standard I pointed you to,
>> you would have seen both that the word used is "grouping", which
>> is clearly intended to distinguish it from execution order, and a
>> clear statement that the order of evaluation is unspecified.  And
>> THAT was the intent of WG14 during the standardisation of C90.
>> 
>> I am fully aware that a lot of people are now claiming that the
>> BNF has always been meant to define  the execution order, but that
>> flatly contradicts large chunks of other wording (especially the
>> side-effect rules).  Whether it is now what compilers do, I don't
>> know (and don't much care, either).
>
> The freedom of C implementations to rearrange the order of evaluation is
> great, but it's not completely unconstrained.
> While there are people who have misinterpreted the BNF as fully
> specifying the execution order, I don't consider that to be a common
> position among those most familiar with the standard. A more common
> position, and IMO fully defensible, is that the BNF implies constraints
> on the execution order. For instance, in ((a+b) + (c+d)), the a+b can be
> executed before or after the (c+d), but both must be executed before the
> final addition can be performed, because that addition requires the
> results of those execution. The standard does not say anything
> explicitly about that fact, because it doesn't need to - it's implicit
> in what the standard does explicitly say about the dependency of the
> final value on the values of the sub-expressions.

And C201X, or at least the N1570 draft, makes this explicit.  6.5p1:

    An _expression_ is a sequence of operators and operands that
    specifies computation of a value, or that designates an object
    or a function, or that generates side effects, or that performs
    a combination thereof. The value computations of the operands
    of an operator are sequenced before the value computation of
    the result of the operator.

The phrase "sequenced before" is new; it's defined in 5.1.2.3.
If there's a sequence point after x and before y, then x is
"sequenced before" y.  But the reverse is not necessarily true; x can
be sequenced before y even if there's no sequence point between them.

In the example "a + b + c", the grammar says that the first "+"
applies to "a" and "b", and the second applies to the result of the
first and to "c"; in other words, "a + b + c" means "(a + b) + c",
*not* "a + (b + c)".

The order of evaluation of the operands of a given operator is
unspecified, and I don't believe anyone here has suggested otherwise.
"a + b" could evaluate "a" and then "b", or "b" and then "a"; it could
even evaluate them in parallel as long as the result is as if they were
evaluated sequentially.

But an operator cannot be applied until after its operands have
been evaluated.  This is implicit in C90 and C99, and explicit (see
above) in C201X.  The result of "a + b" *must* be computed before
the result of "a + b + c" can be computed.  There's no sequence
point between the two additions, but their order is constrained by
the simple fact that you can't perform an operation without first
evaluating the operands.  (The lack of a sequence point means that
the "value computation" of "a" is sequenced before the computation of
"a + b", but any side effects of evaluating "a" are not.)

[...]

(Followups to comp.lang.c only.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-08 02:54 +0200
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-07 18:40 -0700
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-07 22:39 -0400
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) William Elliot <marsh@rdrop.com> - 2011-10-07 19:56 -0700
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) Jasen Betts <jasen@xnet.co.nz> - 2011-10-08 06:45 +0000
      Re: Infinity + Infinity (or NegInfinity - NegInfinity) gordonb.eug6r@burditt.org (Gordon Burditt) - 2011-10-08 02:37 -0500
        Re: Infinity + Infinity (or NegInfinity - NegInfinity) William Elliot <marsh@rdrop.com> - 2011-10-08 21:40 -0700
          Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-09 12:17 +0300
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) Jasen Betts <jasen@xnet.co.nz> - 2011-10-08 06:33 +0000
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-08 16:41 +0200
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) kenney@cix.compulink.co.uk - 2011-10-09 04:01 -0500
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-08 17:43 +0300
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-10-08 10:20 -0700
      Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-08 22:35 +0300
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-08 18:55 +0100
      Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-08 23:03 +0300
        Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-08 21:27 +0100
          Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-08 17:28 -0400
            Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-08 18:15 -0400
            Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-08 23:23 +0100
              Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-08 21:29 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-09 10:01 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-12 06:14 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-12 15:25 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-12 12:10 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-12 18:41 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-12 15:32 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-12 20:44 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-12 18:11 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Andrew Reilly <areilly---@bigpond.net.au> - 2011-10-13 03:48 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-13 09:07 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-13 07:00 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-14 10:54 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Andrew Reilly <areilly---@bigpond.net.au> - 2011-10-14 22:36 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-15 09:26 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Jasen Betts <jasen@xnet.co.nz> - 2011-10-24 02:40 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-27 00:32 +0300
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-27 10:41 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-27 13:10 +0300
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-27 13:53 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Keith Thompson <kst-u@mib.org> - 2011-10-12 12:34 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-13 06:39 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-13 10:34 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-14 00:54 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:25 -0800
            Re: Infinity + Infinity (or NegInfinity - NegInfinity) Larry Hewitt <larryhwtt@comporium.net> - 2011-10-09 21:35 -0400
              Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-10 04:37 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Larry Hewitt <larryhwtt@comporium.net> - 2011-10-10 13:58 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-13 20:17 +0200
        Re: Infinity + Infinity (or NegInfinity - NegInfinity) Arivald <arivald_@AT_interia_DOT_.pl> - 2011-10-13 12:10 +0200
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) Nobody <nobody@nowhere.com> - 2011-10-09 05:05 +0100

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