Re: the language (and the role of Undefined Behavior therein)

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: the language (and the role of Undefined Behavior therein)
Date	2015-08-24 19:49 +0000
Organization	Aioe.org NNTP Server
Message-ID	<mrfsfj$pe1$1@speranza.aioe.org> (permalink)
References	(7 earlier) <mra9p0$4g2$1@speranza.aioe.org> <mrbelf$l6m$1@dont-email.me> <mre3fs$b8d$1@speranza.aioe.org> <mre68i$6h7$2@dont-email.me> <25b983cf-2dc5-47b6-9e52-15d812437a1e@googlegroups.com>

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supercat@casperkitty.com wrote:
> On Sunday, August 23, 2015 at 11:25:39 PM UTC-5, James Kuyper wrote:
>> Personally, on those occasions when I wanted to calculate an integer
>> corresponding to a/b when a and b have opposite signs, I've always
>> wanted "round toward negative infinity", not "round toward 0", so I find
>> this specification inconvenient. However, having it inconveniently
>> specified was a substantial improvement over leaving it unspecified.
 
> Ironically, the one time I can think of where I actually wanted 
> round-toward- zero semantics, I was dividing by two on a compiler 
> which didn't optimize for that case, and the code was time-critical, 
> which meant I had to use a shift along with conditional logic to 
> force the rounding.

Note that the results from signed shift are dependent on the 
representation of signed values.

Fortran originated on sign-magnitude machines.
 
> Otherwise, I would posit that while a lot of hardware division 
> instructions follow FORTRAN's model, the opposite model would have 
> resulted in more efficient code in many cases.  Division by 
> constants is extremely common, and in many cases is most 
> efficiently done without using a "divide" instruction.  In the
> case of division by powers of two, most implementations provide 
> a signed right shift instruction for the task, but for other values 
> the most efficient approach is often a multiply/add/shift 
> combination, which is made less efficient by the requirement to 
> treat negative numbers specially.

(snip)

-- glen

Thread

Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-18 16:19 -0700
  Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-18 21:15 -0400
    Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 10:11 -0700
      Re: the language (and the role of Undefined Behavior therein) Ike Naar <ike@iceland.freeshell.org> - 2015-08-19 19:58 +0000
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 14:09 -0700
      Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-19 16:10 -0400
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 16:01 -0700
          Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-19 20:23 -0400
            Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 10:35 -0700
              Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-20 12:32 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 13:31 -0700
                Re: the language (and the role of Undefined Behavior therein) donalhambra@gmail.com - 2015-08-20 14:23 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 16:13 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-20 17:48 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-21 01:32 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-21 18:34 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-21 22:11 -0700
                Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-22 09:41 -0400
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-22 16:59 +0000
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-22 23:30 -0400
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 03:36 +0000
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 00:25 -0400
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 22:57 -0700
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 19:49 +0000
                Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 14:43 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 16:27 -0700
                Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 21:15 -0400
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 20:18 -0700
                Re: the language (and the role of Undefined Behavior therein) raltbos@xs4all.nl (Richard Bos) - 2015-08-22 14:37 +0000
                Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-22 19:12 -0700
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-22 23:38 -0400
                Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-23 20:01 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-23 21:56 -0700
                Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-24 17:25 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-24 18:21 -0700
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 22:22 -0400
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-25 14:31 +0000
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-25 09:22 -0700
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-25 18:55 +0000
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-25 12:33 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-25 16:23 -0700
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 00:31 -0400
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-24 10:32 -0700
                Re: the language (and the role of Undefined Behavior therein) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-22 23:54 -0700
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 03:45 +0000
                Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 08:11 -0700
      Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 18:46 -0400
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 19:04 -0700
          Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 23:40 -0400
            Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 22:48 -0700
              Re: the language (and the role of Undefined Behavior therein) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-24 00:38 -0700
  Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-18 21:22 -0400
  Re: the language (and the role of Undefined Behavior therein) Ike Naar <ike@iceland.freeshell.org> - 2015-08-20 21:44 +0000
    Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 16:48 -0700
    Re: the language (and the role of Undefined Behavior therein) Philip Lantz <prl@canterey.us> - 2015-08-20 23:41 -0700

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