Re: Arithmetic overflow checking

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Subject	Re: Arithmetic overflow checking
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From	"Greg A. Woods" <woods@robohack.org>
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Date	Wed, 13 Jul 2011 11:28:58 -0700
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Patricia Shanahan <pats@acm.org> writes:
> 
> On 7/13/2011 12:52 AM, tm wrote:
> > 
> > On 13 Jul., 05:03, "Greg A. Woods"<wo...@robohack.org>  wrote:
> ...
> > > As I mentioned in another post just now, a decent compiler can use
> > > library routines for all arithmetic operations and thus allow all
> > > operations to be checked for overflow algorithmically.
> >
> > Yes, but this would have probable more overhead than checking
> > an overflow flag, not to mention hardware that can do this
> > without extra cost (trigger interrupt).
>
> I'm not convinced that there would be extra cost. The main performance
> costs on modern processors tend to be data movement, and mis-predicted
> conditional branches. The overflow check needs no additional data, and
> the associated conditional branches will be about as easy to predict as
> is possible.

I'm not sure what target host environment you're thinking of, but in any
general purpose operating system environment I can think of the extra
cost of the hardware interrupts triggered by arithmetic overflows could
be significant.  Signal delivery and context switching is never free.

Worst would be if the hardware interrupt couldn't easily and efficiently
be disabled temporarily by the OS when running a process which didn't
care to be notified.  Then the OS would have to handle the interrupt and
figure out whether to discard it or not.  CRC algorithms, and especially
those in the kernel, may have to be carefully re-implemented to avoid
triggering overflows unnecessarily, and would thus then incur added
runtime costs themselves.  I.e. avoiding triggering unnecessary
interrupts could also be a significant runtime cost.

When you get down to brass tacks you only want overflow and wraparound
checking when you need it.  Unfortunately C makes it hard for
programmers to easily know when they need it.  I think the AIR proposal
may help, and it seems as if the RICH work identified many of these
issues and dealt with some of them too.  I guess I'll be content to wait
until something like these shows up in a GCC or Clang release I'm using,
as I've been living with overflows and wraparound (and no bounds checks)
well enough so far.

-- 
						Greg A. Woods
						RoboHack

<woods@robohack.ca>     living on the edge      http://www.robohack.ca/

Thread

Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-10 01:47 -0700
  Re: Arithmetic overflow checking China Blue Dolls <chine.bleu@yahoo.com> - 2011-07-10 02:47 -0700
    Re: Arithmetic overflow checking pete <pfiland@mindspring.com> - 2011-07-10 06:04 -0400
      Re: Arithmetic overflow checking China Blue Dolls <chine.bleu@yahoo.com> - 2011-07-10 03:29 -0700
        Re: Arithmetic overflow checking Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-10 20:52 +0300
        Re: Arithmetic overflow checking pete <pfiland@mindspring.com> - 2011-07-10 23:29 -0400
    Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-10 04:44 -0700
      Re: Arithmetic overflow checking "BartC" <bc@freeuk.com> - 2011-07-12 11:33 +0100
        Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 04:17 -0700
          Re: Arithmetic overflow checking "BartC" <bc@freeuk.com> - 2011-07-12 12:33 +0100
            Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 05:24 -0700
              Re: Arithmetic overflow checking Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-12 21:45 -0400
          Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-12 05:25 -0700
            Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 10:21 -0700
              Re: Arithmetic overflow checking Thomas Boell <tboell@domain.invalid> - 2011-07-12 22:39 +0200
                Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 14:15 -0700
                Re: Arithmetic overflow checking Keith Thompson <kst-u@mib.org> - 2011-07-12 14:18 -0700
                Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 23:54 -0700
            Re: Arithmetic overflow checking "BartC" <bc@freeuk.com> - 2011-07-12 19:14 +0100
              Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-13 00:20 -0700
        Re: Arithmetic overflow checking markspace <-@.> - 2011-07-12 09:26 -0700
          Re: Arithmetic overflow checking Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-12 10:52 -0600
            Re: Arithmetic overflow checking Keith Thompson <kst-u@mib.org> - 2011-07-12 10:48 -0700
              Re: Arithmetic overflow checking Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-12 12:36 -0600
                Re: Arithmetic overflow checking Keith Thompson <kst-u@mib.org> - 2011-07-12 12:16 -0700
                Re: Arithmetic overflow checking Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-12 14:09 -0600
                Re: Arithmetic overflow checking Keith Thompson <kst-u@mib.org> - 2011-07-12 14:16 -0700
              Re: Arithmetic overflow checking, not so hard after all "Greg A. Woods" <woods@robohack.org> - 2011-07-12 21:52 -0700
                Re: Arithmetic overflow checking, not so hard after all Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-13 12:50 +0100
                Re: Arithmetic overflow checking, not so hard after all "Greg A. Woods" <woods@robohack.org> - 2011-07-13 11:10 -0700
                Re: Arithmetic overflow checking, not so hard after all Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-14 01:34 +0100
                Re: Arithmetic overflow checking, not so hard after all "Greg A. Woods" <woods@robohack.org> - 2011-07-19 17:10 -0700
                Re: Arithmetic overflow checking, not so hard after all Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-19 20:47 -0600
                Re: Arithmetic overflow checking, not so hard after all Keith Thompson <kst-u@mib.org> - 2011-07-19 20:23 -0700
                Re: Arithmetic overflow checking, not so hard after all "Greg A. Woods" <woods@robohack.org> - 2011-09-06 17:47 -0700
                Re: Arithmetic overflow checking, not so hard after all Todd Carnes <toddcarnes@gmail.com> - 2011-07-20 03:48 +0000
                Re: Arithmetic overflow checking, not so hard after all Keith Thompson <kst-u@mib.org> - 2011-07-13 09:27 -0700
              Re: Arithmetic overflow checking "MikeP" <mp011011@some.org> - 2011-07-14 23:55 -0500
          Re: Arithmetic overflow checking Martin Gregorie <martin@address-in-sig.invalid> - 2011-07-12 16:54 +0000
            Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-12 11:35 -0700
          Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 10:13 -0700
          Re: Arithmetic overflow checking Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-12 21:53 -0400
        Re: Arithmetic overflow checking "MikeP" <mp011011@some.org> - 2011-07-14 23:41 -0500
          Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-15 10:56 -0700
            Re: Arithmetic overflow checking "MikeP" <mp011011@some.org> - 2011-07-15 21:27 -0500
      Re: Arithmetic overflow checking bugbear <bugbear@trim_papermule.co.uk_trim> - 2011-07-20 09:22 +0100
        Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-20 10:51 -0700
          Re: Arithmetic overflow checking gordonb.3urm7@burditt.org (Gordon Burditt) - 2011-07-20 15:39 -0500
          Re: Arithmetic overflow checking "BartC" <bc@freeuk.com> - 2011-07-21 12:12 +0100
    Re: Arithmetic overflow checking Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-10 09:28 -0400
      Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-10 06:52 -0700
        Re: Arithmetic overflow checking Keith Thompson <kst-u@mib.org> - 2011-07-10 14:47 -0700
        Re: Arithmetic overflow checking gordonb.u8vng@burditt.org (Gordon Burditt) - 2011-07-11 18:45 -0500
      Re: Arithmetic overflow checking "MikeP" <mp011011@some.org> - 2011-07-14 23:07 -0500
    Re: Arithmetic overflow checking Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-07-10 12:25 -0400
  Re: Arithmetic overflow checking Robert Wessel <robertwessel2@yahoo.com> - 2011-07-10 10:47 -0500
  Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-11 07:58 -0700
    Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-11 10:48 -0700
      Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-11 14:40 -0700
    Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-11 14:54 -0700
      Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-11 15:55 -0700
      Re: Arithmetic overflow checking Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-11 21:51 -0400
        Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-11 21:31 -0700
          Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-11 23:16 -0700
          Re: Arithmetic overflow checking James Kuyper <jameskuyper@verizon.net> - 2011-07-12 06:28 -0400
          Re: Arithmetic overflow checking David Thompson <dave.thompson2@verizon.net> - 2011-07-24 22:13 -0400
            Re: Arithmetic overflow checking Lew Pitcher <lpitcher@teksavvy.com> - 2011-07-25 10:24 -0400
  Re: Arithmetic overflow checking "io_x" <a@b.c.invalid> - 2011-07-12 09:05 +0200
    Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 02:22 -0700
      Re: Arithmetic overflow checking "io_x" <a@b.c.invalid> - 2011-07-12 11:34 +0200
        Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-12 03:04 -0700
          Re: Arithmetic overflow checking "Greg A. Woods" <woods@robohack.org> - 2011-07-12 19:55 -0700
            Re: Arithmetic overflow checking "Greg A. Woods" <woods@robohack.org> - 2011-07-12 21:54 -0700
            Re: Arithmetic overflow checking "MikeP" <mp011011@some.org> - 2011-07-14 23:28 -0500
              Re: Arithmetic overflow checking "Greg A. Woods" <woods@robohack.org> - 2011-07-19 17:16 -0700
        Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-12 03:33 -0700
        Re: Arithmetic overflow checking David Lamb <dalamb@cs.queensu.ca> - 2011-07-12 08:29 -0400
      Re: Arithmetic overflow checking "io_x" <a@b.c.invalid> - 2011-07-12 13:18 +0200
      Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-12 11:39 -0700
        Re: Arithmetic overflow checking Patricia Shanahan <pats@acm.org> - 2011-07-12 12:38 -0700
          Re: Arithmetic overflow checking markspace <-@.> - 2011-07-12 13:20 -0700
            Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-12 13:23 -0700
              Re: Arithmetic overflow checking Martin Gregorie <martin@address-in-sig.invalid> - 2011-07-12 21:08 +0000
                Re: Arithmetic overflow checking lewbloch <lewbloch@gmail.com> - 2011-07-12 14:48 -0700
                Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-12 15:24 -0700
                Re: Arithmetic overflow checking lewbloch <lewbloch@gmail.com> - 2011-07-12 16:09 -0700
                Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-13 10:38 -0700
                Re: Arithmetic overflow checking Patricia Shanahan <pats@acm.org> - 2011-07-13 11:00 -0700
                Re: Arithmetic overflow checking lewbloch <lewbloch@gmail.com> - 2011-07-13 12:16 -0700
                Re: Arithmetic overflow checking Gene Wirchenko <genew@ocis.net> - 2011-07-13 13:10 -0700
                Re: Arithmetic overflow checking markspace <-@.> - 2011-07-13 13:21 -0700
                Re: Arithmetic overflow checking Keith Thompson <kst-u@mib.org> - 2011-07-13 13:41 -0700
                Re: Arithmetic overflow checking Robert Wessel <robertwessel2@yahoo.com> - 2011-07-14 21:10 -0500
                Re: Arithmetic overflow checking "io_x" <a@b.c.invalid> - 2011-07-15 11:57 +0200
                Re: Arithmetic overflow checking Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-07-15 04:36 -0700
                Re: Arithmetic overflow checking Niklas Holsti <niklas.holsti@tidorum.invalid> - 2011-08-13 21:54 +0300
      Re: Arithmetic overflow checking "Greg A. Woods" <woods@robohack.org> - 2011-07-12 20:03 -0700
        Re: Arithmetic overflow checking tm <thomas.mertes@gmx.at> - 2011-07-13 00:52 -0700
          Re: Arithmetic overflow checking Patricia Shanahan <pats@acm.org> - 2011-07-13 07:45 -0700
            Re: Arithmetic overflow checking "Greg A. Woods" <woods@robohack.org> - 2011-07-13 11:28 -0700
          Re: Arithmetic overflow checking "Greg A. Woods" <woods@robohack.org> - 2011-07-13 11:15 -0700

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