Re: GCC hasn't even gotten around to sequencing argument evaluation

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: GCC hasn't even gotten around to sequencing argument evaluation
Date	2022-04-06 18:21 +0200
Organization	A noiseless patient Spider
Message-ID	<t2kelr$v8t$1@dont-email.me> (permalink)
References	(11 earlier) <t2hmk6$1nno$1@gioia.aioe.org> <t2hnnm$c47$1@dont-email.me> <t2hon3$rje$1@gioia.aioe.org> <t2jm8n$jhl$1@dont-email.me> <t2jr7h$r65$1@gioia.aioe.org>

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On 06/04/2022 12:49, Juha Nieminen wrote:
> David Brown <david.brown@hesbynett.no> wrote:
>> As long as the results are the same for any possible values of the
>> variables for which there are no signed overflows in original
>> expression, any order of evaluation is allowed.
> 
> While not completely relevant to this discussion, this somehow reminded
> me of the gcc (and clang) option -ffast-math, which gives the compiler
> express permission to do (floating point) optimizations that are,
> strictly speaking, not allowed by the standard (because according to
> the standard optimizations must not affect observable behavior, other
> than runtime).
> 

I am no expert on the details of floating point conformance (for my uses 
of floating point, -ffast-math is ideal).  But I believe many aspects 
are implementation-dependent.  So while gcc won't follow IEEE standards 
precisely when -ffast-math is in effect, it may still be conforming to 
the C standards.  I could be wrong here - as I say, it is not something 
I have looked at carefully.

> It can sometimes be a bit surprising what kind of optimizations the
> compiler is *not* allowed to perform, unless explicitly allowed to
> bypass the strictest requirements of the language standard.
> 
> For example, the compiler is not allowed to optimize "x*0.0"
> into "0.0" because that changes observable behavior if x is inf
> or NaN. (-ffast-math allows it to do that optimization.)
> 

This are not observable behaviour in itself (unless the implementation 
guarantees traps or signals).  Results of expressions are only going to 
be "observable" if they are stored in volatile variables, printed out, 
or otherwise affect the observable behaviour of the code.  If the value 
is not used, it is not observable.  (Of course, most calculations /will/ 
affect some observable behaviour - otherwise there would be no point in 
doing them!  But it is not the calculation itself that is observable.)

> Likewise it cannot optimize "(x+y)-x" into "y" because the result
> of the expression may not be exactly "y" (because of the limited
> size of the mantissa, or because "x+y" may result in inf or -inf.)
> The same is true for many other expressions that could mathematically
> be reduced but the compiler isn't allowed to do because with floating
> point the result may not be the same. (Again, -ffast-math gives it
> permission to do that type of optimization.)

Yes, indeed.

Thread

GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-01 20:34 -0700
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-04-02 10:19 +0200
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Bo Persson <bo@bo-persson.se> - 2022-04-02 12:19 +0200
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-02 14:18 +0200
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Christian Gollwitzer <auriocus@gmx.de> - 2022-04-02 12:30 +0200
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? David Brown <david.brown@hesbynett.no> - 2022-04-02 14:12 +0200
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-02 14:20 +0200
      Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-02 14:28 +0000
        Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-02 17:27 +0200
          Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-02 15:39 +0000
            Re: GCC hasn't even gotten around to sequencing argument evaluation Barry Schwarz <schwarzb@delq.com> - 2022-04-02 09:15 -0700
              Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-04 08:21 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 12:02 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-04 14:42 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-04 08:49 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-05 15:18 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-05 08:37 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-05 15:54 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-06 11:24 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Juha Nieminen <nospam@thanks.invalid> - 2022-04-06 10:49 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-06 18:21 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-04-06 15:40 +0100
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-06 15:00 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-04-06 17:03 +0100
                Re: GCC hasn't even gotten around to sequencing argument evaluation Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-04-06 17:18 +0100
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-07 08:58 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-07 11:01 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-04-06 23:13 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-07 09:11 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-06 10:53 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-06 14:50 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation Sams Lara <samlara622@gmail.com> - 2022-04-06 22:27 +0100
                Re: GCC hasn't even gotten around to sequencing argument evaluation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-06 14:51 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Sams Lara <samlara622@gmail.com> - 2022-04-06 22:59 +0100
                Re: GCC hasn't even gotten around to sequencing argument evaluation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-06 15:54 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Öö Tiib <ootiib@hot.ee> - 2022-04-06 15:11 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-09 09:38 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-06 23:48 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-06 18:39 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-06 23:49 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-05 08:55 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-05 10:23 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-05 14:36 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-04 12:27 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-05 15:21 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-05 08:41 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-05 15:50 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-05 10:19 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-05 14:33 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-05 14:28 -0400
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 18:29 +0200
            Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-02 11:27 -0700
              Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-04 08:23 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 12:15 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-04 14:43 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 18:34 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-05 15:23 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-05 08:38 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-05 15:49 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-05 11:31 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation scott@slp53.sl.home (Scott Lurndal) - 2022-04-05 22:45 +0000
            Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-02 23:34 -0400
              Re: GCC hasn't even gotten around to sequencing argument evaluation Manfred <noname@add.invalid> - 2022-04-03 17:43 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-03 15:40 -0400
              Re: GCC hasn't even gotten around to sequencing argument evaluation Muttley@dastardlyhq.com - 2022-04-04 08:24 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-04 12:01 -0400
            Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-03 13:02 +0200
              Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-03 04:46 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Öö Tiib <ootiib@hot.ee> - 2022-04-03 07:02 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-03 11:30 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-03 21:20 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-03 12:49 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-04 00:40 +0300
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-03 15:40 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 09:13 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 09:02 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-04 02:07 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 13:28 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-04 06:01 -0700
                Re: GCC hasn't even gotten around to sequencing argument evaluation David Brown <david.brown@hesbynett.no> - 2022-04-04 19:11 +0200
                Re: GCC hasn't even gotten around to sequencing argument evaluation scott@slp53.sl.home (Scott Lurndal) - 2022-04-04 17:59 +0000
                Re: GCC hasn't even gotten around to sequencing argument evaluation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-05 06:55 -0700
              Re: GCC hasn't even gotten around to sequencing argument evaluation Manfred <noname@add.invalid> - 2022-04-03 18:39 +0200
        Re: GCC hasn't even gotten around to sequencing argument evaluation Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-03 09:09 +0300
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Manfred <noname@add.invalid> - 2022-04-02 19:53 +0200
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? David Brown <david.brown@hesbynett.no> - 2022-04-03 13:45 +0200
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Manfred <noname@add.invalid> - 2022-04-03 17:27 +0200
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-04-02 21:17 +0100
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Manfred <noname@invalid.add> - 2022-04-02 23:04 +0200
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-04-03 00:22 +0100
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-02 17:33 -0700
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-04-03 14:23 +0100
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-06 08:31 -0700
            Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-06 11:05 -0700
              Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-25 03:32 -0700
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-02 11:18 -0700
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? David Brown <david.brown@hesbynett.no> - 2022-04-02 13:11 +0200
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Manu Raju <MR@invalid.invalid> - 2022-04-02 18:26 +0100
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Manu Raju <MR@invalid.invalid> - 2022-04-02 21:46 +0100
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-02 17:35 -0700
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Language Lawyer <language.lawyer@gmail.com> - 2022-04-04 02:28 -0700
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-04 05:52 -0700
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Language Lawyer <language.lawyer@gmail.com> - 2022-06-21 03:36 -0700
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Juha Nieminen <nospam@thanks.invalid> - 2022-04-04 10:48 +0000
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-04 05:49 -0700
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Juha Nieminen <nospam@thanks.invalid> - 2022-04-05 06:34 +0000
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-04-05 10:20 +0100
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-05 07:51 -0700
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Manfred <noname@add.invalid> - 2022-04-05 17:53 +0200
  Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Christian Hanné <the.hanne@gmail.com> - 2022-04-10 06:06 +0200
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-09 21:20 -0700
    Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-04-09 21:39 -0700
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-09 22:34 -0700
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Christian Hanné <the.hanne@gmail.com> - 2022-04-10 09:19 +0200
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? David Brown <david.brown@hesbynett.no> - 2022-04-10 12:21 +0200
            Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Christian Hanné <the.hanne@gmail.com> - 2022-04-10 12:47 +0200
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-04-11 12:18 -0700
            Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-11 12:33 -0700
      Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Christian Hanné <the.hanne@gmail.com> - 2022-04-10 09:20 +0200
        Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-04-11 12:32 -0700
          Re: GCC hasn't even gotten around to sequencing argument evaluation yet??? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-11 15:11 -0700

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