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

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c++
Subject	Re: GCC hasn't even gotten around to sequencing argument evaluation yet???
Date	2022-04-06 08:31 -0700
Organization	A noiseless patient Spider
Message-ID	<86mtgy9qj5.fsf@linuxsc.com> (permalink)
References	<t28g93$cou$1@dont-email.me> <t298jp$duj$1@dont-email.me> <t29ej1$5dk$1@dont-email.me> <87wng7fdea.fsf@bsb.me.uk> <t2aq0o$1td$1@dont-email.me>

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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> On 4/2/2022 1:17 PM, Ben Bacarisse wrote:
>
>> David Brown <david.brown@hesbynett.no> writes:
>>
>>> On 02/04/2022 12:30, Christian Gollwitzer wrote:
>>>
>>>> Am 02.04.22 um 05:34 schrieb Andrey Tarasevich:
>>>>
>>>>>   int r = foo(++i, ++i);
>>>>>
>>>>> This outputs 6 in GCC 11.2 in `-std=c++17` mode and later.  WTF?
>>>>> Adding sequencing to argument evaluation, albeit indeterminate,
>>>>> is a major and a very important change in C++17.  How come it is
>>>>> still not implemented in GCC?  What are they waiting for?  Is
>>>>> there some sort of intentional opposition to that change in the
>>>>> standard?
>>>>
>>>> I'm not an expert on the standard, but according to this page:
>>>> https://en.cppreference.com/w/cpp/language/eval_order
>>>>
>>>> it is not specified.  They have exactly your example down the
>>>> page:
>>>>
>>>> f(++i, ++i);
>>>> // undefined behavior until C++17, unspecified after C++17
>>>>
>>>> To me that sounds like before C+17, the program could segfault,
>>>> whereas with C++17, it can either output 5 or 6, but shold
>>>> consistently do so.
>>>>
>>>> Am I wrong?
>>>
>>> The "unspecified" aspect refers to the ordering of evaluation of
>>> the parameters - they are "indeterminately ordered".
>>>
>>> So given "int foo(int a, int b)", the call "f(++i, ++i)" should be
>>> either:
>>>
>>> 	a = ++i;
>>> 	b = ++i;
>>> 	foo(a, b);
>>>
>>> or
>>>
>>> 	b = ++i;
>>> 	a = ++i;
>>> 	foo(a, b);
>>>
>>> It is thus unspecified whether a and b are 2 and 3, or 3 and 2
>>> respectively.  But their sum will be the same.
>>>
>>> It seems gcc has handled this as:
>>>
>>> 	++i;
>>> 	++i;
>>> 	a = i;
>>> 	b = i;
>>>
>>> and AFAIUI, this interleaving is not allowed in C++17.
>>
>> I am not 100% sure.  The wording (C++20 but similar in C++17) is
>> this:
>>
>>    "The initialization of a parameter, including every associated
>>    value computation and side effect, is indeterminately sequenced
>>    with respect to that of any other parameter."
>>
>> That sounds like the two ++i expressions need not to be thought of
>> as units with one sequenced before the other.  It sounds as if any
>> interleaving of the parts is permitted.  At least that's how I
>> would read it, unfamiliar as I am with the way the C++ authors use
>> language.
>
> No, "sequenced before" is a very strong requirement preventing any
> kind of interleaving.
>
> The modern C++ sequencing terminology abandoned the idea of a
> "sequence point" and switched to some more finely attributed
> variants, which might refer to value computation and side-effects
> separately.  One can say "value computation of the operand is
> sequenced before..." or "side-effects of the argument are
> sequenced after..."
>
> But a plain unattributed "sequenced before (or after)" is
> basically the old-fashioned "sequence point":  a full sequencing
> requirement that covers everything, i.e. the value computation and
> all side-effects together.
>
> The quote you provided completely compartmentalizes and isolates
> argument expression evaluation and parameter initialization.  No
> interleaving is permitted.  This is the intent.

Going just by what is stated in the C++ standard, I would say the
question is clearly ambiguous.

There is no question that the initializations of _parameters_ are
indeterminately sequenced.  But the standard does not say the
evaluations of _argument values_ are indeterminately sequenced.

The standard does use the phrase "every associated value computation
and side effect", but AFAICT there is no definition for what that
phrase means.  Perhaps it is meant to include the evaluation of
argument values, and perhaps it isn't.  What is clear is that the
C++ standard doesn't state which of those possibilities is the case.
Hence, going by just what is stated in the C++ standard, it appears
the question is clearly ambiguous.

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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