Re: Undefined Behavior Optimizations in C

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.compilers
Subject	Re: Undefined Behavior Optimizations in C
Date	2023-01-20 13:54 -0800
Organization	None to speak of
Message-ID	<23-01-068@comp.compilers> (permalink)
References	(2 earlier) <23-01-031@comp.compilers> <23-01-041@comp.compilers> <23-01-062@comp.compilers> <23-01-063@comp.compilers> <23-01-066@comp.compilers>

Show all headers | View raw

gah4 <gah4@u.washington.edu> writes:
[snip]
> For many years, C allowed for sign-magnitude and ones' complement
> representation.

It still does.  The upcoming 2023 ISO C standard mandates 2's-complement
for signed integer types, but it hasn't been published yet.

[...]

> Now, C has unsigned int which you can use when you need specific
> overflow behavior (except on some Unisys machines).

If a Unisys C implementation doesn't behave as the standard requires
with respect to unsigned overflow, then it's a non-conforming
implementation.  (Non-conforming implementations can still be useful.)

[...]

> As C allows for both UB and system-dependent behavior, and it is
> hard for people to remember every case of each one, it is unreasonable
> to me, to assume fixed point overflow is UB.

C has:
- Undefined behavior (the standard imposes no requirements);
- Unspecified behavior (the standard provides 2 or more possibilities
  and implementations can choose arbitrarily; an example is the order of
  evaluation of function arguments); and
- Implementation-defined behavior (unspecified behavior where the
  implementation must document its choice).

Signed integer overflow has undefined behavior -- not because it is or
isn't reasonable, but because the standard says so.

Even in C23, the mandate for 2's-complement representation doesn't
imply a requirement for 2's-complement behavior on overflow.

[...]

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for XCOM Labs
void Void(void) { Void(); } /* The recursive call of the void */

Back to comp.compilers | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

Undefined Behavior Optimizations in C "Lucian Popescu" <lucic71@ctrl-c.club> - 2023-01-05 10:05 +0000
  RE: Undefined Behavior Optimizations in C "Nuno Lopes" <nuno.lopes@tecnico.ulisboa.pt> - 2023-01-05 10:24 +0000
  Re: Undefined Behavior Optimizations in C Spiros Bousbouras <spibou@gmail.com> - 2023-01-05 18:06 +0000
    Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-05 16:22 -0800
      Re: Undefined Behavior Optimizations in C anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2023-01-06 08:41 +0000
      Re: Undefined Behavior Optimizations in C David Brown <david.brown@hesbynett.no> - 2023-01-06 16:12 +0100
        Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-06 10:33 -0800
          Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-06 11:39 -0800
          Re: Undefined Behavior Optimizations in C Spiros Bousbouras <spibou@gmail.com> - 2023-01-07 12:10 +0000
            Re: Undefined Behavior Optimizations in C antispam@math.uni.wroc.pl - 2023-01-13 20:46 +0000
        Re: Undefined Behavior Optimizations in C Kaz Kylheku <864-117-4973@kylheku.com> - 2023-01-09 10:14 +0000
          Re: Re: Undefined Behavior Optimizations in C Jon Chesterfield <jonathanchesterfield@gmail.com> - 2023-01-10 10:46 +0000
            Re: Undefined Behavior Optimizations in C Thomas Koenig <tkoenig@netcologne.de> - 2023-01-11 09:34 +0000
              Re: Undefined Behavior Optimizations in C Kaz Kylheku <864-117-4973@kylheku.com> - 2023-01-12 05:21 +0000
                Re: Undefined Behavior Optimizations in C Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-01-12 12:21 -0800
                Re: Undefined Behavior Optimizations in C Thomas Koenig <tkoenig@netcologne.de> - 2023-01-12 21:50 +0000
                Re: Undefined Behavior Optimizations in C Kaz Kylheku <864-117-4973@kylheku.com> - 2023-01-15 04:17 +0000
            Re: Undefined Behavior Optimizations in C David Brown <david.brown@hesbynett.no> - 2023-01-11 14:20 +0100
              Re: Undefined Behavior Optimizations in C Spiros Bousbouras <spibou@gmail.com> - 2023-01-18 13:14 +0000
                Re: Undefined Behavior Optimizations in C David Brown <david.brown@hesbynett.no> - 2023-01-18 21:14 +0100
                Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-18 21:10 -0800
                Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-20 10:45 -0800
                Re: Undefined Behavior Optimizations in C Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-01-20 13:54 -0800
                Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-23 18:50 -0800
                Re: Undefined Behavior Optimizations in Fortran "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2023-01-26 21:12 +0000
                Re: Undefined Behavior Optimizations in Fortran gah4 <gah4@u.washington.edu> - 2023-01-26 17:50 -0800
                Re: Undefined Behavior Optimizations in C "Alexei A. Frounze" <alexfrunews@gmail.com> - 2023-01-19 21:18 -0800
                Re: Undefined Behavior Optimizations in C Thomas Koenig <tkoenig@netcologne.de> - 2023-01-20 20:42 +0000
                Re: Undefined Behavior Optimizations in C anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2023-01-21 11:54 +0000
                Re: Undefined Behavior Optimizations in C anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2023-01-22 09:56 +0000
                Re: Undefined Behavior Optimizations in C Kaz Kylheku <864-117-4973@kylheku.com> - 2023-01-22 07:04 +0000
                Re: Undefined Behavior Optimizations in C Martin Ward <martin@gkc.org.uk> - 2023-01-23 17:12 +0000
          Re: Undefined Behavior Optimizations in C David Brown <david.brown@hesbynett.no> - 2023-01-10 17:32 +0100
            Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-10 15:57 -0800
              Re: Undefined Behavior Optimizations in C David Brown <david.brown@hesbynett.no> - 2023-01-11 14:40 +0100
                Re: Undefined Behavior Optimizations in C gah4 <gah4@u.washington.edu> - 2023-01-11 16:09 -0800
            Re: Undefined Behavior Optimizations in C dave_thompson_2@comcast.net - 2023-01-28 10:35 -0500
  Re: Undefined Behavior Optimizations in C anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2023-01-06 07:47 +0000
  Re: Undefined Behavior Optimizations in C Kaz Kylheku <864-117-4973@kylheku.com> - 2023-01-09 09:10 +0000

csiph-web