Re: what is defined, was for or against equality

From	Thomas Koenig <tkoenig@netcologne.de>
Newsgroups	comp.compilers
Subject	Re: what is defined, was for or against equality
Date	2022-01-12 19:02 +0000
Organization	news.netcologne.de
Message-ID	<22-01-048@comp.compilers> (permalink)
References	(6 earlier) <22-01-038@comp.compilers> <22-01-041@comp.compilers> <22-01-044@comp.compilers> <22-01-045@comp.compilers> <22-01-046@comp.compilers>

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gah4 <gah4@u.washington.edu> schrieb:
> On Tuesday, January 11, 2022 at 11:47:26 AM UTC-8, Kaz Kylheku wrote:
>
> (big snip)
>
>> This leaves a lot of room for Fortran and C to have entirely different
>> defined/undefined behaviors.
>
>> Even the front end for one single language can have a lot of switches
>> affecting what is defined or not.
>
> I suppose so.  But more usual, the compiler works to the least
> common denominator.
>
> For one, C requires static variables, and especially external ones, to
> initialize to zero, but Fortran doesn't.  Fortran compilers that use C
> compiler middle and back ends, tend to zero such variables.

This is more a matter of operating system and linker conventions
than of compilers.

Looking at the ELF standard, one finds

.bss

This section holds uninitialized data that contribute to the program's
memory image. By definition, the system initializes the data with zeros
when the program begins to run. The section occupies no file space, as
indicated by the section type, SHT_NOBITS.

which, unsurprisingly, matches exactly what C is doing.

Anybody who writes a Fortran compiler for an ELF system will
use .bss for COMMOM blocks, because it is easiest.  Initialization
with zeros then happens automatically.

> I suspect that there are many more that I don't know about.
> As long as the cost is small, and it satisfies both standards,
> not much reason not to do it.
>
> Fortran has stricter rules on aliasing than C.  I don't actually know
> about any effect on C programs, though, but it might be that
> compilers do the same for C.

The rules are different, and unless C is the intermediate language,
a good compiler will hand the corresponding hints to the middle end.
[I have used Fortran systems that initialized otherwise undefined data to a value that would
trap, to help find use-before-set errors.  -John]

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Re: for or against equality, was Why are ambiguous grammars usually a bad idea? Martin Ward <martin@gkc.org.uk> - 2022-01-05 10:25 +0000
  Re: for or against equality, was Why are ambiguous grammars usually a bad idea? David Brown <david.brown@hesbynett.no> - 2022-01-06 09:11 +0100
    Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-06 16:43 +0000
      Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-07 12:06 +0100
      Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-07 13:21 +0000
        Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-08 09:31 +0000
          Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-08 22:28 +0000
            Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-09 00:09 +0000
              Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-09 21:30 +0000
          Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-09 23:00 +0100
            Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-10 12:04 +0000
              Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-11 18:16 +0100
                Re: what is defined, was for or against equality Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-11 19:19 +0000
                Re: what is defined, was for or against equality gah4 <gah4@u.washington.edu> - 2022-01-11 14:18 -0800
                Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-12 19:02 +0000
                Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-13 08:24 +0100
                Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-13 11:17 +0000
          Re: what is defined, was for or against equality gah4 <gah4@u.washington.edu> - 2022-01-10 16:58 -0800
    Re: for or against equality, was Why are ambiguous grammars usually a bad idea? Robert Prins <robert@prino.org> - 2022-01-06 19:07 +0000
    Undefined behaviour, was: for or against equality Martin Ward <martin@gkc.org.uk> - 2022-01-07 14:02 +0000
      Re: Undefined behaviour, was: for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-08 03:41 +0000
    Re: Undefined behaviour, was: for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-07 15:56 +0100
      Re: Undefined behaviour, was: for or against equality anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2022-01-08 17:52 +0000
        Re: Undefined behaviour, was: for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-09 23:53 +0100
        Re: Undefined behaviour, was: for or against equality Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-11 16:55 +0000
          Re: Undefined behaviour, was: for or against equality George Neuner <gneuner2@comcast.net> - 2022-01-11 22:01 -0500

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