Re: what is defined, was for or against equality

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.compilers
Subject	Re: what is defined, was for or against equality
Date	2022-01-13 08:24 +0100
Organization	A noiseless patient Spider
Message-ID	<22-01-050@comp.compilers> (permalink)
References	(7 earlier) <22-01-041@comp.compilers> <22-01-044@comp.compilers> <22-01-045@comp.compilers> <22-01-046@comp.compilers> <22-01-048@comp.compilers>

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On 12/01/2022 20:02, Thomas Koenig wrote:
> gah4 <gah4@u.washington.edu> schrieb:
>> On Tuesday, January 11, 2022 at 11:47:26 AM UTC-8, Kaz Kylheku wrote:

>> 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 was under the impression that FORTRAN compilers typically put data in
the ".common" section of object files.  A key difference between .common
and .bss is that (with standard linker setup) duplicate symbols in .bss
are an error, while duplicate symbols in .common are merged.  But in C
startup code, .common is also zeroed (FORTRAN may have different startup
code here - with no experience of the language, I don't know such details).

The use of ".common" by C compilers such as gcc was common practice
precisely to improve compatibility with FORTRAN in the early days, and
it let people write "int global_x;" in headers and have everything work,
rather than the correct practice of "extern int global_x;" in headers
and a single "int global_x;" in one object file.  The big disadvantages
are that if you have "int local_x;" in two files, and don't use static,
they'll be merged with no error, and if you have "int global_x;" in one
file and "double global_x;" in another, it's a mess.  Modern gcc now
uses "-fno-common" to avoid this.

>
>> 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.

AFAIUI the difference in aliasing rules is that in FORTRAN, pointer or
array parameters are assumed not to alias, while in C the compiler must
assume that they might alias, unless you use "restrict".  Are there
other differences?

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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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