Re: who invented static?

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: who invented static?
Date	2015-09-07 21:25 +0200
Organization	A noiseless patient Spider
Message-ID	<msko74$ueg$1@dont-email.me> (permalink)
References	(8 earlier) <kfnio7nng50.fsf@x-alumni2.alumni.caltech.edu> <msjhi9$2d9$1@dont-email.me> <lny4gisiz6.fsf@kst-u.example.com> <msjodd$qjn$1@dont-email.me> <55EDC3A8.3000804@verizon.net>

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On 07/09/15 19:04, James Kuyper wrote:
> On 09/07/2015 06:22 AM, David Brown wrote:
> ...
>> However, as far as I can see, "void func(int x[static 100])" means
>> almost exactly the same to the compiler as "void func(int x[100])".  The
>> compiler is quite happy to view the declaration with the "static" as
>> being entirely compatible with those you listed without the "static",
>> and it is quite happy to have multiple declarations of the same function
>> with different static sizes of the array.
>
> The key difference is that without the "static", there's undefined
> behavior only if the code for func(p) tries to access x[i] for values of
> i such that p[i] would have undefined behavior. With "static 100", it's
> also undefined behavior to call the function with a pointer to less than
> 100 consecutive accessible 'int' objects, regardless of what func()
> actually does with x.

That's what I thought was the case, but then Tim confused me by saying 
that without the "static" there is still undefined behaviour if you call 
the function with a pointer to less than 100 ints, even if the function 
does not attempt to access them.  If I was merely mistaken in my 
interpretation of Tim (or if Time made a mistake in his description), 
then I believe I understand the point.

> I don't fully understand what opportunities this
> opens up for optimization, but if Tim says there are some, I'm willing
> to guess that he might be right.

Tim's example was that the compiler could issue cache prefetches for 
data without worrying about whether or not it was accessible.  Usually 
prefetches are not dangerous operations - even if, for example, the 
cpu's MMU was set up to trap on accesses beyond the array then a 
prefetch would typically not trap until the addresses are accessed by 
non-prefetch instructions.  But maybe on some cpus it /would/ trap.

However, other possibilities include loop unrolling, converting the loop 
to run backwards (if that happened to be more efficient), or loading 
data in larger lumps into SIMD registers.

I doubt if there are many opportunities to make significant speed 
improvements, but there always some.

> But for me, the only useful aspect of
> this feature is one not guaranteed by the standard - the possibility of
> getting a warning message during compilation of the program.
>

I agree - that's what would be most useful for me.  That is why I find 
it particularly disappointing that not only does the standard not 
specify any diagnostics, it does not even require the code to be 
consistent in the size of the static array parameters.

Thread

who invented static? fir <profesor.fir@gmail.com> - 2015-08-29 11:40 -0700
  Re: who invented static? David Kleinecke <dkleinecke@gmail.com> - 2015-08-29 13:36 -0700
    Re: who invented static? fir <profesor.fir@gmail.com> - 2015-08-29 13:46 -0700
      Re: who invented static? David Kleinecke <dkleinecke@gmail.com> - 2015-08-29 13:53 -0700
        Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-08-29 17:08 -0400
          Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-08-30 16:56 +0200
            Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-08-30 12:18 -0400
              Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-08-30 19:00 +0200
                Re: who invented static? Richard Damon <Richard@Damon-Family.org> - 2015-08-30 13:23 -0400
                Re: who invented static? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-08-30 21:18 +0100
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 19:33 -0700
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 19:26 -0700
                Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-08-30 13:40 -0400
                Re: who invented static? supercat@casperkitty.com - 2015-08-31 06:47 -0700
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 19:20 -0700
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 18:59 -0700
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-07 10:25 +0200
                Re: who invented static? Keith Thompson <kst-u@mib.org> - 2015-09-07 01:59 -0700
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-07 12:22 +0200
                Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-09-07 13:04 -0400
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-07 21:25 +0200
                Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-09-07 15:47 -0400
                Re: who invented static? Philip Lantz <prl@canterey.us> - 2015-09-07 13:27 -0700
                Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-09-07 17:31 -0400
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-08 09:16 +0200
                Re: who invented static? Keith Thompson <kst-u@mib.org> - 2015-09-07 13:27 -0700
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-08 09:12 +0200
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 16:06 -0700
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-08 09:08 +0200
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-12 11:29 -0700
                Re: who invented static? Keith Thompson <kst-u@mib.org> - 2015-09-08 10:18 -0700
                Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-09-08 13:28 -0400
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-11 10:07 -0700
                Re: who invented static? Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-12 12:30 -0700
                Re: who invented static? David Brown <david.brown@hesbynett.no> - 2015-09-13 15:48 +0200
                Re: who invented static? supercat@casperkitty.com - 2015-09-29 09:04 -0700
                Re: who invented static? Keith Thompson <kst-u@mib.org> - 2015-09-29 09:18 -0700
                Re: who invented static? supercat@casperkitty.com - 2015-09-29 09:57 -0700
        Re: who invented static? fir <profesor.fir@gmail.com> - 2015-08-29 14:18 -0700
          Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-01 01:36 -0700
    Re: who invented static? Keith Thompson <kst-u@mib.org> - 2015-08-29 14:27 -0700
  Re: who invented static? Siri Cruz <chine.bleu@yahoo.com> - 2015-08-29 14:32 -0700
    Re: who invented static? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-29 23:03 +0000
    Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-01 01:52 -0700
      Re: who invented static? Robert Wessel <robertwessel2@yahoo.com> - 2015-09-01 23:42 -0500
        Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-02 00:14 -0700
          Re: who invented static? Richard Heathfield <rjh@cpax.org.uk> - 2015-09-02 08:24 +0100
        Re: who invented static? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-09-04 04:25 +0000
          Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-04 00:45 -0700
            Re: who invented static? Robert Wessel <robertwessel2@yahoo.com> - 2015-09-04 04:09 -0500
              Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-04 02:35 -0700
                Re: who invented static? Robert Wessel <robertwessel2@yahoo.com> - 2015-09-04 23:58 -0500
                Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-05 03:02 -0700
                Re: who invented static? wssimms@gmail.com - 2015-09-05 13:14 -0700
                Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-05 13:49 -0700
                Re: who invented static? Siri Cruz <chine.bleu@yahoo.com> - 2015-09-05 14:09 -0700
                Re: who invented static? gazelle@shell.xmission.com (Kenny McCormack) - 2015-09-05 22:30 +0000
                Re: who invented static? wssimms@gmail.com - 2015-09-05 20:41 -0700
                Re: who invented static? gazelle@shell.xmission.com (Kenny McCormack) - 2015-09-06 05:28 +0000
                Re: who invented static? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-09-05 22:40 -0700
                Re: who invented static? James Kuyper <jameskuyper@verizon.net> - 2015-09-06 11:42 -0400
                Re: who invented static? Robert Wessel <robertwessel2@yahoo.com> - 2015-09-05 23:38 -0500
                Re: who invented static? raltbos@xs4all.nl (Richard Bos) - 2015-09-27 21:25 +0000
                Re: who invented static? Siri Cruz <chine.bleu@yahoo.com> - 2015-09-27 14:53 -0700
                Re: who invented static? Thomas Richter <thor@math.tu-berlin.de> - 2015-09-29 01:46 +0200
                Re: who invented static? raltbos@xs4all.nl (Richard Bos) - 2015-10-02 12:52 +0000
            Re: who invented static? Siri Cruz <chine.bleu@yahoo.com> - 2015-09-04 02:34 -0700
              Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-04 02:43 -0700
              Re: who invented static? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-09-05 01:30 +0000
                Re: who invented static? Siri Cruz <chine.bleu@yahoo.com> - 2015-09-04 20:52 -0700
                Re: who invented static? Robert Wessel <robertwessel2@yahoo.com> - 2015-09-05 00:03 -0500
                Re: who invented static? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-09-05 09:04 +0000
                Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-05 03:02 -0700
                Re: who invented static? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-09-05 04:33 -0700
                Re: who invented static? Reinhardt Behm <rbehm@hushmail.com> - 2015-09-05 20:08 +0800
                Re: who invented static? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-09-05 05:21 -0700
                Re: who invented static? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-09-05 06:37 -0700
                Re: who invented static? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-09-05 16:54 +0000
                Re: who invented static? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-05 11:06 +0100
                Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-05 03:17 -0700
                Re: who invented static? raltbos@xs4all.nl (Richard Bos) - 2015-09-05 19:14 +0000
                Re: who invented static? fir <profesor.fir@gmail.com> - 2015-09-05 12:59 -0700
            Re: who invented static? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-09-05 01:24 +0000

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