global/static variables & loops

From	Rob Janecek <robs@invalid.com>
Newsgroups	comp.lang.c
Subject	global/static variables & loops
Date	2012-02-14 19:56 +0000
Organization	Aioe.org NNTP Server
Message-ID	<jhee9b$nm$1@speranza.aioe.org> (permalink)

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Hello,
In some coding advice I saw on the internet, we are urged to "avoid 
referring to global or static variables inside the tightest loops".

I also read (elsewhere) that access time to global/static and
automatic variables can be different on different hardware
(i.e., automatic variables cannot always be counted on being
faster).

What is then the justification for the above quote? Could it be
that the compiler might avoid using registers for global/static
variables? But then why could it not just copy the variable to a
register and restore it's value after the loop's over (assuming,
of course, that there are no function calls or other side
effects in that tight loop)? I can see a problem with multi-
threading but is this not a special case that could be treated
separately by the compiler only when it arises?

[I have no problem following the advice, I'm just curious why it
is true...]

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global/static variables & loops Rob Janecek <robs@invalid.com> - 2012-02-14 19:56 +0000
  Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-14 12:42 -0800
  Re: global/static variables & loops Stephen Sprunk <stephen@sprunk.org> - 2012-02-14 15:01 -0600
  Re: global/static variables & loops Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-14 20:35 -0500
    Re: global/static variables & loops Philip Lantz <prl@canterey.us> - 2012-02-14 21:18 -0800
    Re: global/static variables & loops pete <pfiland@mindspring.com> - 2012-02-15 00:53 -0500
      Re: global/static variables & loops Jonathan Leffler <jleffler@earthlink.net> - 2012-02-18 21:15 -0800
        Re: global/static variables & loops Kaz Kylheku <kaz@kylheku.com> - 2012-02-19 06:05 +0000
          Re: global/static variables & loops Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-22 03:15 +0200
        Re: global/static variables & loops Shao Miller <sha0.miller@gmail.com> - 2012-02-19 04:14 -0500
          Re: global/static variables & loops Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-19 12:32 +0000
            Re: Array 'typedef's and exclusive use of '->' versus '.' Shao Miller <sha0.miller@gmail.com> - 2012-02-19 17:00 -0500
              Re: Array 'typedef's and exclusive use of '->' versus '.' Keith Thompson <kst-u@mib.org> - 2012-02-19 16:43 -0800
                Re: Array 'typedef's and exclusive use of '->' versus '.' Shao Miller <sha0.miller@gmail.com> - 2012-02-19 20:32 -0500
              Re: Array 'typedef's and exclusive use of '->' versus '.' Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-22 03:27 +0200
                Re: Array 'typedef's and exclusive use of '->' versus '.' Shao Miller <sha0.miller@gmail.com> - 2012-02-25 02:19 -0500
                Re: Array 'typedef's and exclusive use of '->' versus '.' Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-25 10:14 +0200
    Re: global/static variables & loops "BartC" <bc@freeuk.com> - 2012-02-15 10:47 +0000
      Re: global/static variables & loops Kaz Kylheku <kaz@kylheku.com> - 2012-02-15 11:42 +0000
        Re: global/static variables & loops "BartC" <bc@freeuk.com> - 2012-02-15 12:54 +0000
        Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-15 05:17 -0800
      Re: global/static variables & loops Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-22 03:28 +0200
    Re: global/static variables & loops August Karlstrom <fusionfile@gmail.com> - 2012-02-15 15:19 +0100
    Re: global/static variables & loops Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-22 03:12 +0200
  Re: global/static variables & loops Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-15 01:23 -0800
    Re: global/static variables & loops August Karlstrom <fusionfile@gmail.com> - 2012-02-15 14:03 +0100
      Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-15 05:20 -0800
        Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 08:41 -0500
          Re: global/static variables & loops Kaz Kylheku <kaz@kylheku.com> - 2012-02-16 00:20 +0000
            Re: global/static variables & loops Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-15 21:02 -0500
              Re: global/static variables & loops "BartC" <bc@freeuk.com> - 2012-02-16 10:03 +0000
                Re: global/static variables & loops Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-16 07:58 -0500
              Re: global/static variables & loops Markus Wichmann <nullplan@gmx.net> - 2012-02-19 20:44 +0100
                Re: global/static variables & loops Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-19 15:37 -0500
                Re: global/static variables & loops Kaz Kylheku <kaz@kylheku.com> - 2012-02-22 05:01 +0000
                Re: global/static variables & loops Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-22 08:12 -0500
    Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 15:55 +0100
      Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 11:05 -0500
      Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-15 09:13 -0800
        Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 18:25 +0100
          Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 13:25 -0500
            Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-15 10:51 -0800
              Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 14:45 -0500
              Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 20:47 +0100
                Re: global/static variables & loops Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-16 04:45 -0800
                Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-16 14:25 +0100
                Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-16 08:10 -0800
                Re: global/static variables & loops Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-17 00:59 -0800
            Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 20:30 +0100
              Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 15:00 -0500
                Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 21:30 +0100
                Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 16:00 -0500
                Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 23:10 +0100
                Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-15 15:50 -0800
                Re: global/static variables & loops James Kuyper <jameskuyper@verizon.net> - 2012-02-15 19:50 -0500
                Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-16 14:36 +0100
            Re: global/static variables & loops Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-16 03:29 -0800
      Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-15 23:37 +0100
      Re: global/static variables & loops Kaz Kylheku <kaz@kylheku.com> - 2012-02-16 00:23 +0000
        Re: global/static variables & loops Anders Wegge Keller <wegge@wegge.dk> - 2012-02-16 01:29 +0100
        Re: global/static variables & loops Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-15 21:20 -0500
      Re: global/static variables & loops Shao Miller <sha0.miller@gmail.com> - 2012-02-15 22:24 -0500
        Re: global/static variables & loops Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-16 01:08 -0800
      Re: global/static variables & loops Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-16 03:17 -0800

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