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Groups > comp.std.c > #1378

Re: Structure Packing.

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Hans-Bernhard Bröker <HBBroeker@t-online.de> writes:
> On 12.06.2012 23:44, Walter Banks wrote:
> > Eric Sosman wrote:
> 
> >>       Perhaps structs are not the right tool for the job?  Instead of
> >>
> >>          struct {
> >>              unsigned char flag;
> >>              double value;
> >>          } data[1000];
> >>
> >> ... you might do better to write
> >>
> >>          unsigned char data_flag[1000];
> >>          double data_value[1000];
> 
> Or the slightly less obscure
> 
> 	struct {
> 	    unsigned char flag[1000];
> 	    double value[1000];
> 	} data;
> 
> I guess one big reason that array-of-structs is favoured over this
> struct-of-arrays approach quite a lot these days is that all those
> caches favour locality over dense packing.

Hopping from index to index or running through linked lists to only
access a limited subset of the fields would have improved locality
as well as dense packing. A locality of reference argument which
assumes that each time you want to access any part of a struct you
want to access it all is a very flawed one. However, that's highly
domain specific, and neither the language nor the compiler can 
assist you post-facto, you simply have to select the data structure
you think (or measure) is best. OK, it helps by letting you 
abstract the data layout via zero-cost accessor functions (or macros
if you're stuck in the 80s), but some might find that too clumsy, 
YMMV.

Phil
-- 
> I'd argue that there is much evidence for the existence of a God.
Pics or it didn't happen.
-- Tom (/. uid 822)

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Thread

Structure Packing. John Kearney <dethrophes@googlemail.com> - 2012-06-06 17:27 -0700
  Re: Structure Packing. James Kuyper <jameskuyper@verizon.net> - 2012-06-06 21:19 -0400
    Re: Structure Packing. Walter Banks <walter@bytecraft.com> - 2012-06-11 22:39 -0400
      Re: Structure Packing. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-12 09:21 -0400
        Re: Structure Packing. Walter Banks <walter@bytecraft.com> - 2012-06-12 11:51 -0400
          Re: Structure Packing. Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2012-06-12 18:37 +0200
            Re: Structure Packing. Walter Banks <walter@bytecraft.com> - 2012-06-12 15:06 -0400
              Re: Structure Packing. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-12 16:22 -0400
                Re: Structure Packing. Walter Banks <walter@bytecraft.com> - 2012-06-12 17:44 -0400
                Re: Structure Packing. Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2012-06-13 15:53 +0200
                Re: Structure Packing. Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-06-18 12:37 +0300
        Re: Structure Packing. Martin Shobe <martin.shobe@yahoo.com> - 2012-06-12 16:22 +0000
          Re: Structure Packing. Walter Banks <walter@bytecraft.com> - 2012-06-12 15:24 -0400
            Re: Structure Packing. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-12 16:33 -0400
              Re: Structure Packing. Walter Banks <walter@bytecraft.com> - 2012-06-12 17:17 -0400
  Re: Structure Packing. Keith Thompson <kst-u@mib.org> - 2012-06-06 18:35 -0700
    Re: Structure Packing. John Kearney <dethrophes@googlemail.com> - 2012-06-06 21:58 -0700
      Re: Structure Packing. Keith Thompson <kst-u@mib.org> - 2012-06-06 23:54 -0700
        Re: Structure Packing. John Kearney <dethrophes@googlemail.com> - 2012-06-07 07:47 -0700
          Re: Structure Packing. James Kuyper <jameskuyper@verizon.net> - 2012-06-07 12:39 -0400
    Re: Structure Packing. jacob navia <jacob@spamsink.net> - 2012-06-07 11:14 +0200
      Re: Structure Packing. Keith Thompson <kst-u@mib.org> - 2012-06-07 09:46 -0700
        Re: Structure Packing. John Kearney <dethrophes@googlemail.com> - 2012-06-07 11:57 -0700

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