Re: Struct with unaligned fields

From	Tim Prince <tprince@computer.org>
Newsgroups	comp.lang.c
Subject	Re: Struct with unaligned fields
Date	2013-08-24 17:07 -0400
Message-ID	<b7slkhFa9ciU1@mid.individual.net> (permalink)
References	(4 earlier) <kv8fsb$a09$1@dont-email.me> <kv989m$ub6$1@dont-email.me> <lnioyvec66.fsf@nuthaus.mib.org> <kvagqf$e88$1@dont-email.me> <lnbo4merqn.fsf@nuthaus.mib.org>

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On 8/24/2013 4:52 PM, Keith Thompson wrote:
> Les Cargill <lcargill99@comcast.com> writes:
>> Keith Thompson wrote:
>>> Les Cargill <lcargill99@comcast.com> writes:
>>>> Stephen Sprunk wrote:
> [...]
>>>>> Some CPUs (e.g. most RISCs) don't allow unaligned accesses at all; if
>>>>> you try it, the program crashes with a bus error.
>>>>
>>>> Perhaps I have lived a charmed life, then.
>>>> I hate to be that guy, but can I have a concrete example?
>>> [...]
>>>
>>> On a SPARC, this program:
>>>
> [snip]
>>>
>>> produces this output:
>>>
>>> ptr  = ffbff351
>>> obj.f.x = 10
>>> Bus error
>>>
>>> (I needed the union to force the `struct foo` object to be word-aligned;
>>> without it, the compiler places it at an odd address, causing the `int x`
>>> member to be word-aligned.)
>>
>> Thanks, Keith. Such sadistic hardware!
>
> Sadistic?  I don't think so.
>
> My understanding is that supporting misaligned access requires
> extra work in designing the CPU hardware.  Not supporting it left
> more resources available, both in design effort and chip area,
> to support other features (lots of registers, for example) and
> better performance.
>
> And 90+% of the time, keeping operands properly aligned isn't much
> of a problem anyway; the compiler takes care of it for you.
>
> Perhaps the tradeoffs have changed since SPARC was designed, and
> supporting unaligned access makes more sense now.  But you can see
> from my (snipped) example how convoluted your code has to be to
> trigger a problem.
>
Several current compilers use split moves to handle what appear to be 
mis-aligned access, in case the hardware support may be even worse than 
using multiple instructions, even though neither choice will fail outright.
For example, initial implementations of AVX-256 perform better when 
mis-aligned 256-bit moves are split into 128-bit move instructions. It's 
impractical to sort out whether a newer hardware platform is running 
which would perform better with a 256-bit unaligned move.
Current 512-bit wide architectures support mis-alignment only by the 
compiler choosing split moves, at a performance penalty.

-- 
Tim Prince

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Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-22 19:06 +0100
  Re: Struct with unaligned fields glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-08-22 18:18 +0000
  Re: Struct with unaligned fields James Kuyper <jameskuyper@verizon.net> - 2013-08-22 14:57 -0400
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-22 21:05 +0100
      Re: Struct with unaligned fields James Kuyper <jameskuyper@verizon.net> - 2013-08-22 16:45 -0400
      Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-22 15:12 -0700
    Re: Struct with unaligned fields glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-08-22 22:22 +0000
    Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-22 22:07 -0500
  Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-22 14:13 -0500
    Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-22 22:08 -0500
      Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 09:36 +0100
        Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 12:48 -0500
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 09:34 +0100
      Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 04:47 -0500
        Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 15:31 +0100
          Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 13:31 -0500
            Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 20:46 +0100
            Re: Struct with unaligned fields glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-08-23 20:48 +0000
  Re: Struct with unaligned fields Jorgen Grahn <grahn+nntp@snipabacken.se> - 2013-08-22 20:01 +0000
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 12:54 +0100
      Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-23 08:17 -0400
      Re: Struct with unaligned fields Jorgen Grahn <grahn+nntp@snipabacken.se> - 2013-08-23 20:00 +0000
      Re: Struct with unaligned fields Tim Rentsch <txr@alumni.caltech.edu> - 2013-08-24 16:50 -0700
  Re: Struct with unaligned fields "BartC" <bc@freeuk.com> - 2013-08-22 21:59 +0100
    Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-22 15:02 -0700
    Re: Struct with unaligned fields Tim Rentsch <txr@alumni.caltech.edu> - 2013-08-24 16:42 -0700
  Re: Struct with unaligned fields Siri Cruise <chine.bleu@yahoo.com> - 2013-08-22 14:21 -0700
  Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-22 17:23 -0400
    Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 13:03 +0100
      Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-23 08:43 -0400
        Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-23 20:20 +0100
          Re: Struct with unaligned fields James Kuyper <jameskuyper@verizon.net> - 2013-08-23 16:15 -0400
          Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-24 11:47 +1200
            Re: Struct with unaligned fields "James Harris" <james.harris.1@gmail.com> - 2013-08-24 14:01 +0100
              Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-24 14:26 -0500
                Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-25 08:55 +1200
                Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-25 04:17 -0500
                Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-25 21:22 +1200
                Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-24 16:16 -0500
                Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-25 04:43 -0500
                Re: Struct with unaligned fields David Thompson <dave.thompson2@verizon.net> - 2013-08-28 22:27 -0400
                Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-29 14:30 -0500
                Re: Struct with unaligned fields Robert Wessel <robertwessel2@yahoo.com> - 2013-08-29 16:41 -0500
      Re: Struct with unaligned fields "BartC" <bc@freeuk.com> - 2013-08-23 23:55 +0100
  Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-22 22:05 -0500
    Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 00:29 -0500
      Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 12:53 -0500
        Re: Struct with unaligned fields Stephen Sprunk <stephen@sprunk.org> - 2013-08-23 15:15 -0500
          Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 22:14 -0500
            Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-24 16:24 +1200
              Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-23 23:47 -0500
            Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-24 01:16 -0700
              Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-24 09:45 -0500
                Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-24 13:52 -0700
                Re: Struct with unaligned fields Tim Prince <tprince@computer.org> - 2013-08-24 17:07 -0400
                Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-24 16:35 -0500
                Re: Struct with unaligned fields Ian Collins <ian-news@hotmail.com> - 2013-08-25 09:51 +1200
                Re: Struct with unaligned fields Keith Thompson <kst-u@mib.org> - 2013-08-24 16:04 -0700
                Re: Struct with unaligned fields Les Cargill <lcargill99@comcast.com> - 2013-08-24 21:03 -0500
          Re: Struct with unaligned fields Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-08-24 08:22 -0400
  Re: Struct with unaligned fields Nobody <nobody@nowhere.com> - 2013-08-23 17:51 +0100
  Re: Struct with unaligned fields falk@rahul.net (Edward A. Falk) - 2013-08-24 01:43 +0000

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