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Re: Alignment problems

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On 12/ 2/11 09:50 AM, jacob navia wrote:
> Le 01/12/11 21:38, Alan Curry a écrit :
>> In article<jb7sfh$221$1@speranza.aioe.org>,
>> jacob navia<jacob@jspamsink.org>   wrote:
>>> Thanks to everybody tht answered. I think I will go the way
>>> Gordon Burditt proposed.
>>
>> There's another answer that we forgot:
>>
>> Put the array at the beginning of the struct. Then if the struct is malloced,
>> the array inherits its "aligned for anything" property.
>>
> I can't do that since I do not know how big the array will be.
> The user calls me with
>
> iList.Create(sizeof(double));
>
> to create a list of doubles.
>
> This is a variable length structure, allocated with
>
> 	malloc(sizeof(ListElement)+list->ElementSize);
>
> The data start immediately after the pointer.

In that case, wouldn't it have been better to declare char Data[]?

But then again, you can't use a flexible array in a nested union.

<snip>

> The problem is further complicated by the fact that under SPARC
> modern versions you do NOT need to align anything since the OS
> catches the unaligned trap, and loads the double with 2 instructions.

That's an expensive option, or at least it used to be on older 
processors.  The Sun compilers support misaligned access (through the 
-xmemalign option), but transferring to trap handler is a significant 
overhead.

> Now the problem is that modern versions of gcc (and that machine has
> 4.2 installed) follow the recommendations of Oracle (ex Sun) and
> emit ldd instead of a sequence of two ld instructions expecting
> the OS to trap that. But since that machine has an OS from May
> 2002 (imagine that, almost ten years old) that apparently does NOT
> catch that unaligned instruction.
>
> Anyway, this is a general problem since the power PC has probably
> similar problems.

The joys of portable code!

-- 
Ian Collins

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Alignment problems jacob navia <jacob@spamsink.net> - 2011-12-01 01:05 +0100
  Re: Alignment problems Ian Collins <ian-news@hotmail.com> - 2011-12-01 13:15 +1300
  Re: Alignment problems pacman@kosh.dhis.org (Alan Curry) - 2011-12-01 02:57 +0000
  Re: Alignment problems gordonb.hpln0@burditt.org (Gordon Burditt) - 2011-11-30 21:27 -0600
  Re: Alignment problems jacob navia <jacob@spamsink.net> - 2011-12-01 13:38 +0100
    Re: Alignment problems pacman@kosh.dhis.org (Alan Curry) - 2011-12-01 20:38 +0000
      Re: Alignment problems jacob navia <jacob@spamsink.net> - 2011-12-01 21:50 +0100
        Re: Alignment problems Ian Collins <ian-news@hotmail.com> - 2011-12-02 10:27 +1300
          Re: Alignment problems Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-04 00:23 +0000
            Re: Alignment problems Ian Collins <ian-news@hotmail.com> - 2011-12-04 13:31 +1300
        Re: Alignment problems ImpalerCore <jadill33@gmail.com> - 2011-12-01 14:34 -0800
  Re: Alignment problems "BartC" <bc@freeuk.com> - 2011-12-01 13:46 +0000
    Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-01 14:15 -0800
      Re: Alignment problems David Thompson <dave.thompson2@verizon.net> - 2012-01-19 02:44 -0500
  Re: Alignment problems "James" <no@spam.invalid> - 2011-12-01 19:43 -0800
  Re: Alignment problems "Brain" <Brian@myhome.com> - 2011-12-04 02:47 -0600
    Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-04 08:26 -0800
      Re: Alignment problems Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-12-08 00:12 +0200
        Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-07 22:31 -0800
          Re: Alignment problems lovecreatesbeauty <lovecreatesbeauty@gmail.com> - 2011-12-08 21:43 -0800
            Re: Alignment problems Keith Thompson <kst-u@mib.org> - 2011-12-09 01:31 -0800
              Re: Alignment problems Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-12-09 14:15 +0200

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