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Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type?

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On 05/02/2012 03:35 PM, Barry Schwarz wrote:
> On Wed, 2 May 2012 09:06:22 -0700 (PDT), John Reye
> <jononanon@googlemail.com> wrote:
> 
>> Assume identical common top struct members:
>>
>> struct a {
>>  int i1;
>>  char c1;
>>  short sa1[3];
>> };
>>
>> struct b {
>>  int i2;
>>  char c2;
>>  short sa2[3];
>>
>>  int differ_here;
>> };
>>
>>
>> struct a tmp;
>>
>>
>>
>> Are the following 2 line always equivalent (as in: yielding the same
>> lvalue) and allowed:
>>
>> tmp.c1
>> ((struct b*)&tmp)->c1
> 
> Since struct b does not contain a member c1, the second line should
> produce a diagnostic. 
> 
> I appear to be in the minority.  If you change the second line to 
>      ((struct b*)&tmp)->c2
> and if the two structures are guaranteed to have the same alignment,
> then I believe the requirement in 6.5.2.3-5 (which technically only
> applies if the two structures are members of a union) would force the
> compiler to generate the appropriate code to yield the same lvalue.

It might generate a retrieval using the same offset from the base of the
struct, but the key issue is whether the value it retrieves from that
location is the one that would, otherwise be considered the "current" value.

> This would probably work everywhere except the DS9000.

So the DS9000 is the only platform that would aggressively optimize
based upon the fact that a "struct a" lvalue could never, with defined
behavior, refer to the same object as a "struct b" lvalue?
I'd thought that the best modern optimizers were more aggressive than
that, at least at their highest levels.

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Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-02 09:06 -0700
  Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? "In a very dark China Blue, China Blue condition." <chine.bleu@yahoo.com> - 2012-05-02 09:33 -0700
  Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-02 12:36 -0400
    Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-02 10:04 -0700
      Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-02 13:23 -0400
    Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 09:20 -0700
      Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 09:25 -0700
        Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 10:16 -0700
        Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 11:05 -0700
        Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 15:21 -0400
      Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 09:46 -0700
      Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Nobody <nobody@nowhere.com> - 2012-05-03 19:14 +0100
        Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 11:45 -0700
          Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-03 21:05 +0200
            Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 12:14 -0700
            Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 15:16 -0400
              Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Lanarcam <lanarcam1@yahoo.fr> - 2012-05-03 21:56 +0200
                Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 16:12 -0400
                Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 13:45 -0700
                Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Nobody <nobody@nowhere.com> - 2012-05-04 01:34 +0100
              Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-03 22:32 +0200
            Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 12:10 -0700
              Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Keith Thompson <kst-u@mib.org> - 2012-05-03 16:14 -0700
            Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:24 -0700
          Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 15:19 -0400
            Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? John Reye <jononanon@googlemail.com> - 2012-05-03 12:25 -0700
      Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-03 17:44 -0400
  Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Barry Schwarz <schwarzb@dqel.com> - 2012-05-02 12:35 -0700
    Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-05-02 14:40 -0700
    Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? James Kuyper <jameskuyper@verizon.net> - 2012-05-02 17:51 -0400
    Re: Can *common* struct-members of 2 different struct-types, that are the same for the first common members, be accessed via pointer cast to either struct-type? Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:16 -0700

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