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Re: Repeated types in union

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Am 12/12/2011 06:49 PM, schrieb christian.bau:
> On Dec 11, 6:04 am, Barry Schwarz <schwa...@dqel.com> wrote:
> You are right, but that seems to have some awful consequences. Take
> this code:
> 
> union {
>        int a;
>        long b;
> } u;
> u.a = 3;
> printf("%ld\n", u.b);
> 
> So on an implementation where int and long have the same size and
> representation, this code would be well-defined and print "3"?
> 
> Now take this code:
> 
> void f (int* a, long* b) { *a = 3; *b = 4; *a = *a + 2; }
> 
> If I call f (&u.a, &u.b) is this required to set both to 6?
> And since the compiler doesn't know that I'm going to make this call,
> lots of optimization goes out of the window?

If I remember correctly the aliasing rules state that the compiler is
allowed to assume that a and b (insided the function) point to different
objects because they are of different types. Thus in the second
assignment to *a the compiler can assume that *a is still 3 and store 5
in place.

Jens

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Repeated types in union Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2011-12-10 22:06 +0000
  Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-11 00:11 +0100
  Re: Repeated types in union Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-10 18:14 -0500
    Re: Repeated types in union Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2011-12-12 20:09 +0000
      Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-12 23:45 +0100
      Re: Repeated types in union Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-12 19:48 -0500
        Re: Repeated types in union "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-12-13 15:17 -0800
          Re: Repeated types in union Keith Thompson <kst-u@mib.org> - 2011-12-13 15:29 -0800
          Re: Repeated types in union James Kuyper <jameskuyper@verizon.net> - 2011-12-13 18:30 -0500
          Re: Repeated types in union Noob <root@127.0.0.1> - 2011-12-14 17:40 +0100
          Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:28 -0800
        Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 15:04 -0800
    Re: Repeated types in union ralph <nt_consulting64@yahoo.net> - 2011-12-12 14:50 -0600
    Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 14:53 -0800
  Re: Repeated types in union Barry Schwarz <schwarzb@dqel.com> - 2011-12-10 22:04 -0800
    Re: Repeated types in union "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-12-12 09:49 -0800
      Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-12 20:09 +0100
        Re: Repeated types in union "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-12-13 15:10 -0800
          Re: Repeated types in union Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-14 00:24 +0100
          Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:46 -0800
      Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:42 -0800
    Re: Repeated types in union Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 13:36 -0800

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