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Re: Repeated types in union

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Hello,

Am 12/14/2011 12:10 AM, schrieb christian.bau:
> On Dec 12, 7:09 pm, Jens Gustedt <jens.gust...@loria.fr> wrote:
> 
>> 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.
> 
> You are right. On the other hand, footnote 82 says:
> 
> "If the member used to access the contents of a union object is not
> the same as the member last used to store a value in the object, the
> appropriate part of the object representation of the value is
> reinterpreted as an object representation in the new type as described
> in 6.2.6 (a process sometimes called "type punning"). "
> 
> Which is a direct contradiction. I am assuming that the rules for
> union members apply in the same way whether the compiler knows that it
> is accessing different members of the same union or not.

I think this assumption can't be made. Generally, inside a function the
compiler has no way to know that the pointers originate from the same
object. In the contrary the aliasing rules were invented to assure that
under the given circumstances the *must* point to different objects.

And these things happen. gcc assumes (or at least there has been some
version of gcc) that they are different, even if the function is inlined
and it could deduce that both point to the same address.

(Also, footnotes in the standard are not normative)

Jens

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Thread

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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