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To: Dale Johannesen <dalej@apple.com>
Cc: Andrew Haley <aph@redhat.com>, "D. Hugh Redelmeier" <hugh@mimosa.com>,
        gcc@gcc.gnu.org, Nathan Sidwell <nathan@codesourcery.com>,
        Mike Stump <mrs@apple.com>
Subject: Re: volatile semantics
References: <851D2CB0-93DF-4C49-A6A8-8895DB1A08F9@apple.com>
	<42778D99.7070904@codesourcery.com>
	<b0f8ff9938df57bbfda0ec3aea4fc02b@apple.com>
	<Pine.LNX.4.61.0507161250120.1478@redshift.mimosa.com>
	<17113.17689.558864.411876@zapata.pink>
	<a315399723d359b46360b94cbdcb1af4@apple.com>
From: Gabriel Dos Reis <gdr@integrable-solutions.net>
In-Reply-To: <a315399723d359b46360b94cbdcb1af4@apple.com>
Date: Sat, 16 Jul 2005 21:17:00 -0000
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X-SW-Source: 2005-07/txt/msg00681.txt.bz2

Dale Johannesen <dalej@apple.com> writes:

| >  the type of an object
| > changes depending on how it is accessed.
| 
| this also makes nonsense of gcc's implementation of type-based aliasing
| rules.
| 
|    *((int *)&x) = 3

No.  That one is specifically covered by the C and C++  standards
(although they use different terminologies).  C uses the notion of
*effective type* to formulate the rule.


       [#6] The effective type of an object for an  access  to  its
       stored value is the declared type of the object, if any.72)
       If a value is stored into an object having no declared  type
       through  an  lvalue  having  a  type that is not a character
       type, then the type of the lvalue becomes the effective type
       of  the  object  for that access and for subsequent accesses
       that do not modify the stored value.  If a value  is  copied
       into  an  object  having  no  declared  type using memcpy or
       memmove, or is copied as an array of  character  type,  then
       the  effective  type  of the modified object for that access
       and for subsequent accesses that do not modify the value  is
       the  effective  type  of  the object from which the value is
       copied, if it has one.  For all other accesses to an  object
       having no declared type, the effective type of the object is
       simply the type of the lvalue used for the access.


-- Gaby