[Bug libstdc++/29286] [4.0/4.1/4.2/4.3 Regression] placement new does not change the dynamic type as it should

[rguenther at suse dot de]

------- Comment #126 from rguenther at suse dot de  2007-05-23 14:43 -------
Subject: Re:  [4.0/4.1/4.2/4.3 Regression] placement
 new does not change the dynamic type as it should

On Wed, 23 May 2007, gdr at cs dot tamu dot edu wrote:

> ------- Comment #125 from gdr at cs dot tamu dot edu  2007-05-23 14:22 -------
> Subject: Re:  [4.0/4.1/4.2/4.3 Regression] placement new does not change the
> dynamic type as it should
> 
> "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org> writes:
> 
> [...]
> 
> | But you can still perform hoisting loads of incoming pointer arguments
> | and sinking stores to incoming pointer arguments.  Please read comment 
> | #105 and come up with a testcase where we wouldn't be allowed to do
> | a useful transformation we do now.  So I believe making placement new
> | work with our current scheme will severely pessimize placement new
> | users, but if we slightly change rules for everyone we'll be all happy.
> 
> Update.
> 
> The only comment I have so far on the -core reflector is to the effect
> that the reading that the program I posted earlier violates NO
> aliasing rule.  I'll follow with the proposal to bring the rules in
> line with recent C99 rules.

Note that it is important to retain the capability to implement
memory allocators in C++ that are allowed to re-use memory for different
typed objects.  Which is not possible with C99 rules.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286