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

["mark at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #111 from mark at codesourcery dot com  2007-05-22 17:37 -------
Subject: Re:  [4.0/4.1/4.2/4.3 Regression] placement
 new does not change the dynamic type as it should

gdr at cs dot tamu dot edu wrote:

> | No, I do not.  And GCC historically has not; you are only allowed to use
> | the union for type-punning if the accesses are through the union
> | directly. 
> 
> I am not talking of the GCC's historical behaviour here, but what the
> standard actually says.  For the object "t", above the last write was
> to the double field, therefore the read is well-defined.

Suffice it to say that I disagree.  I'm not debating that you can read
the standard that way.  But, I don't think the standard contemplated
these issues in sufficient detail to make it useful in this respect.

Pragmatically, I don't think that we should change GCC, after years of
people using it with the current rules, to make it generate inferior
code -- without clear guidance from the standards committee.  IMO, that
needs to go beyond a reading of the current standard; there needs to be
a clear expression from the committee that, indeed, the compiler cannot
use TBAA in the way that GCC has historically used it.

I'm all for bringing G++ into better conformance with the standard, and
agree that correctness is more important than optimization, but I don't
believe that the standard was written with these considerations in mind,
so I don't think it can be relied upon in this respect.


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