[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 #106 from mark at codesourcery dot com  2007-05-22 16:04 -------
Subject: Re:  [4.0/4.1/4.2/4.3 Regression] placement
 new does not change the dynamic type as it should

rguenth at gcc dot gnu dot org wrote:

>  - we _cannot_ sink loads across stores.
> 
>      x = *int;
>      *double = 1.0;
> 
>    the store to double may change the dynamic type of what *int
>    points to.

To be clear, you mean something like this, right:

  int i;
  int *ip = &i;
  double *dp = (double *)&i;
  int x;
  x = *ip;
  *dp = 1.0;

?

I think that considering this code valid, and, therefore, forbidding the
interchange of the last two statements, requires a perverse reading of
the standard.  Placement new allows you to change the dynamic type of
storage; I don't think that just writing through a pointer does.  A key
goal of C++ relative to C was better type-safety.  The placement new
operator provides a facility for explicitly controlling object lifetime,
for programmers that need this.

Before we do anything to support the case above, we should have a
crystal-clear ruling from the committee that says this is valid.
Otherwise, this is exactly the kind of optimization that TBAA was
designed to perform.

For history, the reason I implemented TBAA in GCC was that the SGI
MIPSPro C/C++ compiler did these kinds of optimizations ten years ago,
and I was trying to catch us up when looking at POOMA performance on
IRIX.  G++ has had the freedom to interchange those stores for a long
time, and I believe it should continue to have that choice.


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