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