------- Comment #77 from mark at codesourcery dot com 2007-05-17 00:41 ------- Subject: Re: [4.0/4.1/4.2/4.3 Regression] placement new does not change the dynamic type as it should
ian at airs dot com wrote: > I don't believe that the C++ standards writers really meant to eliminate > TBAA. > And that is the inevitable consequence of the dynamic memory type approach if > you are allowed to change the dynamic type in a function. I agree with Ian. I think there is good evidence that the authors of the standard intended C++ to be *more* typesafe than C, and we should read the standard in that way. It's unfortunate that neither the C or C++ standards is very precise about various aspects of the memory model, but that is what it is. I think trying to read the standard to divine the answers to these questions is essentially futile; this is a situation where we should accept that the standard doesn't say, and do our best to balance performance with existing practice and expectations. I don't fully understand the point of Comment #73. I thought the whole point of this series of patches was to make the compiler understand that memory returned by placement new could alias other memory, or to otherwise introduce a barrier that would prevent the compiler from reordering accesses across the call to operator new. If that's the case, why does the post-patch compiler still think that the writes to "f" and "l" don't alias? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286
