Re: [boost] aligned_storage in unions

Subject: Re: [boost] aligned_storage in unions
From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2010-09-22 16:28:34

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On Wednesday 22 September 2010, Larry Evans wrote:
> On 09/22/10 14:04, Frank Mori Hess wrote:

> > I don't see how that gets around the anti-aliasing rules.  The compiler
> > is allowed to assume incompatible pointers won't _ever_ alias.  So just
> > because your program does things in a certain order doesn't mean the
> > compiler isn't allowed to use the anti-aliasing assumptions to reorder
> > things in a nasty way.  Couldn't it look at the destruction of the old
> > object and the construction of the new object as two completely
> > independent actions it is free to reorder or interleave?
>
> I hadn't thought of the compiler reordering the calls.
> Just to be clear, the compiler, using the anti-aliasing rules,
> could reorder:
>
>   316	        destroy();
>   317	        assign_copy(from);
>
> to:
>
>   317	        assign_copy(from);
>   316	        destroy();
>
> ?  Or maybe just do everything in destroy before the lhs->~Lhs() call,
> then execute assign_copy(from) and then do lhs->~Lhs().  Hmm...
> OK, I think I got it, but am surprised.

I think so, in principle.  I only have a vague idea of what optimizing 
compilers actually do under the hood though, which is why I tend to be very 
conservative about obeying the strict aliasing rules (they've bitten me 
before).

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