Re: strict aliasing

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Wed, Nov 07, 2007 at 02:30:44AM +1100, skaller wrote:
> > The claim was, in the context of the example given, "Without strict
> > aliasing being enabled, the compiler can't assume that that the assignment
> > 'p[i] = 1' won't change 'p'". 
> 
> In this case I think it can. More precisely, IF the assignment 
> changes p, another rules is broken (1 isn't a pointer), and
> all bets are off, so  the compiler can go ahead and assume
> p can't be modified -- on an amd64 anyhow (since int isn't
> intptr_t).

You are implicitly assuming strict aliasing.  Strict aliasing means
that you reject a possible aliasing based on the type, as you are
doing here (1 isn't a pointer).  If you turn it off, the compiler
cannot make that assumption: the user might be doing something
strange with casts.

Now it appears that you want to make some kind of intermediate assumption
(semi-strict aliasing?), where pointers of different types are allowed to
alias while ints can't alias with pointers.  But that's a rule you're
making up yourself, without any support in the standard or in the GCC
implementation.