On Fri, Jun 26, 2015 at 1:23 PM, Davin McCall <dav...@davmac.org> wrote: > On 26/06/15 12:03, Davin McCall wrote: >> >> ... The stored value of 'n' is not accessed by any other type than the >> type of n itself. This value is then cast to a different pointer type. You >> are mistaken if you think that the cast accesses the stored value of n. The >> other "stored value" access that it occurs in that expression is to the >> object pointed at by the result of the cast. [...]: > > > I'm sorry, I think that was phrased somewhat abrasively, which I did not > intend. Let me try this part again. If we by break up the expression in > order of evaluation: > > From: > return ((const struct exec_node **)n)[0] > > In order of evaluation: > > n > - which accesses the stored value of n, i.e. a value of type 'struct exec > node *', via n, which is obviously of that type. > > (const struct exec_node **)n > - which casts that value, after it has been retrieved, to another type. If > this were an aliasing violation, then casting any pointer variable to > another type would be an aliasing violation; this is clearly not the case. > > ((const struct exec_node **)n)[0] > - which de-references the result of the above cast, thereby accessing a > stored value of type 'exec node *' using a glvalue of type 'exec node *'.
I think breaking this up is a mistake, because the strict-aliasing rules is explicitly about the *combination* of these two things. You *are* accessing the underlying memory of 'n' through a different type, and this is what strict aliasing is all about. But it takes two steps, a single step isn't enough to do so. Those other spec-quotes doesn't undo the strict-aliasing definitions; knowing how things are laid out in memory doesn't mean the compiler cannot assume two differently typed variables doesn't overlap. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev