On Wed, 20 May 2009, Mark Mitchell wrote:
> Richard Guenther wrote:
>
> >> void f(float *f, int *n) {
> >> for (int i = 0; i < *n; ++i) {
> >> f[i] *= 2;
> >> }
> >> }
>
> > The difference is if you want to sink a load from *n beyond the
> > store to f[i] - in which case you ask if there is an anti-dependence
> > which we cannot exclude in this case (no TBAA is allowed here).
>
> By "not allowed", you don't mean "would be an invalid optimization", but
> rather "will no longer be done by GCC", right?
Right, not invalid in the above case but nevertheless no longer being
done by GCC. This is to properly support
int i;
float f;
void foo()
{
int *p = (int *)malloc(sizeof(int));
*p = 1;
i = *p;
float *q = (float *)p;
*q = 2.0;
f = *q;
}
where we need to avoid scheduling the store via *q before the load
from *p. The above is valid as I read C99 6.5/6, it is an object
with no declared type (obtained via malloc) and has type int
due to the store via *p "for that access and for subsequent accesses
_that do not modify the stored value_." (emphasis mine). So for
objects without a declared type C can do "placement new" by simply
storing with a different type. In C++ we of course have the
usual placement new situations.
Richard.
