Re: [PATCH][RFC] Adjust the middle-end memory model

[Richard Guenther <rguenther@suse.de>]

On Tue, 19 May 2009, Mark Mitchell wrote:

> Richard Guenther wrote:
> 
> >>  - The tree alias oracle got similar functionality, refs_anti_dependent
> >>    and refs_output_dependent and the tree level data dependence
> >>    analysis code makes use of these.
> 
> Do we still use TBAA for the original motivating reason for adding it,
> e.g.,:
> 
>   void f(float *f, int *n) {
>     for (int i = 0; i < *n; ++i) {
>       f[i] *= 2;
>     }
>   }
> 
> where here you want to know that "f[i]" does not modify "*n"?
> 
> (Yes, that code is kinda hokey, in that real-world code would probably
> not pass n by-reference, but of course this happens with structures and
> such...)

Yes, for the purpose of hoisting the load of *n out of the loop (if
a store to f[i] would clobber *n then you wouldn't be allowed to read
it back as int).  What matters for hoisting loads is whether there
is a true dependence between *n and f[i] which there is not, as we
still disambiguate using TBAA for true dependence queries.

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).

The latter is to make placement new and friends work.

Richard.