[llvm] r203734 - Fix vectorizer docs.

[Zinovy Nis]

Thanks.


2014-03-13 3:54 GMT+04:00 Arnold Schwaighofer <aschwaighofer at apple.com>:

>
> On Mar 12, 2014, at 4:36 PM, Chandler Carruth <chandlerc at google.com>
> wrote:
>
> >
> > On Wed, Mar 12, 2014 at 4:23 PM, Arnold Schwaighofer <
> aschwaighofer at apple.com> wrote:
> > Fix vectorizer docs.
> >
> > This example is not vectorized because LLVM does not prove no-wrapping of
> > "a[i*7] += ...".
> >
> > See my other email, but...
> >
> >
> > -  int foo(int *A, int *B, int n, int k) {
> > -    for (int i = 0; i < n; ++i)
> > -      A[i*7] += B[i*k];
> > +  int foo(int * A, int * B, int n) {
> > +    for (intptr_t i = 0; i < n; ++i)
> > +        A[i] += B[i*4];
> >
>
>
> > I don't believe that this can possibly change whether or not any of this
> pointer arithmetic could possibly wrap. I mean... maybe I'm missing
> something blindly obvious and just have a think-o moment... but I *really*
> don't think this is remotely possible.
> >
>
>
> Wrapping only matters for store to load safety. As I explained in the
> other email, it is a matter of LLVM not being able to prove this atm. Not
> that this example is unsafe. It is not. The page describes examples that
> LLVM can vectorize so I took this example out :).
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