On Wed, Sep 18, 2013 at 1:23 PM, Cong Hou <co...@google.com> wrote:
> First, look as the code below.
>
>
> void foo(int* a, int* b, int n) {
> int i;
> for (i = 0; i < n; ++i)
> a[i] = *b;
> }
>
>
> This loop contains possible aliasing between a[i] and *b, and in order
> to vectorize this loop, GCC produces two versions of the loop, and
> only vectorizes the one in which there is no aliasing between a[i] and
> *b. In this version we can assert *b is a loop variant and thereby can
> hoist the load and shuffle operations outside of the loop. But the
> current implementation does not do this.
>
> If we replace *b by a stack variable then during the vectorization
> pass the load and shuffle are already hoisted. So I think we can just
> do it during the vectorization pass without passing additional
> information to other passes (e.g. pass_lim). Is it safe for us to
> assume that there is no aliasing between a variable accessed via an
> address which is a loop invariant and any other variable modified in
> the loop (the version to be vectorized)?
>
For 0-stride read accesses, it should be safe to do. If vectorizer
does not already do this hoisting for non-aliased scalars, it might be
tricky to pass the info to the aliaser or the following LIM pass.
David
>
> thanks,
> Cong
