Re: [tree-ssa] RFC - tree vectorizer

[Daniel Berlin <dberlin at dberlin dot org>]

On Fri, 26 Dec 2003, Dorit Naishlos wrote:

>
>
>
>
> Hi,
>
> Here is the first implementation of a basic loop vectorizer, as described
> in http://gcc.gnu.org/projects/tree-ssa/vectorization.html. A real patch
> will follow shortly - this one is relative to an old tree-ssa snapshot and
> also includes bits from an old version of Sebastian's IV evolution
> analyzer. But I think it's in a shape that allows people to start to take a
> look and comment (please do!).
>
>       A few comments/questions:
> As I described in http://gcc.gnu.org/ml/gcc/2003-12/msg00908.html, memory
> references are vectorized using a pointer to vector type which is used to
> point to the accessed array; e.g., the following scalar code:
>       (1a) int a[N] = {0,1,2,3};
>       (2a) for (i...) {int x = a[i]; ...} #VUSE a_1
> becomes:
>       (1b) int a[N] = {0,1,2,3}
>       (2b) v4si *ptr = (v4si *)a;
>       (3b) for (i...) {v4si vx = *(ptr + 16 * i); ...}
> Unfortunately, due to the aliasing semantics in gcc, the compiler marks
> reads/writes via 'ptr' as referencing a different alias set than the
> reads/writes via 'a[i]' (because they have different types). Therefore,
> unless I use -fno-strict-aliasing, the resulting code does not run
> correctly (in the example above, the load in (3b) is hoisted before the
> stores that initialize array 'a'...). How do I force '*ptr' references into
> the same alias set as 'a[i]' references?

Using unions.
THe above violates C aliasing rules (you are casting int * to v4si *),
AFAIK.

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