On Tue, Jul 19, 2011 at 8:25 AM, Ira Rosen <ira.ro...@linaro.org> wrote:
> Hi,
>
> The vectorizer performs the following alias checks for data-refs with
> unknown dependence:
>
> ((store_ptr_0 + store_segment_length_0) <= load_ptr_0)
> || (load_ptr_0 + load_segment_length_0) <= store_ptr_0))
>
> where segment_length is data-ref's step in the loop multiplied by the
> loop's number of iterations (in the general case). For invariant
> data-refs segment_length is 0, since the step is 0. This creates
> incorrect check for:
>
> for (i = 0; i < 1000; i++)
> for (j = 0; j < 1000; j++)
> a[j] = a[i] + 1;
>
> We check:
>
> &a + 4000 <= &a + i*4
> || &a + i*4 <= &a
>
> and the second check is wrong for i=0.
>
> This patch makes segment_length to be sizeof (data-ref type) in case
> of zero step, changing the checks into
>
> &a + 4000 <= &a + i*4
> || &a + i*4 + 4 <= &a
>
> Bootstrapped and tested on powerpc64-suse-linux.
> Committed revision 176434.
>
> Ira
>
> ChangeLog:
>
> PR tree-optimization/49771
> * tree-vect-loop-manip.c (vect_vfa_segment_size): In case of
> zero step, set segment length to the size of the data-ref's type.
>
> testsuite/ChangeLog:
>
> PR tree-optimization/49771
> * gcc.dg/vect/pr49771.c: New test.
>
> Index: tree-vect-loop-manip.c
> ===================================================================
> --- tree-vect-loop-manip.c (revision 176433)
> +++ tree-vect-loop-manip.c (working copy)
> @@ -2356,9 +2356,14 @@ static tree
> vect_vfa_segment_size (struct data_reference *dr, tree length_factor)
> {
> tree segment_length;
> - segment_length = size_binop (MULT_EXPR,
> - fold_convert (sizetype, DR_STEP (dr)),
> - fold_convert (sizetype, length_factor));
> +
> + if (!compare_tree_int (DR_STEP (dr), 0))
integer_zerop (DR_STEP (dr))
> + segment_length = TYPE_SIZE_UNIT (TREE_TYPE (DR_REF (dr)));
> + else
> + segment_length = size_binop (MULT_EXPR,
> + fold_convert (sizetype, DR_STEP (dr)),
> + fold_convert (sizetype, length_factor));
> +
> if (vect_supportable_dr_alignment (dr, false)
> == dr_explicit_realign_optimized)
> {
> Index: testsuite/gcc.dg/vect/pr49771.c
> ===================================================================
> --- testsuite/gcc.dg/vect/pr49771.c (revision 0)
> +++ testsuite/gcc.dg/vect/pr49771.c (revision 0)
> @@ -0,0 +1,26 @@
> +#include <stdlib.h>
> +#include <stdarg.h>
> +
> +static int a[1000];
> +
> +int
> +foo (void)
> +{
> + int j;
> + int i;
> + for (i = 0; i < 1000; i++)
> + for (j = 0; j < 1000; j++)
> + a[j] = a[i] + 1;
> + return a[0];
> +}
> +
> +int
> +main (void)
> +{
> + int res = foo ();
> + if (res != 1999)
> + abort ();
> + return 0;
> +}
> +
> +/* { dg-final { cleanup-tree-dump "vect" } } */
>
