On Fri, Jul 20, 2018 at 3:57 AM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> We could vectorise:
>
> for (...)
> {
> a[0] = ...;
> a[1] = ...;
> a[2] = ...;
> a[3] = ...;
> a += stride;
> }
>
> (including the case when stride == 8) but not:
>
> for (...)
> {
> a[0] = ...;
> a[1] = ...;
> a[2] = ...;
> a[3] = ...;
> a += 8;
> }
>
> (where the stride is always 8). The former was treated as a "grouped
> and strided" store, while the latter was treated as grouped store with
> gaps, which we don't support.
>
> This patch makes us treat groups of stores with gaps at the end as
> strided groups too. I tried to go through all uses of STMT_VINFO_STRIDED_P
> and all vector uses of DR_STEP to see whether there were any hard-baked
> assumptions, but couldn't see any. I wondered whether we should relax:
>
> /* We do not have to consider dependences between accesses that belong
> to the same group, unless the stride could be smaller than the
> group size. */
> if (DR_GROUP_FIRST_ELEMENT (stmtinfo_a)
> && (DR_GROUP_FIRST_ELEMENT (stmtinfo_a)
> == DR_GROUP_FIRST_ELEMENT (stmtinfo_b))
> && !STMT_VINFO_STRIDED_P (stmtinfo_a))
> return false;
>
> for cases in which the step is constant and the absolute step is known
> to be greater than the group size, but data dependence analysis should
> already return chrec_known for those cases.
>
> The new test is a version of vect-avg-15.c with the variable step
> replaced by a constant one.
>
> A natural follow-on would be to do the same for groups with gaps in
> the middle:
>
> /* Check that the distance between two accesses is equal to the type
> size. Otherwise, we have gaps. */
> diff = (TREE_INT_CST_LOW (DR_INIT (data_ref))
> - TREE_INT_CST_LOW (prev_init)) / type_size;
> if (diff != 1)
> {
> [...]
> if (DR_IS_WRITE (data_ref))
> {
> if (dump_enabled_p ())
> dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> "interleaved store with gaps\n");
> return false;
> }
>
> But I think we should do that separately and see what the fallout
> from this change is first.
>
> Tested on aarch64-linux-gnu (with and without SVE), aarch64_be-elf
> and x86_64-linux-gnu. OK to install?
>
> Richard
>
>
> 2018-07-20 Richard Sandiford <richard.sandif...@arm.com>
>
> gcc/
> * tree-vect-data-refs.c (vect_analyze_group_access_1): Convert
> grouped stores with gaps to a strided group.
>
This patch caused:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87214
H.J.
