[PATCH GCC][09/13]Simply cost model merges partitions with the same references

[Richard Biener]

On Fri, Jun 23, 2017 at 12:19 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Mon, Jun 19, 2017 at 4:20 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Mon, Jun 19, 2017 at 3:40 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>> On Wed, Jun 14, 2017 at 2:54 PM, Richard Biener
>>> <richard.guenther@gmail.com> wrote:
>>>> On Mon, Jun 12, 2017 at 7:03 PM, Bin Cheng <Bin.Cheng@arm.com> wrote:
>>>>> Hi,
>>>>> Current primitive cost model merges partitions with data references sharing the same
>>>>> base address.  I believe it's designed to maximize data reuse in distribution, but
>>>>> that should be done by dedicated data reusing algorithm.  At this stage of merging,
>>>>> we should be conservative and only merge partitions with the same references.
>>>>> Bootstrap and test on x86_64 and AArch64.  Is it OK?
>>>>
>>>> Well, I'd say "conservative" is merging more, not less.  For example
>>>> splitting a[i+1] from a[i]
>>>> would be bad(?), so I'd see to allow unequal DR_INIT as "equal" for
>>>> merging.  Maybe
>>>> DR_INIT within a cacheline or so.
>>>>
>>>> How many extra distributions in say SPEC do you get from this change alone?
>>> Hi,
>>> I collected data for spec2006 only with/without this patch.  I am a
>>> bit surprised that it doesn't change the number of distributed loops.
>>>>
>>>> It shows also that having partition->reads_and_writes would be nice
>>>> ...  the code duplication
>>> Yeah, I merged read/write data references in previous patch, now this
>>> duplication is gone.  Update patch attached.  Is it OK?
>>
>> +      gcc_assert (i < datarefs_vec.length ());
>> +      dr1 = datarefs_vec[i];
>>
>> these asserts are superfluous -- vec::operator[] does them as well.
>>
>> Ok if you remove them.
> Done.
> I realized I made mistakes when measuring the impact of this patch.
> This patch only apparently causes failure of
> gcc.dg/tree-ssa/ldist-6.c, so here is the updated patch.  I also
> collected the number of distributed loops in spec2k6 as below:
>      trunk:  5882
>      only this patch: 7130
>      whole patch series: 5237
> So the conclusion is, this patch does aggressive distribution like
> ldist-6.c, which means worse data-locality.  The following patch does
> more fusion which mitigates impact of this patch and results in
> conservative distribution overall.

What changed in the patch?  Did you attach the correct one?

I'm not sure ldist-6.c is a "valid" testcase but I didn't try to see
where it was reduced from.

>   But as we lack of data locality
> cost model, ldist-6.c remains failed even after applying whole patch
> series.  Hmm, a cache-sensitive cost model is need for several passes
> now, distribution, prefetch and (possible) interchange.
> Richard, do you have second comment based on the new data?

I expected the "only this patch" result somewhat, as said, I'd have
allowed "related" references to fuse by not requiring equal
DR_INIT for example.

I suggest to go forward with it in its current form.  We can tweak the
cost model later.

Thanks,
Richard.

> Thanks,
> bin
> 2017-06-20  Bin Cheng  <bin.cheng@arm.com>
>
>     * tree-loop-distribution.c (ref_base_address): Delete.
>     (similar_memory_accesses): Rename ...
>     (share_memory_accesses): ... to this.  Check if partitions access
>     the same memory reference.
>     (distribute_loop): Call share_memory_accesses.
>
> gcc/testsuite/ChangeLog
> 2017-06-20  Bin Cheng  <bin.cheng@arm.com>
>
>     * gcc.dg/tree-ssa/ldist-6.c: XFAIL.