Re: [PING] [PATCH] Optional alternative base_expr in finding basis for CAND_REFs

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Dec 3, 2013 at 11:04 PM, Bill Schmidt
<wschmidt@linux.vnet.ibm.com> wrote:
> On Tue, 2013-12-03 at 21:35 +0100, Richard Biener wrote:
>> Yufeng Zhang <Yufeng.Zhang@arm.com> wrote:
>> >On 12/03/13 14:20, Richard Biener wrote:
>> >> On Tue, Dec 3, 2013 at 1:50 PM, Yufeng Zhang<Yufeng.Zhang@arm.com>
>> >wrote:
>> >>> On 12/03/13 06:48, Jeff Law wrote:
>> >>>>
>> >>>> On 12/02/13 08:47, Yufeng Zhang wrote:
>> >>>>>
>> >>>>> Ping~
>> >>>>>
>> >>>>> http://gcc.gnu.org/ml/gcc-patches/2013-11/msg03360.html
>> >>>>
>> >>>>
>> >>>>>
>> >>>>> Thanks,
>> >>>>> Yufeng
>> >>>>>
>> >>>>> On 11/26/13 15:02, Yufeng Zhang wrote:
>> >>>>>>
>> >>>>>> On 11/26/13 12:45, Richard Biener wrote:
>> >>>>>>>
>> >>>>>>> On Thu, Nov 14, 2013 at 12:25 AM, Yufeng
>> >>>>>>> Zhang<Yufeng.Zhang@arm.com>     wrote:
>> >>>>>>>>
>> >>>>>>>> On 11/13/13 20:54, Bill Schmidt wrote:
>> >>>>>>>>>
>> >>>>>>>>> The second version of your original patch is ok with me with
>> >the
>> >>>>>>>>> following changes.  Sorry for the little side adventure into
>> >the
>> >>>>>>>>> next-interp logic; in the end that's going to hurt more than
>> >it
>> >>>>>>>>> helps in
>> >>>>>>>>> this case.  Thanks for having a look at it, anyway.  Thanks
>> >also for
>> >>>>>>>>> cleaning up this version to be less intrusive to common
>> >interfaces; I
>> >>>>>>>>> appreciate it.
>> >>>>>>>>
>> >>>>>>>>
>> >>>>>>>>
>> >>>>>>>> Thanks a lot for the review.  I've attached an updated patch
>> >with the
>> >>>>>>>> suggested changes incorporated.
>> >>>>>>>>
>> >>>>>>>> For the next-interp adventure, I was quite happy to do the
>> >>>>>>>> experiment; it's
>> >>>>>>>> a good chance of gaining insight into the pass.  Many thanks
>> >for
>> >>>>>>>> your prompt
>> >>>>>>>> replies and patience in guiding!
>> >>>>>>>>
>> >>>>>>>>
>> >>>>>>>>> Everything else looks OK to me.  Please ask Richard for final
>> >>>>>>>>> approval,
>> >>>>>>>>> as I'm not a maintainer.
>> >>>>
>> >>>> First a note, I need to check on voting for Bill as the slsr
>> >maintainer
>> >>>> from the steering committee.   Voting was in progress just before
>> >the
>> >>>> close of stage1 development so I haven't tallied the results :-)
>> >>>
>> >>>
>> >>> Looking forward to some good news! :)
>> >>>
>> >>>
>> >>>>>>
>> >>>>>> Yes, you are right about the non-trivial 'base' tree are rarely
>> >shared.
>> >>>>>>      The cached is introduced mainly because get_alternative_base
>> >() may
>> >>>>>> be
>> >>>>>> called twice on the same 'base' tree, once in the
>> >>>>>> find_basis_for_candidate () for look-up and the other time in
>> >>>>>> alloc_cand_and_find_basis () for record_potential_basis ().  I'm
>> >happy
>> >>>>>> to leave out the cache if you think the benefit is trivial.
>> >>>>
>> >>>> Without some sense of how expensive the lookups are vs how often
>> >the
>> >>>> cache hits it's awful hard to know if the cache is worth it.
>> >>>>
>> >>>> I'd say take it out unless you have some sense it's really saving
>> >time.
>> >>>>     It's a pretty minor implementation detail either way.
>> >>>
>> >>>
>> >>> I think the affine tree routines are generally expensive; it is
>> >worth having
>> >>> a cache to avoid calling them too many times.  I run the slsr-*.c
>> >tests
>> >>> under gcc.dg/tree-ssa/ and find out that the cache hit rates range
>> >from
>> >>> 55.6% to 90%, with 73.5% as the average.  The samples may not well
>> >represent
>> >>> the real world scenario, but they do show the fact that the 'base'
>> >tree can
>> >>> be shared to some extent.  So I'd like to have the cache in the
>> >patch.
>> >>>
>> >>>
>> >>>>
>> >>>>>>
>> >>>>>>> +/* { dg-do compile } */
>> >>>>>>> +/* { dg-options "-O2 -fdump-tree-slsr" } */
>> >>>>>>> +
>> >>>>>>> +typedef int arr_2[50][50];
>> >>>>>>> +
>> >>>>>>> +void foo (arr_2 a2, int v1)
>> >>>>>>> +{
>> >>>>>>> +  int i, j;
>> >>>>>>> +
>> >>>>>>> +  i = v1 + 5;
>> >>>>>>> +  j = i;
>> >>>>>>> +  a2 [i-10] [j] = 2;
>> >>>>>>> +  a2 [i] [j++] = i;
>> >>>>>>> +  a2 [i+20] [j++] = i;
>> >>>>>>> +  a2 [i-3] [i-1] += 1;
>> >>>>>>> +  return;
>> >>>>>>> +}
>> >>>>>>> +
>> >>>>>>> +/* { dg-final { scan-tree-dump-times "MEM" 5 "slsr" } } */
>> >>>>>>> +/* { dg-final { cleanup-tree-dump "slsr" } } */
>> >>>>>>>
>> >>>>>>> scanning for 5 MEMs looks non-sensical.  What transform do
>> >>>>>>> you expect?  I see other slsr testcases do similar non-sensical
>> >>>>>>> checking which is bad, too.
>> >>>>>>
>> >>>>>>
>> >>>>>> As the slsr optimizes CAND_REF candidates by simply lowering them
>> >to
>> >>>>>> MEM_REF from e.g. ARRAY_REF, I think scanning for the number of
>> >MEM_REFs
>> >>>>>> is an effective check.  Alternatively, I can add a follow-up
>> >patch to
>> >>>>>> add some dumping facility in replace_ref () to print out the
>> >replacing
>> >>>>>> actions when -fdump-tree-slsr-details is on.
>> >>>>
>> >>>> I think adding some details to the dump and scanning for them would
>> >be
>> >>>> better.  That's the only change that is required for this to move
>> >forward.
>> >>>
>> >>>
>> >>> I've updated to patch to dump more details when
>> >-fdump-tree-slsr-details is
>> >>> on.  The tests have also been updated to scan for these new dumps
>> >instead of
>> >>> MEMs.
>> >>>
>> >>>
>> >>>>
>> >>>> I suggest doing it quickly.  We're well past stage1 close at this
>> >point.
>> >>>
>> >>>
>> >>> The bootstrapping on x86_64 is still running.  OK to commit if it
>> >succeeds?
>> >>
>> >> I still don't like it.  It's using the wrong and too expensive tools
>> >to do
>> >> stuff.  What kind of bases are we ultimately interested in?  Browsing
>> >> the code it looks like we're having
>> >>
>> >>    /* Base expression for the chain of candidates:  often, but not
>> >>       always, an SSA name.  */
>> >>    tree base_expr;
>> >>
>> >> which isn't really too informative but I suppose they are all
>> >> kind-of-gimple_val()s?  That said, I wonder if you can simply
>> >> use get_addr_base_and_unit_offset in place of get_alternative_base
>> >(),
>> >> ignoring the returned offset.
>> >
>> >'base_expr' is essentially the base address of a handled_component_p,
>> >e.g. ARRAY_REF, COMPONENT_REF, etc.  In most case, it is the address of
>> >
>> >the object returned by get_inner_reference ().
>> >
>> >Given a test case like the following:
>> >
>> >typedef int arr_2[20][20];
>> >
>> >void foo (arr_2 a2, int i, int j)
>> >{
>> >   a2[i+10][j] = 1;
>> >   a2[i+10][j+1] = 1;
>> >   a2[i+20][j] = 1;
>> >}
>> >
>> >The IR before SLSR is (on x86_64):
>> >
>> >   _2 = (long unsigned int) i_1(D);
>> >   _3 = _2 * 80;
>> >   _4 = _3 + 800;
>> >   _6 = a2_5(D) + _4;
>> >   *_6[j_8(D)] = 1;
>> >   _10 = j_8(D) + 1;
>> >   *_6[_10] = 1;
>> >   _12 = _3 + 1600;
>> >   _13 = a2_5(D) + _12;
>> >   *_13[j_8(D)] = 1;
>> >
>> >The base_expr for the 1st and 2nd memory reference are the same, i.e.
>> >_6, while the base_expr for a2[i+20][j] is _13.
>> >
>> >_13 is essentially (_6 + 800), so all of the three memory references
>> >essentially share the same base address.  As their strides are also the
>> >
>> >same (MULT_EXPR (j, 4)), the three references can all be lowered to
>> >MEM_REFs.  What this patch does is to use the tree affine tools to help
>> >
>> >recognize the underlying base address expression; as it requires
>> >looking
>> >into the definitions of SSA_NAMEs, get_addr_base_and_unit_offset ()
>> >won't help here.
>> >
>> >Bill has helped me exploit other ways of achieving this in SLSR, but so
>> >
>> >far we think this is the best way to proceed.  The use of tree affine
>> >routines has been restricted to CAND_REFs only and there is the
>> >aforementioned cache facility to help reduce the overhead.
>> >
>> >Thanks,
>> >Yufeng
>> >
>> >P.S. some more details what the patch does:
>> >
>> >The CAND_REF for the three memory references are:
>> >
>> >  6  [2] *_6[j_8(D)] = 1;
>> >      REF  : _6 + ((sizetype) j_8(D) * 4) + 0 : int[20] *
>> >      basis: 0  dependent: 8  sibling: 0
>> >      next-interp: 0  dead-savings: 0
>> >
>> >   8  [2] *_6[_10] = 1;
>> >      REF  : _6 + ((sizetype) j_8(D) * 4) + 4 : int[20] *
>> >      basis: 6  dependent: 11  sibling: 0
>> >      next-interp: 0  dead-savings: 0
>> >
>> >  11  [2] *_13[j_8(D)] = 1;
>> >      REF  : _13 + ((sizetype) j_8(D) * 4) + 0 : int[20] *
>> >      basis: 8  dependent: 0  sibling: 0
>> >      next-interp: 0  dead-savings: 0
>> >
>> >Before the patch, the strength reduction candidate chains for the three
>> >
>> >CAND_REFs are:
>> >
>> >   _6 -> 6 -> 8
>> >   _13 -> 11
>> >
>> >i.e. SLSR recognizes the first two references share the same basis,
>> >while the last one is on it own.
>> >
>> >With the patch, an extra candidate chain can be recognized:
>> >
>> >   a2_5(D) + (sizetype) i_1(D) * 80 -> 6 -> 11 -> 8
>> >
>> >i.e. all of the three references are found to have the same basis
>> >(a2_5(D) + (sizetype) i_1(D) * 80), which is essentially the expanded
>> >_6
>> >or _13, with the immediate offset removed.  The pass is now able to
>> >lower all of the three references, instead of the first two only, to
>> >MEM_REFs.
>>
>> Ok, so slsr handles arbitrary complex bases and figures out common components? If so, then why not just use get_inner_reference? After all slsr does not use tree-affine as representation for bases (which it could?)
>
> I think that's overstating SLSR's current capabilities a bit. :)  We do
> use get_inner_reference to come up with the base expression for
> reference candidates (based on some of your suggestions a couple of
> years back).  However, in the case of multiple levels of array
> references, we miss opportunities because get_inner_reference stops at
> an SSA name that could be further expanded by following its definition
> back to a more fundamental base expression.

Using tree-affine.c to_affine_comb / affine_comb_to_tree has exactly the
same problem.

> Part of the issue here is that reference candidates are basis for a more
> specific optimization than the mult and add candidates.  The latter have
> a more general framework for building up a recording of simple affine
> expressions that can be strength-reduced.  Ultimately we ought to be
> able to do something similar for reference candidates, building up
> simple affine expressions from base expressions, so that everything is
> done in a forward order and the tree-affine interfaces aren't needed.
> But that will take some more fundamental design changes, and since this
> provides some good improvements for important cases, I feel it's
> reasonable to get this into the release.

But I fail to see what is special about doing the dance to affine and
then back to trees just to drop the constant offset which would be
done by get_inner_reference as well and cheaper if you just ignore
bitpos.

?!

Richard.

> Thanks,
> Bill
>
>>
>> Richard.
>>
>>
>