On Tue, Dec 3, 2013 at 11:04 PM, Bill Schmidt <wschm...@linux.vnet.ibm.com> wrote: > On Tue, 2013-12-03 at 21:35 +0100, Richard Biener wrote: >> Yufeng Zhang <yufeng.zh...@arm.com> wrote: >> >On 12/03/13 14:20, Richard Biener wrote: >> >> On Tue, Dec 3, 2013 at 1:50 PM, Yufeng Zhang<yufeng.zh...@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.zh...@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. >> >> >
