On Wed, 2013-12-04 at 07:13 -0600, Bill Schmidt wrote: > On Wed, 2013-12-04 at 11:30 +0100, Richard Biener wrote: > > On Wed, Dec 4, 2013 at 11:26 AM, Richard Biener > > <richard.guent...@gmail.com> wrote: > > > 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. > > > > Oh, you're using affine combination expansion ... which is even more > > expensive. So why isn't that then done for all ref candidates? That is, > > why do two different things, get_inner_reference _and_ affine-combination > > dances. And why build back trees from that instead of storing the > > affine combination. > > Well, the original design had no desire to use the expensive machinery > of affine combination expansion. For what was envisioned, the simpler > mechanisms of get_inner_reference have been plenty. > > My thought, and please correct me if I'm wrong, is that once we've > already reduced to an SSA name from get_inner_reference, the affine > machinery will terminate fairly quickly -- we shouldn't get into too > deep a search on underlying pointer arithmetic in most cases. But > compile time testing will tell us whether this is reasonable.
As a middle ground, may I suggest that we only do the extra tree_affine expansion at -O2 and above? Any extra compile time should be a blip at those levels. At -O1 there could be legitimate issues, though. Bill > > Bill > > > > > I'll bet we come back with compile-time issues after this patch > > went in. I'll count on you two to fix them then. > > > > Richard. > > > > >> 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. > > >>> > > >>> > > >> > > >
