Hi Alan, I probably wasn't clear enough. The implementation in the vectorizer is fine and I'm not asking that to change per target. What I'm objecting to is the equivalence between a REDUC_MAX_EXPR and a cost associated with vec_to_scalar. This assumes that the back end will implement a REDUC_MAX_EXPR in a specific way that at least some back ends cannot. But those back ends should be free to model the cost of the REDUC_MAX_EXPR appropriately. Therefore I am asking for a new vect_cost_for_stmt type to represent the cost of a REDUC_MAX_EXPR. For ARM, this cost will be the same as a vec_to_scalar. For others, it may not be; for powerpc, it certainly will not be.
We can produce a perfectly fine sequence for a REDUC_MAX_EXPR during RTL expansion, and therefore it is not correct for us to explode this in tree-vect-generic. This would expand the code size without providing any significant optimization opportunity, and could reduce the ability to, for instance, common REDUC_MAX_EXPRs. It would also slow down the gimple vectorizers. I apologize if my loose use of language confused the issue. It isn't the whole COND_REDUCTION I'm concerned with, but the REDUC_MAX_EXPRs that are used by it. (The costs in powerpc won't be enormous, but they are definitely mode-dependent in a way that vec_to_scalar is not. We'll need 2*log(n) instructions, where n is the number of elements in the mode being vectorized.) A secondary concern for powerpc is that REDUC_MAX_EXPR produces a scalar that has to be broadcast back to a vector, and the best way to implement it for us already has the max value in all positions of a vector. But that is something we should be able to fix with simplify-rtx in the back end. Thanks, Bill On Fri, 2015-09-11 at 10:15 +0100, Alan Hayward wrote: > Hi Bill, > > I’d be a bit worried about asking the backend for the cost of a > COND_REDUCTION, as that will rely on the backend understanding the > implementation the vectorizer is using - every time the vectorizer > changed, the backends would need to be updated too. I’m hoping soon to get > together a patch to reduce the stmts produced on the simpler cases, which > would require a different set of costings. I can also imagine further > improvements being added for other special cases over time. Having the > backends understand every variation would be a little cumbersome. > > As it stands today, we correctly exit the optimisation if max reduction > isn’t supported in hardware, which is what the cost model is expecting. > > > If power wanted to use this implementation, then I think it’d probably > need some code in tree-vect-generic.c to implement on emulated max > reduction, which would then require updates to the costs modelling of > anything that uses max reduction (not just cond reduction). All of that is > outside the scope of this patch. > > > Thanks, > Alan. > > On 10/09/2015 23:14, "Bill Schmidt" <wschm...@linux.vnet.ibm.com> wrote: > > >Hi Alan, > > > >The cost modeling of the epilogue code seems pretty target-specific ("An > >EQ stmt and an AND stmt, reduction of the max index and a reduction of > >the found values, a broadcast of the max value," resulting in two > >vector_stmts, one vec_to_scalar, and two scalar_to_vecs). On powerpc, > >this will not represent the cost accurately, and the cost will indeed be > >quite different depending on the mode (logarithmic in the number of > >elements). I think that you need to create a new entry in > >vect_cost_for_stmt to represent the cost of a COND_REDUCTION, and allow > >each target to calculate the cost appropriately. > > > >(Powerpc doesn't have a max-reduction hardware instruction, but because > >the reduction will be only in the epilogue code, it may still be > >profitable for us to generate the somewhat expensive reduction sequence > >in order to vectorize the loop. But we definitely want to model it as > >costly in and of itself. Also, the sequence will produce the maximum > >value in all positions without a separate broadcast.) > > > >Thanks, > >Bill > > > >On Thu, 2015-09-10 at 15:51 +0100, Alan Hayward wrote: > >> Hi, > >> This patch (attached) adds support for vectorizing conditional > >>expressions > >> (PR 65947), for example: > >> > >> int condition_reduction (int *a, int min_v) > >> { > >> int last = 0; > >> for (int i = 0; i < N; i++) > >> if (a[i] < min_v) > >> last = a[i]; > >> return last; > >> } > >> > >> To do this the loop is vectorised to create a vector of data results (ie > >> of matching a[i] values). Using an induction variable, an additional > >> vector is added containing the indexes where the matches occured. In the > >> function epilogue this is reduced to a single max value and then used to > >> index into the vector of data results. > >> When no values are matched in the loop, the indexes vector will contain > >> all zeroes, eventually matching the first entry in the data results > >>vector. > >> > >> To vectorize sucessfully, support is required for REDUC_MAX_EXPR. This > >>is > >> supported by aarch64 and arm. On X86 and powerpc, gcc will complain that > >> REDUC_MAX_EXPR is not supported for the required modes, failing the > >> vectorization. On mips it complains that the required vcond expression > >>is > >> not supported. It is suggested the relevant backend experts add the > >> required backend support. > >> > >> Using a simple testcase based around a large number of N and run on an > >> aarch64 juno board, with the patch in use, the runtime reduced to 0.8 of > >> it's original time. > >> > >> This patch caused binary differences in three spec2006 binaries on > >>aarch64 > >> - 4.16.gamess, 435.gromacs and 456.hmmer. Running them on a juno board > >> showed no improvement or degregation in runtime. > >> > >> > >> In the near future I hope to submit a further patch (as PR 66558) which > >> optimises the case where the result is simply the index of the loop, for > >> example: > >> int condition_reduction (int *a, int min_v) > >> { > >> int last = 0; > >> for (int i = 0; i < N; i++) > >> if (a[i] < min_v) > >> last = i; > >> return last; > >> } > >> In this case a lot of the new code can be optimized away. > >> > >> I have run check for aarch64, arm and x86 and have seen no regressions. > >> > >> > >> Changelog: > >> > >> 2015-08-28 Alan Hayward <alan.hayw...@arm.com> > >> > >> PR tree-optimization/65947 > >> * tree-vect-loop.c > >> (vect_is_simple_reduction_1): Find condition reductions. > >> (vect_model_reduction_cost): Add condition reduction costs. > >> (get_initial_def_for_reduction): Add condition reduction initial > >> var. > >> (vect_create_epilog_for_reduction): Add condition reduction > >>epilog. > >> (vectorizable_reduction): Condition reduction support. > >> * tree-vect-stmts.c > >> (vectorizable_condition): Add vect reduction arg > >> * doc/sourcebuild.texi (Vector-specific attributes): Document > >> vect_max_reduc > >> > >> testsuite/Changelog: > >> > >> PR tree-optimization/65947 > >> * lib/target-supports.exp > >> (check_effective_target_vect_max_reduc): Add. > >> * gcc.dg/vect/pr65947-1.c: New test. > >> * gcc.dg/vect/pr65947-2.c: New test. > >> * gcc.dg/vect/pr65947-3.c: New test. > >> * gcc.dg/vect/pr65947-4.c: New test. > >> * gcc.dg/vect/pr65947-5.c: New test. > >> * gcc.dg/vect/pr65947-6.c: New test. > >> * gcc.dg/vect/pr65947-7.c: New test. > >> * gcc.dg/vect/pr65947-8.c: New test. > >> * gcc.dg/vect/pr65947-9.c: New test. > >> * gcc.dg/vect/pr65947-10.c: New test. > >> * gcc.dg/vect/pr65947-11.c: New test. > >> > >> > >> > >> Thanks, > >> Alan > >> > >> > > > > > >