On 4 July 2016 at 13:51, Andrew Pinski <pins...@gmail.com> wrote: > On Mon, Jul 4, 2016 at 12:58 AM, Prathamesh Kulkarni > <prathamesh.kulka...@linaro.org> wrote: >> Hi, >> I have attached a "quick and dirty" prototype patch (var-partition-1.diff), >> that attempts to partition variables to reduce number of >> external references and to increase usage of section-anchors >> to CSE address computation of global variables. >> >> We could put a variable in a partition that has max references for it, >> however it doesn't lend itself directly to section anchor optimization. >> For instance if a partition has max references for variables 'a' and 'b', >> but no function in that partition references both 'a', and 'b' then AFAIU >> it doesn't make any difference from section anchors perspective to have them >> in same partition. >> >> The patch tries to assign a set of variables (>= 2) >> to a partition whose functions have maximum references for that set. >> Functions within the partition that reference the variables >> in the set can take advantage of section-anchors. Functions >> referencing the variables in the set outside the partition >> would need to load them as external references (using movw/movt), >> however since we are placing the set in partition that has maximal >> references for it, number of external references should be overall >> reduced. >> >> Partitioning is gated by -flto-var-partition and enabled >> only for arm and aarch64. > > Why only for arm and aarch64? Shouldn't it be enabled for all section > anchor targets? AFAIK the only targets supporting section anchors are arm, aarch64 and powerpc. I didn't enable it for ppc64 because I am not sure how much profitable it is for that target. Honza mentioned to me some time back that effect of partitioning on powerpc was nearly zero.
Thanks, Prathamesh > > Thanks, > Andrew > >> As per previous discussion [1], I haven't >> touched function partitioning. Does this approach look ok >> especially regarding correctness ? >> So far, I have cross-tested patch on arm*-*-*, aarch64*-*-*. >> >> I haven't yet managed to benchmark the patch. >> As a cheap measurement, I tried to measure number of external >> references with and without patch by writing a small ipa pass >> which is run during ltrans and simply walks over varpool nodes >> and counts number of varpool_nodes for which DECL_EXTERNAL (vnode->decl) is >> true >> and vnode->definition is 0. Is that sufficient condition to determine >> if variable is externally defined ? I have attached the pass >> (count-external-refs.diff) >> and the comparison done with it for for SPEC2000 [2]. The entries >> in "before" and "after" column contain summation of number of >> external refs (total_count) across all partitions before and after applying >> the patch. Does the comparison hold any merit ? >> I was wondering if we could we use a better way for >> measuring statically the effects of variable partitioning ? >> I hope also to get done with benchmarking soon. >> >> I have not yet figured out how to integrate it with existing cost metrics for >> balanced partitioning, I am looking into that. >> I would be grateful for suggestions on the patch. >> >> [1] https://gcc.gnu.org/ml/gcc/2016-04/msg00090.html >> >> [2] SPEC2000 comparison: >> https://docs.google.com/spreadsheets/d/1xnszyw04ksoyBspmCVYesq6KARLw-PA2n3T4aoaKdYw/edit?usp=sharing
