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? 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
