http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46590
--- Comment #26 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-08-21 14:56:41 UTC --- For a somewhat reduced testcase I now get at -O1: alias stmt walking : 105.51 (45%) usr 0.33 (24%) sys tree SSA rewrite : 22.01 ( 9%) usr 0.04 ( 3%) sys tree SSA incremental : 25.25 (11%) usr 0.07 ( 5%) sys dominance frontiers : 35.35 (15%) usr 0.02 ( 1%) sys dominance computation : 14.60 ( 6%) usr 0.09 ( 7%) sys TOTAL : 234.28 1.38 as previously said most of the non-alias-stmt walk time is spent on loop header copying. WIth -O1 -fno-tree-ch we have alias stmt walking : 101.52 (68%) usr 0.37 (34%) sys tree SSA rewrite : 4.14 ( 3%) usr 0.01 ( 1%) sys tree SSA incremental : 8.00 ( 5%) usr 0.02 ( 2%) sys dominance frontiers : 6.14 ( 4%) usr 0.01 ( 1%) sys dominance computation : 4.74 ( 3%) usr 0.06 ( 6%) sys TOTAL : 150.14 1.09 limiting stmt walk results in the ability to arbitrarily scale down its cost with a param (we can either limit alias oracle query numbers or SCCVN table lookups). With 100 alias oracle queries per load/store we end up with alias stmt walking : 1.60 ( 3%) usr 0.05 ( 5%) sys with 100 SCCVN table lookups the figure is alias stmt walking : 1.60 ( 3%) usr 0.06 ( 6%) sys one assumes the lookups are expensive, the other one assumes the walk itself is. Increasing the latter to 1000 SCCVN table lookup produces alias stmt walking : 9.24 (16%) usr 0.18 (19%) sys which is around the expected 10-fold increase (but still reasonable given the artificial nature of the testcase). We could also, instead of limiting each walk to a constant cost, have a per-function budget that we can use up first before limiting further walks individually (helps to not regress reasonably sized cases).
