On 03/18/2015 12:04 PM, Richard Biener wrote: > On Tue, Mar 17, 2015 at 7:29 PM, Jeff Law <l...@redhat.com> wrote: >> On 03/17/2015 02:17 AM, Andreas Krebbel wrote: >>> >>> >>> Just to have some numbers I did run a -j1 GCC bootstrap twice with and >>> without the patch on x86_64. >>> Best results for both are: >>> >>> clean: 21459s >>> patched: 21314s >>> >>> There rather appears to be a trend towards reduced compile time perhaps >>> due to the reduced number of >>> INSNs to be processed in the RTL passes between the two ifcvt runs (loop >>> optimization, combine, >>> fwprop, dse,...)?! >>> >>> I also tried to measure the testsuite runs but the results show a big >>> variance. So what I have right >>> now does not qualify as a benchmark. >> >> And reality is it's getting harder and harder to benchmark this kind of >> thing with turbo modes and such. A single run isn't sufficient unless >> you've locked the box into a particular cpu frequency. > > For the particular patch I wonder if you really need to change all > three if-conversion pass instances or if changing the one before > combine (pass_rtl_ifcvt, thus rest_of_handle_if_conversion) is enough. Right. For this particular case it would be good enough to do it only in the first ifcvt run. But perhaps there are cases where later passes get confused by the leftovers from ifcvt?
> That already runs an unconditonal (huh...) cleanup_cfg (0) at the end > which could be changed so that DCE is performed (CLEANUP_EXPENSIVE, > runs delete_trivially_dead_insns). > > At least that makes the patch smaller and its impact restricted to > one of the three ifcvt passes. This does not seem to work. The DCE run by cleanup_cfg only deals with dead pseudos. In my case it is a set to the CC hard reg which becomes dead. > OTOH ifcvt performs a DCE at its start (to be not confused by dead > instructions I guess), so why doesn't combine do that as well > (oh, it does!?)? When reaching combine the LR solution is clean and therefore also DCE isn't performed. This is because ifcvt disables running DCE with LR. So LR is always clean after ifcvt although there are still dead insns left. > And maybe _that_ DCE can be removed as if_convert () > already performs a DF_LR_RUN_DCE on the first pass. You mean removing the DCE run in combine? That one probably can go away then given that the passes running between ifcvt and combine (loop and fwprop) get rid of dead insns properly. -Andreas- > > Richard. > >> jeff >>> >>> >> >
