On Fri, Aug 9, 2013 at 1:24 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> I have not looked at the details. One high level question: this form >> seems to only support one indirect target case. LIPO uses TOPN >> indirect target profiling (tracking multiple targets), which can be >> used by LTO as well (when the topn profiling gets into trunk). > > Well, adding multiple direct edges for given call will need extension into > cgraph_turn_edge_to_speculative and cgraph_speculative_call_info APIs (to > allow > multple direct edges) to indirect_info common_target datastructure, to > profiling histograms and to the gimple_ic code. Otherwise there is nothing > really hard coded about single direct target. > > How much benefits do you see from having multiple direct targets?
It can be large depending on applications. For some apps, there are very callsites with 2 or 3 very hot targets. > I would > expect them to be quite quickly disappearing as N increases... For the large apps I see, N rarely exceeds 4. For most of the cases, the hot targets are 2 or 3 with 2 being very common. In LIPO, we track 4 targets, but only use top 2. > How the TOPN profiling counter is implemented? Currently FDO's value profiler is one value based -- it is quite faulty because it can not handle cases where hot targets alternates. For instance for sequence 1, 2, 1, 2, 1, 2, 3,3, 1 -- the winner will be 3 instead of 1. TOPN algorithm is a LFU based --- when match is not found, the least frequent used entry will be evicted. To avoid ping-pong effect, total eviction count is also tracked -- when it exceeds a threshold, the bottom counter will be clear to make room for hot entries (instead of letting them killing each other). thanks, David > > Honza
