On Jun 18, 2011, at 5:15 AM, Rémi Forax wrote: > On 06/18/2011 03:15 AM, Tom Rodriguez wrote: >> On Jun 17, 2011, at 5:00 PM, John Rose wrote: >> >>> On Jun 17, 2011, at 10:21 AM, Tom Rodriguez wrote: >>> >>>>> Sorry, I was thinking recording which branch of the GWT is taken and >>>>> storing them in the GWT. >>>>> Two GWTs should not share the same metadata. >>>> This is the major problem with GWT/selectAlternative. Previously when GWT >>>> was bytecodes we at least had a chance to get some profile information on >>>> which way the branch was likely to go but with the ricochet frame version >>>> we have no knowledge so each side of the if has equal probability. The >>>> were no guarantees that the GWT would always be used as a fast/slow idiom >>>> but in practice it was so we used to get good data. We'll have to find >>>> someway to capture profiles for this if we can to treat fast/slow in a >>>> more aggressive way. >>> selectAlternative has a branch profile. As long as GWT is being used as >>> expected (for fast/slow splits), selectAlternative will have a fast/slow >>> profile, just like the original GWT invokers. >>> >>> The PROB_FAIR in CallGenerator::for_method_handle_inline could be made >>> "smarter", by feeding from the control inputs of the Phi; that would be a >>> good start. >> Well duh on me. When I wrote that my brain was thinking cmove instruction, >> not a real If diamond with probability. I don't think fixing this will >> improve the inlining in the fast case but it should reduce useless inlining >> in the slow path. I've got part of this working. >> >> tom > > But all GWT share the same selectAlternative, so the probability > of the inputs of the Phi is the propability of all GWTs. > I think it's better to artificially mark the fallback as never been > called instead of relying > on a global shared profile.
Are you suggesting that MethodHandles.guardWithTest should always assume that the second choice is effectively never called? That seems like a strong assumption. It unfortunate that the method handles interface doesn't give better hinting for something like this. Can someone explain the idiom being used here a little more? It seems like selectAlternative is being used to build a caching call site but I would have expected that to be a more first class object, like a CallSite subclass with a guard and fallback. tom > > Also, the fallback will be called a few times (usually at least once) > before the target > of a callsite becomes stable so a global profile will be artificially > pollutedp > by the initialization phases of all GWTs. > > Another way to solve the problem is to generate the code twice as the > tiered compilation does. > The first generation will include MDOs that will be used by the second > generation. > > Rémi > > _______________________________________________ > mlvm-dev mailing list > mlvm-dev@openjdk.java.net > http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev _______________________________________________ mlvm-dev mailing list mlvm-dev@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev
