>> When you compile with -Os, the inlining happens only when code size reduces. >> Thus we pretty much care about the code size metrics only. I suspect the >> problem here might be that normal C++ code needs some inlining to make >> abstraction penalty go away. GCC -Os implementation is generally tuned for >> CSiBE and it is somewhat C centric (that makes sense for embedded world). As >> a >> result we might get quite noticeable slowdowns on C++ apps compiled with -Os >> (and code size growth too since abstraction is never eliminated). It can be >> seen also at tramp3d (Pooma testcase) where -Os produces a lot bigger and a >> lot >> slower code. > Looks like -Os needs tweaking for C++. It would be awesome if Mozilla > could serve a testcase for that. >> I would be very interested to know the most obvious cases where we miss >> inlining and should not. It would be most helpful to directly know >> -fdump-tree-inline_param-details for those or have self contained testcase. > I posted such a testcase under "Crucial C++ inlining broken under Os" > subject.
Thanks, I will check it out tomorrow. Testcases are really important here. > Would be nice to ensure that code-size-reducing inlining has testsuite > coverage and then move on to more ambitious targets. We have CSiBE benchmark to test -Os, but it does not contain that much of inlining heavy testcases as far as I am aware. > > For example, it would be nice to mix -O2/-Os via pgo. Mozilla carries > support for a lot of special-cases that are rarely used, would be nice > to aggressively minimize binary size for those features. I'm hoping we > can reduce binary size further via LTO. With PGO everything at average fewer than once per execution in train run is optimized for size. > > I would be happy to see a streamlined feedback loop between GCC and > Mozilla. I think setting up regular Mozilla benchmarking is a great > idea. I volunteer to be the Mozilla contact for that. > > Mozilla build instructions are here: > https://developer.mozilla.org/En/Simple_Firefox_build > > PGO instructions are at > https://developer.mozilla.org/en/Building_with_Profile-Guided_Optimization > > Our testsuite is pretty easy to setup now, see > https://wiki.mozilla.org/StandaloneTalos > It takes over an hour to run on slower hardware. I'm not aware of any > good documentation on what the tests do and what they are measured in. > https://wiki.mozilla.org/Performance:Tinderbox_Tests is an older > reference on the matter. Ping me on irc if you aren't sure what a test > is doing. I will check it out tomorrow and see how far I get. Automated nightly tester would be very good for this. Honza > > Cheers, > Taras
