Performance of Scope.getSymbolsByName()
Ron Shapiro
ronshapiro at google.com
Wed May 22 15:00:23 UTC 2019
I think there still would be benefit in that as well, as I'm seeing that
come up in other contexts (as referenced in the bug).
On Wed, May 22, 2019 at 9:06 AM Jan Lahoda <jan.lahoda at oracle.com> wrote:
> Sorry, I was too busy last few days.
>
> I was peeking at some possible improvements, but I think I like Ron's
> first (.00) patch - that caches what can be cached nicely.
>
> Looking at the testcase generated by Ron's reproducer using:
> python test.py 7
>
> and the (biggest) size of the outcome of:
> types.membersClosure(site, false).getSymbolsByName(sym.name, cf)
>
> seems to be 13700 elements - which means the Scope lookup and iteration
> runs ~13700 times, so avoiding these additional lookup costs seems like
> a clear win.
>
> I have an idea that might speed up the iterations through deeply nested
> CompoundScopes, although the effect of that in combination with Ron's is
> going to be fairly limited, if any, I think.
>
> Jan
>
> On 22. 05. 19 12:24, Maurizio Cimadamore wrote:
> > This doesn't work. You are basically relying on the order in which
> > symbols are entered in the members closure scope.
> >
> > In simple example like these:
> >
> > class A {
> > int m(List<String> l) {return 0;}
> > }
> >
> > class B extends A {
> > int m(List<Integer> l) {return 0;}
> > }
> >
> >
> > The logic you proposed will not work. That's because we first see B::m -
> > and 'symbolByName' is empty at that stage; so we add it there. Then we
> > do another round and see A::m - but we don't really look there - given
> > that we first check to see if the symbol we're considering (sym) is
> > override-equivalent with B::m (the only symbol in symbolByName). And
> > that happens to be the case, since they are the same symbol. So we exit
> > the loop, w/o having found any clash.
> >
> > In other words, symbolByName would need to also contain A::m for the
> > code to see the clash - but that never happens; by the time A::m is
> > added, is already too late.
> >
> >
> > I think caching the result of
> >
> > types.membersClosure(site, false).getSymbolsByName(sym.name, cf)
> >
> > is a good measure.
> >
> > I'm a bit surprised that iteration is so slow (membersClosure is slow to
> > set up, but once you do it the results are cached). So, rather than
> > tweaking the algorithm, I think it'd be better to investigate the reason
> > was to why asking a compound scope iterator is so slow, which then would
> > yield dividends for the rest of the code as well.
> >
> > Maurizio
> >
> >
> > On 21/05/2019 21:21, Maurizio Cimadamore wrote:
> >>
> >> I see what you have done - I have to think about it a bit to see if I
> >> can come up with some counter example.
> >>
> >> Thanks
> >> Maurizio
> >>
> >> On 21/05/2019 17:39, Ron Shapiro wrote:
> >>> Are the checks of the inner loop symmetrical?
> >>>
> >>> Currently it's checking m_i against (m_0..n - m_i ). This second
> >>> webrev below would check it against just (m_0..i-1 ), which albeit
> >>> still n^2, it divides by a factor of 2.
> >>>
> >>> (sorry if the subscripting here doesn't display correctly)
> >>>
> >>> http://cr.openjdk.java.net/~ronsh/8224161/webrev.01/
> >>>
> >>> This feels conceptually logical to me, but I'm not seeing a
> >>> measurable change by it. It looks a little bit cleaner to me, but I'm
> >>> fine with either webrev given the benefits they both bring.
> >>>
> >>> I can take a look in another thread about speeding up CompoundScope
> >>> iteration.
> >>>
> >>> On Tue, May 21, 2019 at 8:05 AM Maurizio Cimadamore
> >>> <maurizio.cimadamore at oracle.com
> >>> <mailto:maurizio.cimadamore at oracle.com>> wrote:
> >>>
> >>>
> >>> On 21/05/2019 12:16, Ron Shapiro wrote:
> >>>> I still think that something to optimize the actual ScopeImpl
> >>>> Iterable is a worthwhile endeavor, as that would alleviate the
> >>>> need to materialize here (and solve hopefully the other issues
> >>>> I'm seeing), but I was having trouble figuring out how to do
> >>>> that. This may be a good interim option without much cost.
> >>>
> >>> Sure - I'm not opposed to optimizing the iteration process - I
> >>> was expressing my skepticism w.r.t. making checkOverrideClash
> >>> simpler/non quadratic.
> >>>
> >>> Maurizio
> >>>
> >>>>
> >>>>
> >>>>
> >>>> On Tue, May 21, 2019, 5:59 AM Maurizio Cimadamore
> >>>> <maurizio.cimadamore at oracle.com
> >>>> <mailto:maurizio.cimadamore at oracle.com>> wrote:
> >>>>
> >>>> I think your fix is a good one. We spent some cycles
> >>>> optimizing this, a bit odd we have missed this :-)
> >>>>
> >>>> I'm very skeptical you can collapse into a single loop, as
> >>>> this implement the logic in JLS 8.4.8.3 [1] which, as you
> >>>> can see, is inherently quadratic (for each method, we have
> >>>> to scan all methods with same name in supertypes to see if
> >>>> there is an override clash). The algorithm that was there
> >>>> before wasn't - and it lead to the wrong answers in tricky
> >>>> cases - so while you can get 80% there with a non-quadratic
> >>>> algorithm, you will miss issues by doing so.
> >>>>
> >>>> One thing that would help would be, instead, to limit the
> >>>> analysis only in cases where it adds value - for instance,
> >>>> if your hierarchy is just non-generic classes (as in your
> >>>> example), then there's no way for you to accidentally
> >>>> override a 'bridge' method, since no bridges will be
> >>>> generated! But when looking at this, I couldn't find great
> >>>> ways to detect these conditions w/o spending more time than
> >>>> the check itself.
> >>>>
> >>>> [1] -
> >>>>
> https://docs.oracle.com/javase/specs/jls/se12/html/jls-8.html#jls-8.4.8.3
> >>>>
> >>>> Maurizio
> >>>>
> >>>> On 20/05/2019 21:58, Ron Shapiro wrote:
> >>>>> In the real world example, I'm seeing the 40s that was
> >>>>> previously spent in Check.checkOverrideClashes drop to to
> >>>>> 9.5s when using this patch. Of that 9.5, 8.9 is spent in
> >>>>> iterating through the CompoundIterator and calling
> >>>>> getSymbolsByName.
> >>>>>
> >>>>> On Mon, May 20, 2019 at 4:34 PM Ron Shapiro
> >>>>> <ronshapiro at google.com <mailto:ronshapiro at google.com>>
> wrote:
> >>>>>
> >>>>> This patch, which materializes the duplicate outer and
> >>>>> inner Iterables first into a list. It removes the
> >>>>> entire section of the CompoundIterator iteration from
> >>>>> the profile.
> >>>>>
> >>>>> webrev:
> >>>>>
> http://cr.openjdk.java.net/~ronsh/8224161/webrev.00/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java.sdiff.html
> >>>>>
> >>>>> I'm not sure it's the absolutely correct solution as it
> >>>>> possibly masks an underlying issue.
> >>>>>
> >>>>> I'm still seeing some time spent in
> >>>>> MethodSymbol.overrides, Types.isSubSignature, and
> >>>>> Types.memberType, all of which happen in the inner
> >>>>> loop. If we can remove those and collapse the nested
> >>>>> loops into one, then this solution isn't necessary and
> >>>>> it would also solve that performance issue.
> >>>>>
> >>>>> On Fri, May 17, 2019 at 5:55 PM Ron Shapiro
> >>>>> <ronshapiro at google.com <mailto:ronshapiro at google.com>>
> >>>>> wrote:
> >>>>>
> >>>>> I still have more to investigate to fully wrap my
> >>>>> head around it, but I finally found a sample
> >>>>> program that exhibits this. Filed a bug here:
> >>>>> https://bugs.openjdk.java.net/browse/JDK-8224161
> >>>>>
> >>>>> On Fri, May 17, 2019 at 11:21 AM Jan Lahoda
> >>>>> <jan.lahoda at oracle.com
> >>>>> <mailto:jan.lahoda at oracle.com>> wrote:
> >>>>>
> >>>>> Hi Ron,
> >>>>>
> >>>>> I am afraid it is hard to guess what is the
> >>>>> problem without some
> >>>>> testcase. So, at least to me, having a sample
> >>>>> would be helpful.
> >>>>>
> >>>>> Thanks,
> >>>>> Jan
> >>>>>
> >>>>> On 17. 05. 19 0:41, Ron Shapiro wrote:
> >>>>> > Hi,
> >>>>> >
> >>>>> > I'm observing a particularly bizarre
> >>>>> compilation. It's a single file
> >>>>> > with annotation processing, and the type that
> >>>>> is being compiled and
> >>>>> > processed has ~1000 declared and inherited
> >>>>> methods combined. The total
> >>>>> > compilation is 3 minutes, but 65% of the
> >>>>> entire compilation is spent in
> >>>>> > 3 methods:
> >>>>> >
> >>>>>
> Check.checkOverrideClashes(), Resolve.findInheritedMemberType(),
> >>>>>
> >>>>> > and Resolve.findField().
> >>>>> >
> >>>>> > Looking at profiles, it looks like
> >>>>> getSymbolsByName() is the major
> >>>>> > culprit here. I initially thought the reason
> >>>>> was that there were far too
> >>>>> > many overloads (this type had >600
> >>>>> overloads...) and that that was
> >>>>> > causing a bad regression for the
> >>>>> pseudo-hashmap that ScopeImpl uses.
> >>>>> > However, renaming the methods did not
> >>>>> alleviate the build pain and these
> >>>>> > methods continue to be taking long amounts of
> >>>>> time.
> >>>>> >
> >>>>> > I was wondering what could be done to improve
> >>>>> the performance of this
> >>>>> > code. It seemed to me that something like a
> >>>>> Map<Name, List<Symbol>>
> >>>>> > could be a reasonable+modern replacement for
> >>>>> this table, which would
> >>>>> > naturally have a fast getSymbolsForName()
> >>>>> implementation. I'm having
> >>>>> > some trouble implementing it correctly, and I
> >>>>> believe it's partially
> >>>>> > related to not fully understanding some of
> >>>>> the semantics of the class.
> >>>>> >
> >>>>> > Does what I wrote make sense to anyone, and
> >>>>> maybe spark a lightbulb?
> >>>>> >
> >>>>> > I'm trying to put together a repro in case
> >>>>> that helps, but I'm not 100%
> >>>>> > sure I even understand what the regression
> >>>>> case is.
> >>>>> >
> >>>>> > Thanks for you help!
> >>>>>
>
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