Performance of Scope.getSymbolsByName()
Maurizio Cimadamore
maurizio.cimadamore at oracle.com
Wed May 22 10:24:02 UTC 2019
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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