JDK-8219713 : Reduce work in DefaultMethods::generate_default_methods
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: runtime
  • Affected Version: 8u192,11.0.2,12,13
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2019-02-26
  • Updated: 2020-08-18
  • Resolved: 2019-03-06
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JDK 13
13 b12Fixed
Related Reports
Relates :  
JDK-8251990 - Improve default method handling of private static methods in java.lang.Object
Description
The default methods generation scans the hierarchy of classes during load to determine what default and overpass methods to generate into the class being loaded. There are a few inefficiencies:

- java.lang.Object is scanned at least once for every interface in the hierarchy
- classes that implement multiple interfaces that extend each other will visit shared interfaces multiple times
- private static methods in java.lang.Object are added in the build-up, but then explicitly filtered out later: filter them out early

Using -Xlog:defaultmethods=debug we can see some of these inefficiencies in action (X is a noop class implementing AutoCloseable, Closeable, Predicate<Integer>):

[0,655s][debug][defaultmethods] Class X requires default method processing
[0,655s][debug][defaultmethods] X
[0,655s][debug][defaultmethods]   java/lang/Object
[0,655s][debug][defaultmethods]   java/lang/AutoCloseable
[0,655s][debug][defaultmethods]     java/lang/Object
[0,655s][debug][defaultmethods]   java/io/Closeable
[0,655s][debug][defaultmethods]     java/lang/Object
[0,655s][debug][defaultmethods]     java/lang/AutoCloseable
[0,655s][debug][defaultmethods]       java/lang/Object
[0,655s][debug][defaultmethods]   java/util/function/Predicate
[0,655s][debug][defaultmethods]     java/lang/Object
[0,655s][debug][defaultmethods] Slots that need filling:
[0,655s][debug][defaultmethods]   or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,655s][debug][defaultmethods]   and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,655s][debug][defaultmethods]   negate()Ljava/util/function/Predicate;
[0,655s][debug][defaultmethods]   <clinit>()V
[0,655s][debug][defaultmethods]   registerNatives()V
[0,655s][debug][defaultmethods]   Looking for default methods for slot or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods]   Looking for default methods for slot and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods]   Looking for default methods for slot negate()Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods]   Looking for default methods for slot <clinit>()V
[0,656s][debug][defaultmethods]   Looking for default methods for slot registerNatives()V
[0,656s][debug][defaultmethods] Creating defaults and overpasses...
[0,656s][debug][defaultmethods] for slot: or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods]   Selected method: java/util/function/Predicate.or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods] for slot: and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods]   Selected method: java/util/function/Predicate.and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods] for slot: negate()Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods]   Selected method: java/util/function/Predicate.negate()Ljava/util/function/Predicate;
[0,656s][debug][defaultmethods] Created 0 overpass methods
[0,656s][debug][defaultmethods] Created 3 default  methods
[0,656s][debug][defaultmethods] Default method processing complete

With experimental optimization we resolve the issue with Object being scanned a multitude of times, along with adding <clinit> / registerNatives to the candidate list for every class considered: http://cr.openjdk.java.net/~redestad/scratch/defmeth_opt.00/ 

[0,657s][debug][defaultmethods] Class X requires default method processing
[0,657s][debug][defaultmethods] X
[0,657s][debug][defaultmethods]   java/lang/Object
[0,657s][debug][defaultmethods]   java/lang/AutoCloseable
[0,657s][debug][defaultmethods]   java/io/Closeable
[0,657s][debug][defaultmethods]     java/lang/AutoCloseable
[0,657s][debug][defaultmethods]   java/util/function/Predicate
[0,657s][debug][defaultmethods] Slots that need filling:
[0,657s][debug][defaultmethods]   or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   negate()Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   Looking for default methods for slot or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   Looking for default methods for slot and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   Looking for default methods for slot negate()Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods] Creating defaults and overpasses...
[0,657s][debug][defaultmethods] for slot: or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   Selected method: java/util/function/Predicate.or(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods] for slot: and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   Selected method: java/util/function/Predicate.and(Ljava/util/function/Predicate;)Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods] for slot: negate()Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods]   Selected method: java/util/function/Predicate.negate()Ljava/util/function/Predicate;
[0,657s][debug][defaultmethods] Created 0 overpass methods
[0,657s][debug][defaultmethods] Created 3 default  methods
[0,657s][debug][defaultmethods] Default method processing complete

This translates to a small but scaling speedup in various startup tests.
Comments
Might be there's no way around the need to visit interfaces inherited via different classes/interfaces multiple times, but getting rid of multiple visits to Object and avoiding queuing up clinit()V and Object::registerNatives()V (which will be filtered out later on) is passing the extensive testing and means a substantial gain.
26-02-2019
There are very specific rules about how default methods are resolved and the order in which it must happen. I think you will find some of these apparent inefficiencies are a result of those strict rules.
26-02-2019