Merge pull request jruby#8480 from headius/optimize_subclasses_walk

Provide concrete-only traversal for Class#subclasses

Author: headius

bench/bench_subclasses.rb

@@ -1,5 +1,10 @@
 require 'benchmark/ips'
 
+Custom = Class.new(Object)
+# create these once and hold references
+SINGLETON_COUNT = 1000
+many_singletons = SINGLETON_COUNT.times.map { c = Custom.new; class << c; end; c }
+
 Benchmark.ips do |bm|
   [1, 5, 10, 50].each do |count|
     bm.report("#{count} thread Numeric.subclasses") do
@@ -24,5 +29,16 @@
         }
       }.each(&:join)
     end
+    bm.report("#{count} thread Custom.subclasses with #{SINGLETON_COUNT} singletons") do
+      count.times.map {
+        Thread.new {
+          i = 10_000 / count
+          while i > 0
+            Custom.subclasses
+            i-=1
+          end
+        }
+      }.each(&:join)
+    end
   end
 end

core/src/main/java/org/jruby/RubyClass.java

@@ -1106,7 +1106,7 @@ private int concreteSubclasses(RubyArray<RubyClass> subs) {
         int clearedCount = 0;
         while (subclassNode != null) {
             RubyClass klass = subclassNode.ref.get();
-            subclassNode = subclassNode.next;
+            subclassNode = subclassNode.nextConcrete;
 
             if (klass == null) {
                 clearedCount++;
@@ -1153,22 +1153,25 @@ private static SubclassNode rebuildSubclasses(SubclassNode subclassNode) {
             RubyClass klass = ref.get();
             subclassNode = subclassNode.next;
             if (klass == null) continue;
-            newTop = new SubclassNode(ref, newTop);
+            newTop = new SubclassNode(klass, ref, newTop);
         }
         return newTop;
     }
 
     // TODO: make into a Record
     static class SubclassNode {
         final SubclassNode next;
+        final SubclassNode nextConcrete;
+        final boolean concrete;
         final WeakReference<RubyClass> ref;
         SubclassNode(RubyClass klass, SubclassNode next) {
-            ref = new WeakReference<>(klass);
-            this.next = next;
+            this(klass, new WeakReference<>(klass), next);
         }
-        SubclassNode(WeakReference<RubyClass> ref, SubclassNode next) {
+        SubclassNode(RubyClass klass, WeakReference<RubyClass> ref, SubclassNode next) {
             this.ref = ref;
             this.next = next;
+            this.nextConcrete = next == null ? null : next.concrete ? next : next.nextConcrete;
+            this.concrete = !klass.isSingleton();
         }
     }