Is it possible to make java.lang.invoke.MethodHand

问题:

I'm comparing performance of MethodHandle::invoke and direct static method invokation. Here is the static method:

public class IntSum {
    public static int sum(int a, int b){
        return a + b;
    }
}

And here is my benchmark:

@State(Scope.Benchmark)
public class MyBenchmark {

    public int first;
    public int second;
    public final MethodHandle mhh;

    @Benchmark
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    @BenchmarkMode(Mode.AverageTime)
    public int directMethodCall() {
        return IntSum.sum(first, second);
    }

    @Benchmark
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    @BenchmarkMode(Mode.AverageTime)
    public int finalMethodHandle() throws Throwable {
        return (int) mhh.invoke(first, second);
    }

    public MyBenchmark() {
        MethodHandle mhhh = null;

        try {
            mhhh = MethodHandles.lookup().findStatic(IntSum.class, "sum", MethodType.methodType(int.class, int.class, int.class));
        } catch (NoSuchMethodException | IllegalAccessException e) {
            e.printStackTrace();
        }

        mhh = mhhh;
    }

    @Setup
    public void setup() throws Exception {
        first = 9857893;
        second = 893274;
    }
}

I got the following result:

Benchmark                      Mode  Cnt  Score   Error  Units
MyBenchmark.directMethodCall   avgt    5  3.069 ± 0.077  ns/op
MyBenchmark.finalMethodHandle  avgt    5  6.234 ± 0.150  ns/op

MethodHandle has some performance degradation.

Running it with -prof perfasm shows this:

....[Hottest Regions]...............................................................................
 31.21%   31.98%         C2, level 4  java.lang.invoke.LambdaForm$DMH::invokeStatic_II_I, version 490 (27 bytes) 
 26.57%   28.02%         C2, level 4  org.sample.generated.MyBenchmark_finalMethodHandle_jmhTest::finalMethodHandle_avgt_jmhStub, version 514 (84 bytes) 
 20.98%   28.15%         C2, level 4  org.openjdk.jmh.infra.Blackhole::consume, version 497 (44 bytes) 

As far as I could figure out the reason for the benchmark result is that the Hottest Region 2 org.sample.generated.MyBenchmark_finalMethodHandle_jmhTest::finalMethodHandle_avgt_jmhStub contains all the type-checks performed by the MethodHandle::invoke inside the JHM loop. Assembly output fragment (some code ommitted):

....[Hottest Region 2]..............................................................................
C2, level 4, org.sample.generated.MyBenchmark_finalMethodHandle_jmhTest::finalMethodHandle_avgt_jmhStub, version 519 (84 bytes) 
;...
0x00007fa2112119b0: mov     0x60(%rsp),%r10
;...
0x00007fa2112119d4: mov     0x14(%r12,%r11,8),%r8d  ;*getfield form
0x00007fa2112119d9: mov     0x1c(%r12,%r8,8),%r10d  ;*getfield customized
0x00007fa2112119de: test    %r10d,%r10d
0x00007fa2112119e1: je      0x7fa211211a65    ;*ifnonnull
0x00007fa2112119e7: lea     (%r12,%r11,8),%rsi
0x00007fa2112119eb: callq   0x7fa211046020    ;*invokevirtual invokeBasic
;...
0x00007fa211211a01: movzbl  0x94(%r10),%r10d  ;*getfield isDone
;...
0x00007fa211211a13: test    %r10d,%r10d
;jumping at the begging of jmh loop if not done
0x00007fa211211a16: je      0x7fa2112119b0    ;*aload_1 
;...

Before calling the invokeBasic we perform the type-checking (inside the jmh loop) which affects the output avgt.

QUESTION: Why isn't all the type-check moved outside of the loop? I declared public final MethodHandle mhh; inside the benchmark. So I expected the compiler can figured it out and eliminate the same type-checks. How to make the same typechecks eliminated? Is it possible?

回答1:

You use reflective invocation of MethodHandle. It works roughly like Method.invoke, but with less run-time checks and without boxing/unboxing. Since this MethodHandle is not static final, JVM does not treat it as constant, that is, MethodHandle's target is a black box and cannot be inlined.

Even though mhh is final, it contains instance fields like MethodType type and LambdaForm form that are reloaded on each iteration. These loads are not hoisted out of the loop because of a black-box call inside (see above). Furthermore, LambdaForm of a MethodHandle can be changed (customized) in run-time between calls, so it needs to be reloaded.

How to make the call faster?

1. Use static final MethodHandle. JIT will know the target of such MethodHandle and thus may inline it at the call site.

2. Even if you have non-static MethodHandle, you may bind it to a static CallSite and invoke it as fast as direct methods. This is similar to how lambdas are called.

   private static final MutableCallSite callSite = new MutableCallSite(
           MethodType.methodType(int.class, int.class, int.class));
   private static final MethodHandle invoker = callSite.dynamicInvoker();
   
   public MethodHandle mh;
   
   public MyBenchmark() {
       mh = ...;
       callSite.setTarget(mh);
   }
   
   @Benchmark
   public int boundMethodHandle() throws Throwable {
       return (int) invoker.invokeExact(first, second);
   }
   

   3. Use regular invokeinterface instead of MethodHandle.invoke as @Holger suggested. An instance of interface for calling given MethodHandle can be generated with LambdaMetafactory.metafactory().

回答2:

Make MethodHandle mhh static:

Benchmark            Mode  Samples  Score   Error  Units
directMethodCall     avgt        5  0,942 ± 0,095  ns/op
finalMethodHandle    avgt        5  0,906 ± 0,078  ns/op

Non-static:

Benchmark            Mode  Samples  Score   Error  Units
directMethodCall     avgt        5  0,897 ± 0,059  ns/op
finalMethodHandle    avgt        5  4,041 ± 0,463  ns/op