Re: Why is "checkcast MethodHandle" necessary in methodHandleInvokeLinkerMethod?

[Ioi Lam]

Hi John,

That was my suspicion and thanks for confirming it.

I wrote a simple benchmark to try to compare the speed of a 'real' 
method invocation vs MH.invokeExact


http://cr.openjdk.java.net/~iklam/misc/method_handle_bench/BenchMH.java

Basically

    private static void loopMH(int loops, MethodHandle mh) throws 
Throwable {

    for (int i=0; i> wrote:


So why is this first argument not declared as a MethodHandle?



The short answer is to simplify the plumbing of lambda forms.

Method handles are strongly typed but the underlying IR of
lambda forms is weakly typed.  This keeps the IR simple.
To make up the difference, casts are inserted where necessary.
Most (not all) uses of MHs are inlined and/or customized,
in which case the JIT simply drops the casts.

— John


___ mlvm-dev mailing list 
mlvm-dev@openjdk.java.net 
http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev


___
mlvm-dev mailing list
mlvm-dev@openjdk.java.net
http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev

Re: Why is "checkcast MethodHandle" necessary in methodHandleInvokeLinkerMethod?

[Ioi Lam]

So why is this first argument not declared as a MethodHandle?

Thanks

- Ioi


On 4/9/18 1:00 PM, Attila Szegedi wrote:

invokeExact_MT000_LFL_V is a static method and its first declared argument type 
is java.lang.Object, not MethodHandle. So aload_0 will load a reference with 
static type of Object, hence making a checkcast necessary to cast it to a 
MethodHandle.

HTH,
   Attila.


On 2018. Apr 9., at 21:49, Ioi Lam  wrote:

I am looking at the code generated by MethodHandleNatives.linkMethod:

(note the output was edited for brevity)

$ cat HelloMH.java
import java.lang.invoke.*;

public class HelloMH {
 public static void main(String ...args) throws Throwable {
 MethodHandles.Lookup lookup = MethodHandles.lookup();
 MethodType mt = MethodType.methodType(void.class, float.class);
 MethodHandle mh = lookup.findStatic(HelloMH.class, "callme", mt);
 mh.invokeExact(4.0f);
 }

 private static void callme(float x) {
 System.out.println("Hello MH.invoke: " + x);
 Thread.dumpStack();
 }
}

$ javac HelloMH.java

$ java -Djava.lang.invoke.MethodHandle.DUMP_CLASS_FILES=true \
 -XX:+UnlockDiagnosticVMOptions -XX:+ShowHiddenFrames -cp . HelloMH
[...]
Hello MH.invoke: 4.0
java.lang.Exception: Stack trace
 at java.base/java.lang.Thread.dumpStack(Thread.java:1434)
 at HelloMH.callme(HelloMH.java:13)
 at java.base/java.lang.invoke.LambdaForm$DM.invokeStatic_LF_V()
 at java.base/java.lang.invoke.LambdaForm$MH.invokeExact_MT_LFL_V()
 at HelloMH.main(HelloMH.java:8)


$ javap -c 'DUMP_CLASS_FILES/java/lang/invoke/LambdaForm$MH.class'
final class java.lang.invoke.LambdaForm$MH {
   static void invokeExact_MT000_LFL_V(java.lang.Object, float, 
java.lang.Object);
 Code:
0: aload_0
1: checkcast #14// class java/lang/invoke/MethodHandle
4: dup
...


Why is the checkcast necessary here?

I thought the verifier would ensure that local#0 must of type MethodHandle.

Is this checkcast needed only for reflective invocation of 
MethodHandle.invokeExact()?

Thanks
- Ioi

___
mlvm-dev mailing list
mlvm-dev@openjdk.java.net
http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev

___
mlvm-dev mailing list
mlvm-dev@openjdk.java.net
http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev


___
mlvm-dev mailing list
mlvm-dev@openjdk.java.net
http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev

Re: Why is "checkcast MethodHandle" necessary in methodHandleInvokeLinkerMethod?

[Attila Szegedi]

invokeExact_MT000_LFL_V is a static method and its first declared argument type 
is java.lang.Object, not MethodHandle. So aload_0 will load a reference with 
static type of Object, hence making a checkcast necessary to cast it to a 
MethodHandle.

HTH,
  Attila.

> On 2018. Apr 9., at 21:49, Ioi Lam  wrote:
> 
> I am looking at the code generated by MethodHandleNatives.linkMethod:
> 
> (note the output was edited for brevity)
> 
> $ cat HelloMH.java
> import java.lang.invoke.*;
> 
> public class HelloMH {
> public static void main(String ...args) throws Throwable {
> MethodHandles.Lookup lookup = MethodHandles.lookup();
> MethodType mt = MethodType.methodType(void.class, float.class);
> MethodHandle mh = lookup.findStatic(HelloMH.class, "callme", mt);
> mh.invokeExact(4.0f);
> }
> 
> private static void callme(float x) {
> System.out.println("Hello MH.invoke: " + x);
> Thread.dumpStack();
> }
> }
> 
> $ javac HelloMH.java
> 
> $ java -Djava.lang.invoke.MethodHandle.DUMP_CLASS_FILES=true \
> -XX:+UnlockDiagnosticVMOptions -XX:+ShowHiddenFrames -cp . HelloMH
> [...]
> Hello MH.invoke: 4.0
> java.lang.Exception: Stack trace
> at java.base/java.lang.Thread.dumpStack(Thread.java:1434)
> at HelloMH.callme(HelloMH.java:13)
> at java.base/java.lang.invoke.LambdaForm$DM.invokeStatic_LF_V()
> at java.base/java.lang.invoke.LambdaForm$MH.invokeExact_MT_LFL_V()
> at HelloMH.main(HelloMH.java:8)
> 
> 
> $ javap -c 'DUMP_CLASS_FILES/java/lang/invoke/LambdaForm$MH.class'
> final class java.lang.invoke.LambdaForm$MH {
>   static void invokeExact_MT000_LFL_V(java.lang.Object, float, 
> java.lang.Object);
> Code:
>0: aload_0
>1: checkcast #14// class java/lang/invoke/MethodHandle
>4: dup
>...
> 
> 
> Why is the checkcast necessary here?
> 
> I thought the verifier would ensure that local#0 must of type MethodHandle.
> 
> Is this checkcast needed only for reflective invocation of 
> MethodHandle.invokeExact()?
> 
> Thanks
> - Ioi
> 
> ___
> mlvm-dev mailing list
> mlvm-dev@openjdk.java.net
> http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev

___
mlvm-dev mailing list
mlvm-dev@openjdk.java.net
http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev