InvokeDynamic for Mere Mortals [JavaOne 2015 CON7682]

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Invokedynamic
for Mere Mortals
David Buck
Principal Member of Technical Staff
Java SE
October 26, 2015

Safe Harbor Statement
The following is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
6

Program Agenda
Introduction
java.lang.invoke
invokedynamic instruction
Other stuff
1
2
3
4
7

Introduction
8

Target Audience
• Not compiler writers
• Curious
9

Motivation
• Understand javap output better
• Understand the value JVM has as a multi-language JVM
10

Da Vinci Machine Project
• The JVM is a great platform for running all sorts of languages
– Great performance
– Portability
– Security (sandbox)
– Pre-existing libraries and frameworks
11

• JVM-specific
– Scala
– Clojure
– Groovy
– Ceylon
– Fortress
– Gosu
– Kotlin
• Ported to JVM
– JRuby
– Jython
– Smalltalk
– Ada
– Scheme
– REXX
– Prolog
– Pascal
– Common LISP
(a small subset of) JVM languages
12

Java Code
Language Runtime
13
JVM
OS
Java
Class Library
JRuby Runtime
JVM
OS
Java
Class Library
Ruby Code

non-Java language wish list
• Continuations
• Dynamic invocation
• Tail recursion
• Interface injection
• Other stuff
14

non-Java language wish list
• Continuations
• Dynamic invocation
• Tail recursion
• Interface injection
• Other stuff
15

What is dynamic typing?
16

def addtwo(a, b)
a + b;
end
17

We do not know what the types are until runtime
18

statically-typed vs. dynamically-typed
When do we type check / link?
– Compilation time (javac)
– Runtime
19

Compile-time checking / linking
• Catch errors early
• Limits the type of code we can write (false positives)
20

Run time checking / linking
• Allow more freedom of programming (less false positives)
• Less guarantees about runtime behavior
21

dynamic typing != type inference
object InferenceTest1 extends App {
val x = 1 + 2 * 3 // the type of x is Int
val y = x.toString() // the type of y is String
def succ(x: Int) = x + 1 // succ returns Int values
}
(Shamelessly copied from http://docs.scala-lang.org/tutorials/tour/local-
type-inference.html)
22

dynamic typing != week typing
a = "40"
b = a + 2
23

Dynamically-typed languages
• Allow more programs, but have to do more runtime checking.
• No perfect type information at compile time
24

Polymorphism != Dynamic typing (?!)
public String bar(Object o) {
return "You passed me " + o.toString();
}
25

The original invocation lineup
• invokestatic
– Class method
• invokevirtual
– Instance method
• invokeinterface
– Interface method
• Invokespecial
• Everything else (private, super class, constructors)
26

• invokestatic
– Class method
• invokevirtual
– Instance method
• invokeinterface
• Invokespecial
27

invokestatic
public class InvokeStaticExample {
public static void main(String[] args) {
InvokeStaticExample.foo();
}
public static void foo() {
System.out.println("I am foo!");
}
}
28

• invokestatic
– Class method
• invokevirtual
– Instance method
• invokeinterface
• Invokespecial
29

invokevirtual
public class InvokeVirtualExample {
InvokeVirtualExample ive = new InvokeVirtualExample();
ive.foo();
}
public void foo() {
}
}
30

• invokestatic
– Class method
• invokevirtual
– Instance method
• invokeinterface
• Invokespecial
31

public class InvokeInterfaceExample
implements MyInterface {
public static void main(String[] args)
{
MyInterface iie = new
InvokeInterfaceExample();
iie.foo();
}
public void foo() {
}
}
interface MyInterface {
public void foo();
}
32
invokeinterface

• invokestatic
– Class method
• invokevirtual
– Instance method
• invokeinterface
• Invokespecial
33

invokespecial
public class InvokeSpecialExample {
InvokeSpecialExample ise = new InvokeSpecialExample();
ise.foo();
}
private void foo() {
}
}
34

Poor dynamic languages on JVM?
• invocation logic is not baked into the JVM like it is for Java
• we need to fall back on reflection
35

Reflection is slow
• security check on each invocation
• all arguments are Objects (boxing)
36

What the JVM doesn’t know can hurt it
37
Caller Reflection Magic! Callee

Reflection prevents inlining!
38

No one writes code like this
if (false) {
// do some important stuff...
System.out.println("I'm important!");
}
39

Or this…
boolean cond = true;
if (cond) {
// do some important stuff...
System.out.println("I'm important!");
}
40

public void methodB() {
// ...
methodA(false);
// ...
}
public void methodA(boolean
optionalStuff) {
// ...
if (optionalStuff) {
// do some optional, but
important stuff...
System.out.println("I'm important
sometimes!");
}
// ...
}
41
But we do write stuff like

JSR-292
• java.lang.invoke API
A “better reflection”
• invokedynamic bytecode
Allows us to dispatch to linkage logic defined by invoke API
42

invokedynamic
• We call it “indy”
• No clear way to express in Java language
• Important milestone for JVM
– First new instruction in decades
– First new JVM feature to only (mainly) target non-java languages
43

java.lang.invoke API
• MethodHandle
• CallSite
• Bootstrap Method (BSM)
44

MethodHandle
45
int foo()Method Handle

MethodHandle
• Points to a method
• Is a “function pointer” (am I allowed to say this?)
• Polymorphic signature
46

MethodHandle Performance
47

• Early performance was not ideal
48

• Performance improved tremendously with lambda forms
49

• Is now often significantly faster than reflection
50

• Is now often significantly faster than reflection
• Can be used independently of invokedynamic
51

CallSite
52
private void doStuff();
descriptor: ()V
flags: ACC_PRIVATE
Code:
stack=2, locals=2, args_size=1
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
10: invokedynamic #9, 0
15: invokevirtual #10
18: return
CS Method Handle int foo()

CallSite
53
descriptor: ()V
flags: ACC_PRIVATE
Code:
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
18: return
CS
int bar()
int foo()

CallSite
• Reifies Indy invocation side
• Has a MethodHandle
54

Bootstrapping Step 1
55
descriptor: ()V
flags: ACC_PRIVATE
Code:
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
18: return

56
descriptor: ()V
flags: ACC_PRIVATE
Code:
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
18: return BootStrap Method

57
descriptor: ()V
flags: ACC_PRIVATE
Code:
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
int foo()

58
descriptor: ()V
flags: ACC_PRIVATE
Code:
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
int foo()
CS

59
descriptor: ()V
flags: ACC_PRIVATE
Code:
0: new #7
3: dup
4: invokespecial #8
7: astore_1
8: aload_1
9: aload_0
18: return
int foo()
CS

Bootstrap Method
• Only called on the first invocation of each indy bytecode
• Returns a CallSite
60
"Dr Martens, black, old" by Tarquin
is licensed under CC BY-SA 3.0

Indy lifecycle
Initial Invocation
1. A specific indy invocation is executed for the first time
2. Bootstrap method is called and if finds (generates?!) a method to run
3. Botstrap method returns a permanent CallSite object for this indy
invocation
4. We jump to the method pointed to by the CallSite
61

Indy Lifecycle
All subsequent calls
We jump to the method pointed to by the CallSite
62
Picture from
National Archives and Records Administration

This performance tragedy becomes
63

Linkage != Invocation
65

• Linkage (i.e. bootstrap)
– Usually only needs to be done once
– Is expensive
66

• Linkage (i.e. bootstrap)
– Usually only needs to be done once
– Is expensive
• Invocation
– Done a lot
– Only needs a jmp/call (and possibly a guard)
67

Linkage != Dispatch
• Avoid the cost of linkage on almost every call
68

Takeaways
69

Takeaways
• Invokedynamic lets us programmatically alter linkage
70

Takeaways
• Then it gets out of the way! (linkage != invocation)
71

Takeaways
• The Invoke API can often be used without indy
72

Takeaways
• The Invoke API can often be used without indy
• JVM is a great platform for just about any language!
73

Resources
• JVM Language Summit
http://openjdk.java.net/projects/mlvm/jvmlangsummit/
• Linkers & Loaders book
http://linker.iecc.com/
• John Rose’s Blog
https://blogs.oracle.com/jrose/
74

Thank You!
75

InvokeDynamic for Mere Mortals [JavaOne 2015 CON7682]

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