Download Multi-Dispatch in the Java™ Virtual Machine

Multi-Dispatch in the Java™
Virtual Machine
Design, Implementation, and
Evaluation
Christopher Dutchyn
Computing Science
University of Alberta
5/24/2017
Multi-Dispatch Java
1
The Problem
• Production OO languages (C++, Java)
select the method to run based on the
dynamic type of a single argument only:
}
Color
HPJet
PScript
.draw(Shape)
• We call this uni-dispatch
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Multi-Dispatch Java
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The Goal
• We want method selection based on the
dynamic types of more than one, and
possibly all of the arguments:
}(
Color
HPJet
PScript
.print
)
Circle
Ellipse
Square
Rectangle
• We call this multiple dispatch
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Multi-Dispatch Java
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Double Dispatch Solutions
• Type fields – switch on constant integers
– maintain types encoded as obscure numbers
– risk that some type fields might be omitted
• Typecases – if
…instanceof …else if…else…
– risk that some types might not be tested for
– instanceof tests contain order dependencies
• Visitor pattern – sequence of uni-dispatches
– duplicates dispatcher code into many classes
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Multi-Dispatch Java
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Another Solution
• Perform a single
dynamic dispatch
operation that
considers the
dynamic types of all
of the arguments
• We call this
multi-dispatch.
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interface Printer implements
VirtualMultiDispatchable {
void print(Shape s);
}
class Color implements Printer {
void print(Shape s) {/*null*/}
void print(Circle c) {/*.cc.*/}
void print(Ellipse e){/*.ce.*/}
void print(Rectangle r){/*cr*/}
}
class HPJet implements Printer {
void print(Shape s) {/*null*/}
void print(Circle c) {/*.hc.*/}
void print(Ellipse e){/*.he.*/}
void print(Rectangle r){/*hr*/}
// similar for other Shapes
}
// similar for PScript
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Research Contributions
• Added dynamic multi-dispatch to Java
• without changing syntax or compiler,
• allowing programmer to select classes supporting multidispatch,
• without penalizing existing uni-dispatch,
• and maintaining reflection and existing APIs.
• Realizes the following benefits
• shorter, simpler, and more maintainable code,
• with equal or better performance than complex,
error-prone double dispatch.
• Published COOTS 2001
• Best student paper award.
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Multi-Dispatch At a Call Site
1. Look up the uni-dispatch method
2. If multi-dispatchable
a) Walk the operand stack to obtain precise types
HPJet and Circle instead of Printer and Shape
b) Select the method that most closely matches the
argument types:
HPJet.print(Circle)
c) Verify return types conform
3. Invoke new method
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Evaluation: Two Criteria
• Compatibility with uni-dispatch
– Do existing uni-dispatch Java programs continue
to run correctly?
– What performance penalty does our additions
impose on pure uni-dispatch programs?
• Performance versus double dispatch
– How does multi-dispatch compare with existing
double dispatch techniques?
– Does multi-dispatch scale to full applications?
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Uni-Dispatch Java Support
• The java compiler, javac, is a large Java
program that runs over a Java Virtual
Machine.
• As part of constructing the JDK, javac
runs on the just-constructed JVM to
compile 5000+ classes comprising the
Java Class Libraries.
• Our multi-dispatch JVMs host those
compilations.
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Uni-Dispatch Overhead
Uni-dispatch Timing
– multi-invoker adds zero
– Inlined tests adds 2.5%
• sun.tools.*
MSA-MI
0.02
Times (us)
[lower is better]
• individual uni-dispatch
times
MSA-INL
SRP-MI
0.015
SRP-INL
0.01
0.005
0
compile
sun.tools Compile Times
UNI
MSA-MI
• cache contention due to
larger data structures
• class load overhead
exaggerated by
repeated loading
Times (s)
[lower is better]
– multi-dispatch adds 2-3%
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UNI
Multi-Dispatch Java
50
MSA-INL
40
SRP-MI
SRP-INL
30
20
10
0
Com pile Tim es (s)
10
Double vs. Multi-Dispatch
Double vs. Multi-Dispatch
• Using existing event
processing kernel from
0.6
sun.awt.component
• Comparison of
–
–
–
–
–
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original kernel
typecases
type fields
Visitor
multi-dispatch (SRP)
0.5
Time (us) [lower is better]
– 7 event types
– 7 components
Double
Multi-
0.4
0.3
0.2
0.1
0
TypeCase
Kernel
Typefield
Visitor
SRP
Dispatcher
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Multi-Swing (and AWT)
1. Modified 92 of 846 classes (11%)
2. Replaced 171 conditionals (5%)
3. Mean number of decision points reduced
from 3.8 to 2.0 per method
4. Added 57 new event subclasses
5. Added 123 new multimethods
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Multi-Swing Results
Dispatches
Execution Times
34.00
32.00
30.00
28.00
26.00
24.00
22.00
20.00
18.00
16.00
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
Swing
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16.00
Uni-dispatches
Seconds [lower is better]
Dispatches (millions)
Multi-dispatches
Swing
Multi-Swing
Multi-Dispatch Java
Multi-Swing
13
Research Contributions
• Added dynamic multi-dispatch to Java
• without changing syntax or compiler,
• allowing programmer to select classes supporting multidispatch,
• without penalizing existing uni-dispatch,
• and maintaining reflection and existing APIs.
• Realizes the following benefits
• shorter, simpler, and more maintainable code,
• with equal or better performance than complex,
error-prone double dispatch.
• Published COOTS 2001
• Best student paper award.
5/24/2017
Multi-Dispatch Java
14