OVERVIEW OF THE IBM JAVA JUST-IN-TIME COMPILER Presenters: Zhenhua Liu, Sanjeev Singh

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REFERENCE “Overview of the IBM Java Just-in-Time”,

T. Suganuma, T. Ogasawara, M. Takeuchi, T. Yasue, M. Kawahito, K. Ishizaki, H. Komatsu, T. Nakatani IBM Systems Journal, Volume 39 , No. 1, Pages: 175 – 193, ISSN:0018-8670

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OUTLINEIntroduction to JVMModifications to JVM(IBM JIT compiler)

Optimization on extended bytecodeCode generation detailsExample

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INTRODUCTION TO JVM What’s JVM(Java Virtual Machine)?

Pros & Cons Advantage:

Platform-neutrality, flexibility, reusability Disadvantage:

Performance penalty Run-time overhead for bytecode instruction fetch &

decode

Source Code Bytecode Machine

CodeJVM

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WHY IS JAVA SLOW? Many relative small method can lead to more

frequent method invocation than other non-OOP languages

Requires run-time exception checking to ensure validity of accesses to objects and arrays

Synchronized methods or synchronized blocks cause run-time overhead by locking a given object for the duration of execution

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HOW TO SPEED UP JVM? Just-in-time (JIT) compiler

Converts the given bytecode instruction sequence at runtime before executing it natively

Pros & Cons of JIT compilerReduce overall Run-time overhead Introduce first-time Compilation overhead,

but not afterwards6

STRUCTURE OF JIT COMPILER

Bytecode-level Optimization

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BASE JVM MODIFICATION (1/2) Object layout

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BASE JVM MODIFICATION (2/2) Execution of static initializer

Separate the resolution of a class from the execution of its static initializer

Before Change A class is resolved at run-time, together with the

execution of static initializer. After Change

A class is resolved at compile-time. The static initializer is executed upon run-time calls.

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SELECTIVE COMPILATION Mixed execution

Interpretation + Compiled Code(JIT-generated code)

Rules (using JIT or not?)

Yes Hot methods(frequently invoked methods) Hot branches (frequently traversed branches) Loops included

No Only executed once No loop included 10

BYTECODE-LEVEL OPTIMIZATION Method inlining

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BYTECODE-LEVEL OPTIMIZATION Exception check elimination

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BYTECODE-LEVEL OPTIMIZATION Common sub-expression elimination

Scalar ReplacementReplace subscripted variables by local variable references and them available for register allocation

Common effective address generationReduce register pressure

Partial redundancy eliminationReduce the number of accesses to instance variables 13

CODE GENERATION DETAILS

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The JIT Compiler uses a table with pointers to the methods in the class.

This internal table is used to compile the native code.

Whenever the method is called the address is called.

There is another table which maintains the addresses of the bytecode in case it is needed to be compiled.

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REGISTER ALLOCATION

The code is broken into tiles. Each loop is a tile and the code in between loops is another tile.

Local variable usage is collected within each tile and a local variable table is prepared.

3 types of registers Stack variable registers Permanent cached local variable registers Temporary cached local variable registers

Circular allocation policy Least recently used register 16

IDIOMS IN BYTECODE SEQUENCES A table of frequently appearing bytecode

sequences as idioms. Reduce the stack height of expression evaluation. Exploits local variable cache registers by avoiding

unnecessary move instructions between local and stack variables.

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TYPE INCLUSION TEST (1/2) Checks whether two given types are related

by a subclass relationship. Runtime overhead. Cache mechanism for type inclusion testing. Given an object

Null check. Else, compared with the value of the cache that

holds the successful class from the previous test. Else , JVM runtime library is checked and cache

updated.

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TYPE INCLUSION TEST (2/2)

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EXCEPTION HANDLING (1/2) When an exception occurs, the JVM searches

for the handler of the current method. If not found, it pops the last frame off the

stack and searches for the handler of the next method on the stack

Till a handler is found or no more frames on the stack.

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EXCEPTION HANDLING (2/2) For known exceptions: Each method is registered in an exception

registration record. It is done for only those methods which have

a try/catch block. It maintains an exception chain. During exceptions,a conditional jump is

created to the bottom of the code.

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CODE SCHEDULING Most code scheduling algorithms for static

compilation, when the compilation time is not important.

A code scheduler works with the code generator for a basic block.

When a native code is generated, it is given to the scheduler with some attributes, like reference registers, exception flag, address of memory access etc.

The scheduler considers the dependencies and arranges the code.

Has to guarantee proper execution. Cannot reorder two instructions which may raise exceptions. 22

EXAMPLE

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Questions

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