Introduction to jvm

“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

• “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

Introduction to JVM

• Introduction to JVM

• Modifications to JVM(IBM JIT compiler)

• Optimization on extended bytecode

• Code generation details

• Example

What’s JVM(Java Virtual Machine)?

• What’s JVM(Java Virtual Machine)?

• Pros & Cons

• Advantage:
• Platform-neutrality, flexibility, reusability
• Disadvantage:
• Performance penalty
• Run-time overhead for bytecode instruction fetch & decode

Many relative small method can lead to more frequent method invocation than other non-OOP languages

• 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

Just-in-time (JIT) compiler

• Just-in-time (JIT) compiler

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

• Pros & Cons of JIT compiler

• Reduce overall Run-time overhead
• Introduce first-time Compilation overhead, but not afterwards

Object layout

• Object layout

Execution of static initializer

• 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.

Mixed execution

• 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

Method inlining

• Method inlining

Exception check elimination

• Exception check elimination

Common sub-expression elimination

• Common sub-expression elimination

• Scalar Replacement
• Replace subscripted variables by local variable references and them available for register allocation
• Common effective address generation
• Reduce register pressure
• Partial redundancy elimination
• Reduce the number of accesses to instance variables

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

• 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.

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

• 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

A table of frequently appearing bytecode sequences as idioms.

• 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.

Checks whether two given types are related by a subclass relationship.

• 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.

When an exception occurs, the JVM searches for the handler of the current method.

• 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.

For known exceptions:

• 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.

Most code scheduling algorithms for static compilation, when the compilation time is not important.

• 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.

• Questions