Understanding JIT compiler (just-in-time compiler)

he Just-In-Time (JIT) compiler is a component of the Java Runtime Environment that improves the performance of Java applications at run time. Nothing in the JVM affects performance more than the compiler, and choosing a compiler is one of the first decisions made when running a Java application—whether you are a Java developer or an end-user.

Java JIT Compiler: An Overview

The key of java power "Write once, run everywhere" is bytecode. The way bytecodes get converted to the appropriate native instructions for an application has a huge impact on the speed of an application. These bytecode can be interpreted, compiled to native code or directly executed on a processor whose Instruction Set Architecture is the bytecode specification. Interpreting the bytecode which is the standard implementation of the Java Virtual Machine (JVM) makes execution of programs slow. To improve performance, JIT compilers interact with the JVM at run time and compile appropriate bytecode sequences into native machine code. When using a JIT compiler, the hardware can execute the native code, as opposed to having the JVM interpret the same sequence of bytecode repeatedly and incurring the penalty of a relatively lengthy translation process. This can lead to performance gains in the execution speed, unless methods are executed less frequently. The time that a JIT compiler takes to compile the bytecode is added to the overall execution time, and could lead to a higher execution time than an interpreter for executing the bytecode if the methods that are compiled by the JIT are not invoked frequently. The JIT compiler performs certain optimizations when compiling the bytecode to native code. Since the JIT compiler translates a series of bytecode into native instructions, it can perform some simple optimizations. Some of the common optimizations performed by JIT compilers are data-analysis, translation from stack operations to register operations, reduction of memory accesses by register allocation, elimination of common sub-expressions etc. The higher the degree of optimization done by a JIT compiler, the more time it spends in the execution stage. Therefore a JIT compiler cannot afford to do all the optimizations that is done by a static compiler, both because of the overhead added to the execution time and because it has only a restricted view of the program.

How does it work ?!

The Just-In-Time (JIT) compiler is a component of the Java Runtime Environment that improves the performance of Java applications at run time. Java programs consists of classes, which contain platform neutral bytecode that can be interpreted by a JVM on many different computer architectures. At run time, the JVM loads the class files, determines the semantics of each individual bytecode, and performs the appropriate computation. The additional processor and memory usage during interpretation means that a Java application performs more slowly than a native application. The JIT compiler helps improve the performance of Java programs by compiling bytecode into native machine code at run time.

The JIT compiler is enabled by default, and is activated when a Java method is called. The JIT compiler compiles the bytecode of that method into native machine code, compiling it "just in time" to run. When a method has been compiled, the JVM calls the compiled code of that method directly instead of interpreting it. Theoretically, if compilation did not require processor time and memory usage, compiling every method could allow the speed of the Java program to approach that of a native application.
JIT compilation does require processor time and memory usage. When the JVM first starts up, thousands of methods are called. Compiling all of these methods can significantly affect startup time, even if the program eventually achieves very good peak performance.

Different compilers for different applications

The JIT compiler comes in two flavors, and the choice of which compiler to use is often the only compiler tuning that needs to be made when running an application. In fact, knowing which compiler you want to choose is something that must be considered even before Java is installed, since different Java binaries contain different compilers.

Client-side compilers

A well-known optimizing compiler is C1, the compiler that is enabled through the -client JVM startup option. As its startup name suggests, C1 is a client-side compiler. It is designed for client-side applications that have fewer resources available and are, in many cases, sensitive to application startup time. C1 use performance counters for code profiling to enable simple, relatively unintrusive optimizations.

Server-side compilers

For long-running applications such as server-side enterprise Java applications, a client-side compiler might not be enough. A server-side compiler like C2 could be used instead. C2 is usually enabled by adding the JVM startup option -server to your startup command-line. Since most server-side programs are expected to run for a long time, enabling C2 means that you will be able to gather more profiling data than you would with a short-running light-weight client application. So you'll be able to apply more advanced optimization techniques and algorithms.

Tiered compilation

Tiered compilation combines client-side and server-side compilation. Tiered compilation takes advantage of both client and server compiler advantages in your JVM. The client compiler is most active during application startup and handles optimizations triggered by lower performance-counter thresholds. The client-side compiler also inserts performance counters and prepares instruction sets for more advanced optimizations, which will be addressed at a later stage by the server-side compiler. Tiered compilation is a very resource-efficient way of profiling because the compiler is able to collect data during low-impact compiler activity, which can be used for more advanced optimizations later. This approach also yields more information than you'll get from using interpreted code profile counters alone.

Code optimization

When a method is chosen for compilation, the JVM feeds its bytecode to the Just-In-Time compiler (JIT). The JIT needs to understand the semantics and syntax of the bytecode before it can compile the method correctly. To help the JIT compiler analyze the method, its bytecode are first reformulated in an internal representation called trees, which resembles machine code more closely than bytecode. Analysis and optimizations are then performed on the trees of the method. At the end, the trees are translated into native code. The JIT compiler can use more than one compilation thread to perform JIT compilation tasks. Using multiple threads can potentially help Java applications to start faster. In practice, multiple JIT compilation threads show performance improvements only where there are unused processing cores in the system. The default number of compilation threads is identified by the JVM, and is dependent on the system configuration. If the resulting
number of threads is not optimum, you can override the JVM decision by using the XcompilationThreads option. For information on using this option, see JIT and AOT command-line options

The compilation consists of the following phases:


Inlining is the process by which the trees of smaller methods are merged, or "inlined", into the trees of their callers. This speeds up frequently executed method calls.

Local optimizations

Local optimizations analyze and improve a small section of the code at a time. Many local optimizations implement tried and tested techniques used in classic static compilers.

Control flow optimizations

Control flow optimizations analyze the flow of control inside a method (or specific sections of it) and rearrange code paths to improve their efficiency.

Global optimizations

Global optimizations work on the entire method at once. They are more "expensive", requiring larger amounts of compilation time, but can provide a great increase in performance.

Native code generation

Native code generation processes vary, depending on the platform architecture. Generally, during this phase of the compilation, the trees of a method are translated into machine code instructions; some small optimizations are performed according to architecture characteristics.

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