just in time JIT compiler


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just in time JIT compiler

  1. 1. JUST IN TIME COMPILER (JIT) Term paper submitted By To Department of Computer Science and Engineering In partial fulfilment of the Requirement for the CA of System Software To Ms. Mam (Lecturer) (December 2013)
  2. 2. CONTENTS  Acknowledgement  Introduction to JUST IN TIME COMPILERS  Time space trade off (JIT)  Functioning of JIT  Classification of JUST IN TIME COMPILERS  Conclusion  References
  3. 3. ACKNOWLEDGEMENT The satisfaction that accompanies the successful completion of any task would be incomplete without the mention of people whose ceaseless cooperation made it possible, whose constant guidance and encouragement crown all efforts with success. I am extremely grateful to my teacher ‘Ms. Chavi Ralhan Mam’ for being a source of inspiration and for her constant support in the design, implementation and evaluation of this term paper. I am thankful to him for his constant constructive criticism and valuable suggestions, which benefited me a lot while developing the term paper on ‘JUST IN TIME COMPILER’. Through this column, it would be my utmost pleasure to express my warm thanks to him for his encouragement, co-operation and consent as without which I mightn’t be able to accomplish this term paper. I would also like to express my thanks to almighty god for his grace and mercy. Above all, I am extremely thankful to my friends who always remained aside me.
  4. 4. INTRODUCTION TO JUST IN TIME COMPILER In computing, just-in-time compilation (JIT), also known as dynamic translation, is a method to improve the runtime performance of computer programs based on byte code (virtual machine code). Since byte code is interpreted it executes slower than compiled machine code, unless it is actually compiled to machine code, which could be performed before the execution – making the program loading slow – or during the execution. In this latter case – which is the basis for JIT compilation – the program is stored in memory as byte code, but the code segment currently running is preparatively compiled to physical machine code in order to run faster. JIT compilers represent a hybrid approach, with translation occurring continuously, as with interpreters, but with caching of translated code to minimize performance degradation. It also offers other advantages over statically compiled code at development time, such as handling of late-bound data types and the ability to enforce security guarantees. JIT builds upon two earlier ideas in run-time environments: byte code compilation and dynamic compilation. It converts code at runtime prior to executing it natively, for example byte code into native machine code. Several modern runtime environments, such as Microsoft's .NET Framework and most implementations of Java, rely on JIT compilation for high-speed code execution. In the Java programming language and environment, a just-in-time (JIT) compiler is a program that turns Java byte code (a program that contains instructions that must be interpreted) into instructions that can be sent directly to the processor. After you've written a Java program, the source language statements are compiled by the Java compiler into byte code rather than into code that contains instructions that match a particular hardware platform's processor (for example, an Intel Pentium microprocessor or an IBM System/390 processor). The byte code is platform-independent code that can be sent to any platform and run on that platform. In the past, most programs written in any language have had to be recompiled, and sometimes, rewritten for each computer platform. One of the biggest advantages of Java is that you only
  5. 5. have to write and compile a program once. The Java on any platform will interpret the compiled byte code into instructions understandable by the particular processor. However, the virtual machine handles one byte code instruction at a time. Using the Java just-in-time compiler (really a second compiler) at the particular system platform compiles the byte code into the particular system code (as though the program had been compiled initially on that platform). Once the code has been (re-)compiled by the JIT compiler, it will usually run more quickly in the computer. The just-in-time compiler comes with the virtual machine and is used optionally. It compiles the byte code into platform-specific executable code that is immediately executed. Sun Microsystems suggests that it's usually faster to select the JIT compiler option, especially if the method executable is repeatedly reused. TIME SPACE TRADE OFF (JIT)
  6. 6. FUNCTIONING OF JUST IN TIME COMPILERS In a byte code-compiled system, source code is translated to an intermediate representation known as byte code. Byte code is not the machine code for any particular computer, and may be portable among computer architectures. The byte code may then be interpreted by, or run on, a virtual machine. The JIT compiler reads the byte codes in many sections (or in full, rarely) and compiles them dynamically into machine language so the program can run faster. Java performs runtime checks on various sections of the code and this is the reason the entire code is not compiled at once.[1] This can be done per-file, per-function or even on any arbitrary code fragment; the code can be compiled when it is about to be executed (hence the name "just-in- time"), and then cached and reused later without needing to be recompiled. In contrast, a traditional interpreted virtual machine will simply interpret the bytecode, generally with much lower performance. Some interpreters even interpret source code, without the step of first compiling to byte code, with even worse performance. Statically compiled code or native code is compiled prior to deployment. A dynamic compilation environment is one in which the compiler can be used during execution. For instance, most Common Lisp systems have a compile function which can compile new functions created during the run. This provides many of the advantages of JIT, but the programmer, rather than the runtime, is in control of what parts of the code are compiled. This can also compile dynamically generated code, which can, in many scenarios, provide substantial performance advantages over statically compiled code [citation needed], as well as over most JIT systems. A common goal of using JIT techniques is to reach or surpass the performance of static compilation, while maintaining the advantages of byte code interpretation: Much of the "heavy lifting" of parsing the original source code and performing basic optimization is often handled at compile time, prior to deployment: compilation from byte code to machine code is much faster than compiling from source. The deployed byte code is portable, unlike native code. Since the runtime has control over the compilation, like interpreted byte code, it can run in a secure sandbox. Compilers from byte code to machine code are easier to write, because the portable byte code compiler has already done much of the work.
  7. 7. JIT code generally offers far better performance than interpreters. In addition, it can in some cases offer better performance than static compilation, as many optimizations are only feasible at run-time:  The compilation can be optimized to the targeted CPU and the operating system model where the application runs. For example JIT can choose SSE2 vector CPU instructions when it detects that the CPU supports them. However there is currently no mainstream JIT that implements this. To obtain this level of optimization specificity with a static compiler, one must either compile a binary for each intended platform/architecture, or else include multiple versions of portions of the code within a single binary.  The system is able to collect statistics about how the program is actually running in the environment it is in, and it can rearrange and recompile for optimum performance. However, some static compilers can also take profile information as input.  The system can do global code optimizations (e.g. in lining of library functions) without losing the advantages of dynamic linking and without the overheads inherent to static compilers and linkers. Specifically, when doing global inline substitutions, a static compilation process may need run-time checks and ensure that a virtual call would occur if the actual class of the object overrides the inlined method, and boundary condition checks on array accesses may need to be processed within loops. With just-in-time compilation in many cases this processing can be moved out of loops, often giving large increases of speed.  Although this is possible with statically compiled garbage collected languages, a byte code system can more easily rearrange executed code for better cache utilization.
  8. 8. CLASSIFICATION OF JUST IN TIME COMPILERS In the course of surveying JIT work, some common attributes emerged. We propose that JIT systems can be classified according to three properties: (1) Invocation:- A JIT compiler is explicitly invoked if the user must take some action to cause compilation at runtime. An implicitly invoked JIT compiler is transparent to the user. (2) Executability:- JIT systems typically involve two languages: a source language to translate from, and a target language to translate to (although14 As opposed to the ongoing optimization of Kistler’s [2001] “continuous optimization,” only compilation occurred concurrently using “continuous compilation,”and only happened once. these languages can be the same, if the JIT system is only performing optimization on-the-fly). We call a JITsystem monoexecutable if it can only execute one of these languages, and polyexecutable if can execute more than one. Polyexecutable JIT systemshave the luxury of deciding when compiler invocation is warranted, since either program representation can be used. (3) Concurrency:- This property characterizes how the JIT compiler executes, relative to the program itself. If program execution pauses under its own volition to permit compilation, it is not concurrent; the JIT compiler in this case may be invoked via subroutine call, message transmission, or transfer of control to a coroutine. In contrast, a concurrent JIT compiler can operate as the program executes concurrently: in a separate thread or process, even on a different processor. JIT systems that function in hard real time may constitute a fourth classifying property, but there seems to be little research in the area at present; it is unclear if hard real-time constraints pose any unique problems to JIT systems. Some trends are apparent. For instance, implicitly invoked JIT compilers are definitely predominant in recent work. Executability varies from system to system, but this is more an issue of design than an issue of JIT technology. Work on concurrent JIT compilers is currently only beginning, and will likely increase in importance as processor technology evolves.
  9. 9. CONCLUSION Here, I conclude my lines of my term paper on the topic ‘JUST IN TIME COMPILER’ with the extreme satisfaction and contentment. This term paper contains brief definition of Just IN Time Compiler together with its features and functions. Added to this, my term paper contains the basic description about Just IN Time Compiler and its working. It also includes practical implementation of compilation process step by step. Also I have sincerely included the references from where I have made my term paper. This term paper is the outcome of my hard and laborious work and contains a complete knowledge on the path independent line integral. Here, I end my lines with the hope that my term paper will be equally appreciated and heartily accepted by all. Also, all my faults and mistakes would be forgiven.
  10. 10. REFERENCES :-  http://searchcio-midmarket.techtarget.com/definition/just-in-time-compiler  http://en.wikipedia.org/wiki/Just-in-time_compilation  http://stackoverflow.com/questions/95635/what-does-a-just-in-time-jit-compiler-do  https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved =0CCgQFjAA&url=http%3A%2F%2Fweb.csie.cgu.edu.tw%2F~jhchen%2Fcourse%2FP L2%2FA%2520brief%2520history%2520of%2520just-in- time.pdf&ei=G_uIUqfPJI6qrAfX4ICQBA&usg=AFQjCNHFGgTcsPOUm4_hZnuF9ggi d-szmA&bvm=bv.56643336,d.bmk