In the beginning, a compiler was responsible for turning a high-level language defined as higher level than assembler into object code machine instructions , which would then be linked by a linker into an executable.
At one point in the evolution of languages, compilers would compile a high-level language into pseudo-code, which would then be interpreted by an interpreter to run your program. This eliminated the object code and executables, and allowed these languages to be portable to multiple operating systems and hardware platforms.
Pascal which compiled to P-Code was one of the first; Java and C are more recent examples. Eventually the term P-Code was replaced with bytecode, since most of the pseudo-operations are a byte long.
A Just-In-Time JIT compiler is a feature of the run-time interpreter, that instead of interpreting bytecode every time a method is invoked, will compile the bytecode into the machine code instructions of the running machine, and then invoke this object code instead. Ideally the efficiency of running object code will overcome the inefficiency of recompiling the program every time it runs. The source code will be converted into assembly language like structure [for ex IL intermediate language for C , ByteCode for java].
The intermediate code is converted into machine language only when the application needs that is required codes are only converted to machine code. In JIT not all the code is converted into machine code first a part of the code that is necessary will be converted into machine code then if a method or functionality called is not in machine then that will be turned into machine code, which reduces burden on the CPU.
The JIT compiler can use more than one compilation thread, which typically improves startup performance. The number of threads is determined by the VM, depending on the system configuration.
You can turn off multiple threads by using the -XcompilationThreads option, which simplifies the output in the verbose log. When the invocation count is set to 0 , the JIT compiles every method and your application will fail immediately when the method causing the problem is reached.
You can alter the threshold with the count suboption. Inlining is a complex process that generates larger and more complex code.
To eliminate errors caused by these operations, use the disableInlining suboption. Use the optlevel suboption to gradually decrease the compiler optimization levels to see whether you can isolate the level at which your problem occurs.
More information about these suboptions and the command line syntax is covered in -Xjit. At first glance, a JIT verbose log can look very complex. To help you understand the log we'll look at JIT compiler operations when you run the java -version command. The following option turns on verbose logging and directs output to a log file called vlogfile :. The first section of the log includes lines that start with INFO: , which provides information about the environment that the JIT is operating in.
The last few lines detail the start time of the compilation activity, how much free physical memory is available to the process, and the CPU entitlement. The information section is followed by a sequence of lines that describe the methods that are being compiled, as well as other events significant to the operation of the JIT compiler.
The following example requests information about the performance of JIT compiler threads, with output written to vlogfile. Code compiled at the second tier has to identify when a method becomes hot, so it also has to increment invocation and back-branch counters.
Note that this description is simplified: The implementation tries to not overwhelm compiler threads with requests, and to balance execution speed with compilation load. As a consequence, thresholds that trigger compilations are not fixed and the second tier is actually split into several sub-tiers.
In HotSpot, for historical reasons, the second tier is known as C1 or the client compiler and the optimizing tier is known as C2, or the server compiler. In the HelloWorld example, the third column of numbers in the diagnostic output identifies the tier at which code is compiled. Tiers 1 to 3 are subtiers of the low-tier compiler. Tier 4 is the optimizing compiler. As can be seen in the output, that example is so short-lived that no method reaches the optimizing compiler.
State changes happen left to right. This is incomplete as, perhaps surprisingly, state transitions also exist right to left that is, from more optimized code to less optimized code , as shown in Figure 2. In HotSpot jargon, that process is called deoptimization.
When a thread deoptimizes, it stops executing a compiled method at some point in the method and resumes execution in the same Java method at the exact same point, but in the interpreter. Why would a thread stop executing compiled code to switch to much slower interpreted code? There are two reasons. First, it is sometimes convenient to not overcomplicate the compiler with support for some feature's uncommon corner case. Rather, when that particular corner case is encountered, the thread deoptimizes and switches to the interpreter.
The second and main reason is that deoptimization allows the JIT compilers to speculate. When speculating, the compiler makes assumptions that should prove correct given the current state of the virtual machine, and that should let it generate better code.
However, the compiler can't prove its assumptions are true. If an assumption is invalidated, then the thread that executes a method that makes the assumption deoptimizes in order to not execute code that's erroneous being based on wrong assumptions.
An example of speculation that C2 uses extensively is its handling of null checks. In Java, every field or array access is guarded by a null check. Here is an example in pseudocode:. Here's that speculation in pseudocode:. If NPEs never occur, all the logic for exception creation, throwing, and handling is not needed.
What if a null object is seen at the field access in the pseudocode? The thread deoptimizes, a record is made of the failed speculation, and the compiled method's code is dropped. On the next JIT compilation of that same method, C2 will check for a failed speculation record before speculating again that no null object is seen at the field access. The call in compiledMethod is a virtual call. With only class C loaded but none of its potential subclasses, that call can only invoke C.
When compiledMethod is JIT compiled, the compiler could take advantage of that fact to devirtualize the call. By submitting your email, you agree to the Terms of Use and Privacy Policy. In order to translate human-readable code into something your CPU can run, it must be converted. A computer usually does this through one of two methods—compilation or interpretation. Compilation involves running a compiler, a program that takes source code and converts it into binary machine code, before running the application.
Most executables that you download are compiled at some level to run on your machine, and compiled code is usually pretty fast because the compiler can make optimizations for your particular machine. However, compilers have a few downsides.
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