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Are there any practical uses of the Java Virtual Machine's NOP opcode in today's JVM? If so, what are some scenarios in which NOPs would be generated in bytecode?
I would even be interested to see an example of Java code that compiles into bytecode with NOPs.
Update
BCEL's MethodGen class says,
While generating code it may be necessary to insert NOP operations.
I am guessing other Bytecode generation libraries are in the same boat, as was pointed out in the accepted answer.
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Motivation. Currently, the Java Virtual Machine is constrained to a single byte of instructions. This leaves a mere 256 opcodes for implementations to work with. Of these, approximately 202 are already in use and 3 are reserved for special use cases, leaving just 55 remaining for use in the future.
A Java Virtual Machine instruction consists of an opcode specifying the operation to be performed, followed by zero or more operands embodying values to be operated upon. This chapter gives details about the format of each Java Virtual Machine instruction and the operation it performs.
They can be executed by intepretation, just-in-time compiling, or any other technique that was chosen by the designer of a particular JVM. A method's bytecode stream is a sequence of instructions for the Java virtual machine. Each instruction consists of a one-byte opcode followed by zero or more operands.
Advantages of Bytecode Bytecodes are non-runnable codes that rely on the availability of an interpreter, this is where JVM comes into play. It is a machine-level language code that runs on the JVM. It adds portability to Java which resonates with the saying, “write once, read anywhere”.
Some NOP bytecode use cases are for class file transformations, optimizations and static analysis performed by tools such as Apache BCEL, ASM, FindBugs, PMD, etc. The Apache BCEL manual touches on some uses of NOP for analysis and optimization purposes.
A JVM may use NOP bytecodes for JIT optimizations to ensure code blocks that are at synchronization safepoints are properly aligned to avoid false sharing.
As for some sample code compiled using the JDK javac compiler that contains NOP bytecodes, that's an interesting challenge. However, I doubt the compiler will generate any class file containing NOP bytecodes since the bytecode instruction stream is only single-byte aligned. I would be curious to see such an example, but I can't think of any myself.
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