Why doesn't the JVM compile "incrementing an int variable" as an atom operation of Fetch-and-Increment?

Question

I found out that incrementing an int variable in Java is NOT an atomic operation, however I found that processors support atomic operation Fetch-and-Increment.

So my question is: why does the JVM not compile the operation incrementing a int variablefor the atomic Fetch-and-Incrementoperation supported by the CPU, which may be useful in multi-threaded programming.

Early processors had atomic tests and settings, Fetch-and-Incrementor swap commands, sufficient to implement mutexes, which, in turn, could be used to implement more complex parallel objects.

- Java Concurrency in Practice

Answer 1

Because the Java Standard (JLS) does not require this and because it is an expensive operation that should only be used when necessary.

Answer 2

So my question is why the JVM does not compile the increment of an operation with int variables using the atomic Fetch-and-Increment operation supported by the CPU, which can be useful in multi-threaded programming.

Since on typical modern processors atomic read-modify-write operations (for example, increments) are tens of times more expensive than their corresponding non-atomic operations. And this will not give any advantage - the code cannot rely on atomic operations, because they are not guaranteed to be atomic. So what will be the benefit?

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Check answer Why am I ++ not atomic? mentioned by one of the colleagues in the comments

Answer 7

Because:

• Java compilers are not compiled to machine code, they are compiled to bytecode and

• There is no fetch-and-increment atom instruction in the JVM bytecode, with the exception of local variables.