CODEWITHSUNDEEP
javajvmbytecode
Patrick Collins
Patrick Collins

Reputation: 10574

What's the purpose of the method bytecode limit?

Following up on this question: there is a 64kb bytecode limit on Java methods.

What benefit does this provide to the compiler? GCC will happily compile multi-megabyte C methods -- what's the reasoning behind this design decision? Is it just, as suggested in this question, to limit poor programming practices, since 64kb methods are massive? Or is there some specific benefit to the limit being 64kb rather than some other number?

Upvotes: 2

Views: 1679

Answers (1)

Ingo Kegel
Ingo Kegel

Reputation: 47975

Offsets in the method bytecode are 2 bytes long (called "u2" in the class file format specification). The maximum offset that can be expressed with u2 is 64kB.

The offsets appear in the actual instructions, such as the if* bytecodes, that are followed by two bytes containing the offset delta of the branch. Also, other class file attributes like the StackMapTable, LocalVariableTable, Exceptions and others contain offsets into the bytecode.

If offsets were u4, then methods could be longer, but all class files would also be larger. It's a trade-off.

Upvotes: 9

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