Bytecode Vs. Interpreted

Question

I remember a professor once saying that interpreted code was about 10 times slower than compiled. What's the speed difference between interpreted and bytecode? (assuming that the bytecode isn't JIT compiled)

I ask because some folks have been kicking around the idea of compiling vim script into bytecode and I just wonder what kind of performance boost that will get.

Answer 1

When you compile things down to bytecode, you have the opportunity to first perform a bunch of expensive high-level optimizations. You design the byte-code to be very easily compiled to machine code and run all the optimizations and flow analysis ahead of time.

The speed-increase is thus potentially quite substantial - not only do you skip the whole lexing/parsing stages at runtime, but you also have more opportunity to apply optimizations and generate better machine code.

Answer 2

You could see a pretty good boost. However, there are a lot of factors. You can't just say that compiled code is always about 10 times faster than interpreted code, or that bytecode is n times faster than interpreted code.

Factors include the complexity and verbosity of the language for example. If a keyword in the language is several characters, and the bytecode is one, it should be quite a bit faster to load the bytecode, and jump to the routine that handles that bytecode, than it is to read the keyword string, then figure out where to go. But, if you're interpreting one of the exotic languages that has a one-byte keyword, the difference might be less noticeable.

I've seen this performance boost in practice, so it might worth it for you. Besides, it's fun to write such a thing, gives you a feel for how language interpreters and compilers work, and that will make you a better coder.

Answer 3

Are there actually any mainstream "interpreters" these days that don't actually compile their code? (Either to bytecode or something similar.)

For instance, when you use use a Perl program directly from its source code, the first thing it does is compile the source into a syntax tree, which it then optimizes and uses to execute the program. In normal situations the time spent compiling is tiny compared to the time actually running the program.

Sticking to this example, obviously Perl cannot be faster than well-optimized C code, as it is written in C. In practice, for most things you would normally do with Perl (like text processing), it will be as fast as you could reasonably code it in C, and orders of magnitude easier to write. On the other hand, I certainly wouldn't try to write a high performance math routine directly in Perl.