为什么在执行时而不是在安装时编译字节码JIT？

Question

Compiling a program to bytecode instead of native code enables a certain level of portability, so long a fitting Virtual Machine exists.

But I'm kinda wondering, why delay the compilation? Why not simply compile the byte code when installing an application?

And if that is done, why not adopt it to languages that directly compile to native code? Compile them to an intermediate format, distribute a "JIT" compiler with the installer and compile it on the target machine.

The only thing I can think of is runtime optimization. That's about the only major thing that can't be done at installation time. Thoughts?

Answer 1

Often it is precompiled. Consider, for example, precompiling .NET code with NGEN .

One reason for not precompiling everything would be extensibility. Consider those languages which allow use of reflection to load additional code at runtime.

Answer 2

Some JIT Compilers (Java HotSpot, for example) use type feedback based inlining. They track which types are actually used in the program, and inline function calls based on the assumption that what they saw earlier is what they will see later. In order for this to work, they need to run the program through a number of iterations of its "hot loop" in order to know what types are used.

This optimization is totally unavailable at install time.

Answer 3

The bytecode has been compiled just as well as the C++ code has been compiled.

Also the JIT compiler, ie .NET and the Java runtimes are massive and dynamic; And you can't foresee in a program which parts the apps use so you need the entire runtime.

Also one has to realize that a language targeted to a virtual machine has very different design goals than a language targeted to bare metal.

Take C++ vs. Java.

• C++ wouldn't work on a VM, In particular a lot of the C++ language design is geared towards RAII.
• Java wouldn't work on bare metal for so many reasons. primitive types for one.

EDIT: As delnan points out correctly; JIT and similar technologies, though hugely benificial to bytecode performance, would likely not be available at install time. Also compiling for a VM is very different from compiling to native code.