Bytecode interoperability: the good and bad

[Rocky Bernstein <rocky@gnu.org>]

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In documenting the ELisp Bytcode format and the compilation process, I have
a question.

In what I am calling a "Bytecode Function Literal" -- the vector of 4-6
objects that contains the parameter list, bytecode instructions, max stack,
constants vector, docstring , and interactive specification -  there is no
notion of what bytecode version is in effect.

(Is there a better term for Bytecode Function Literal?)

 The next larger kind of grouping of code is a bytecode file. A bytecode
file does contain a comment indicating the version of Emacs that was used
in compilation is recorded. However I am not sure that fact is made use of
such as to decide if a bytecode file can be run or not.

I have been able to run bytecode compiled in Emacs 25 on Emacs 24 and vice
versa, I think using load-file.

Is there a determination made in advance of whether a bytecode file
compatible with the current version of Emacs in effect? If so, how is that
done?

This is probably obvious, but I'll mention it anyway. The good side of
allowing bytecode across different releases of Emacs is that it allows one
to run bytecode from older and newer versions interoperably - when it
works. And that's the rub. Unless there is some check made, you don't know
if it will work. The program could crash, or worse do something unintended.

In other kinds of bytecode such as the one for C Python, a bytecode version
number is stored in the bytecode file. When there is a change to the
bytecode, that number is changed.

How does this work in Emacs Lisp?

Thanks.

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