If I define a function in my vimrc (or in a plugin), Vim will load it and I can use it. But what's happening under the hood after my vimscript is parsed?
Is it transcoded down to C and compiled?
Is it put on the heap at runtime and referenced through a function pointer?
Is it transcoded down to C and compiled?
Vimscript is interpreted, and in fact you can do basically the same things in vimscript as you could at the : command (and the other way around), plus or minus some bar and escape characters to make things sane.
The evaluation code is mainly located in eval.c, which you can browse if you are brave.
Is it put on the heap at runtime and referenced through a function pointer?
While a built-in function is ultimately a function pointer to the C implementation, a user function is represented by a ufunc_T structure (trimmed of some stuff for brevity):
struct ufunc
{
int uf_varargs;
int uf_flags;
int uf_calls;
garray_T uf_args;
garray_T uf_lines;
scid_T uf_script_ID;
int uf_refcount;
char_u uf_name[1];
};
The function call_user_func is used to actually make the call to a user function. After doing a ton of setup to initialize arguments, variables, profiling information and state management, call_user_func does
do_cmdline(NULL, get_func_line, (void *)fc, DOCMD_NOWAIT|DOCMD_VERBOSE|DOCMD_REPEAT);
to execute each line of the function as a : command line (the implementation of do_cmdline in in ex_docmd.c).