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(elisp)Inline Functions

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Inline Functions
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   You can define an "inline function" by using `defsubst' instead of
`defun'.  An inline function works just like an ordinary function
except for one thing: when you compile a call to the function, the
function's definition is open-coded into the caller.

   Making a function inline makes explicit calls run faster.  But it
also has disadvantages.  For one thing, it reduces flexibility; if you
change the definition of the function, calls already inlined still use
the old definition until you recompile them.  Since the flexibility of
redefining functions is an important feature of Emacs, you should not
make a function inline unless its speed is really crucial.

   Another disadvantage is that making a large function inline can
increase the size of compiled code both in files and in memory.  Since
the speed advantage of inline functions is greatest for small
functions, you generally should not make large functions inline.

   It's possible to define a macro to expand into the same code that an
inline function would execute.  (Note: Macros.)  But the macro would
be limited to direct use in expressions--a macro cannot be called with
`apply', `mapcar' and so on.  Also, it takes some work to convert an
ordinary function into a macro.  To convert it into an inline function
is very easy; simply replace `defun' with `defsubst'.  Since each
argument of an inline function is evaluated exactly once, you needn't
worry about how many times the body uses the arguments, as you do for
macros.  (Note: Argument Evaluation.)

   Inline functions can be used and open-coded later on in the same
file, following the definition, just like macros.