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An inline function is a function that works just like an
ordinary function, except for one thing: when you byte-compile a call
to the function (see Byte Compilation), the function’s definition
is expanded into the caller. To define an inline function, use
defsubst instead of defun.
This macro defines an inline function. Its syntax is exactly the same
as defun (see Defining Functions).
Making a function inline often makes its function 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.
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.
Also, inline functions do not behave well with respect to debugging,
tracing, and advising (see Advising Functions). Since ease of
debugging and the flexibility of redefining functions are important
features of Emacs, you should not make a function inline, even if it’s
small, unless its speed is really crucial, and you’ve timed the code
to verify that using defun actually has performance problems.
It’s possible to define a macro to expand into the same code that an
inline function would execute (see 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 easy; just 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.
After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros.
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