Re: defsubst VS defun or defmacro [was RE: Compiled code in Emacs-26 will fail in Emacs-25 if use pcase]

[Eric Abrahamsen]

Tino Calancha <address@hidden> writes:

> On Thu, 23 Feb 2017, T.V Raman wrote:
>
>> Please separate your personal opinion from fact. And asserting that
>> using defsubst is "misguided "  is not a useful way of carrying on a
>> conversation.
>>
>> Drew Adams writes:
>> > > Really? What should we be using, and when was defsubst deprecated?
>> > > defun or defmacro
>> > It has not been deprecated.
>> > I was expressing my opinion: It's rarely, if ever, needed,
>> > and misguided uses of it instead of defun are bothersome.
>
> Hi Raman,
> I guess Drew meant that defsubst makes debugging harder.  It also
> complicates advising.  I know he does extensively use of advising
> in his own libraries.
>
> The manual mention some disadvantages on using defsubst
> in `(elisp) inline functions':
>
> "Also, inline functions do not behave well with respect to debugging,
> tracing, and advising (*note 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."
>
> I find worth to read the whole node.  I used to be confused about when
> to use (to not) defsubst.

I read that, and still have one question -- is there any practical
difference in the compiled performance of a defsubst vs a macro? I'd
been under the (probably unfounded) impression that if you were using a
macro just to DRY a code snippet, and not using any of its other
features, you'd get better performance with a defsubst.

But probably I just made that up? After byte compilation, is there any
difference between the two (again, assuming you're not using backquotes
or any special macro functionality)?

Eric