bug#58513: describe-function should say that defconst variables are constant

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Lars Ingebrigtsen [2022-10-14 16:25:30] wrote:
> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> FWIW, my own local Emacs has `defconst` create actually
>> constant variables by adding a `SYMBOL_DEFCONST` to `enum
>> symbol_trapped_write`.  Admittedly I didn't make them really constant:
>> I allow a subsequent `defconst` to change the value set by a previous
>> `defconst`, but `setq` signals an error just like it does if you try to
>> `setq` on `enable-multibyte-characters` or on `nil`.
>
> Hm...  do you know if there's a performance impact from doing this?

In terms of memory use, we already use 2 bits for that `enum
symbol_trapped_write`, so adding a new value doesn't cost anything
at all.

In terms of run-time, it adds a 4th branch to a few 3-branch `switch`
statements, so it can have a performance impact, most importantly in
`set_internal`, I guess, but I'd be very surprised if it's ever not lost
in the noise:

- it only affects `setq` on global/dynamic vars which should be a small
  fraction of the overall `setq` we perform nowadays.
- since that new branch always signals an error (well, in my own code,
  that is: I don't know what that branch would end up doing on `master`
  where we'd want to be more careful about backward compatibility), it's
  expected to be more-or-less never taken and even the most naive branch
  predictors should handle it "perfectly".


        Stefan