bug#56641: Deprecate `lsh`

[Mattias Engdegård]

21 juli 2022 kl. 03.43 skrev Po Lu <luangruo@yahoo.com>:

> Which is the obvious use? For compatibility with programs (not
> necessarily within Emacs) that expect different values based on the
> number of VALBITs in the Emacs machine file?

Honestly I don't know. Even before Emacs 27, `lsh` seems a bit questionable 
since the width is both platform-dependent and unlike C, say, isn't a 
particularly useful size. It doesn't correspond to anything in hardware, isn't 
a power of two or anything like that.

It could be code that expects (lsh x -29) to return the sign bit as 1 or 0, as 
some kind of 'optimisation', what do I know.

Also see bug#32463 for some discussion about lsh during the transition to 
bignums. Basically everyone agreed that it's useless but that it probably needs 
to be kept for compatibility in some way.