bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?

[Andrea Corallo]

Mattias Engdegård <mattiase@acm.org> writes:

> 7 juli 2020 kl. 18.24 skrev Andrea Corallo <andrea_corallo@yahoo.it>:
>
>> Sure I'm.  The native compiler does it already but I'm curious to see
>> how you do it at source level and how generic it is.
>
> Not very, but doing it at source level has some advantages since it can 
> enable other source-level transformations.
> It's mainly a proof of concept -- for simplicity, it doesn't attempt to be 
> overly clever in the face of loops or setq.
>
> One snag is that because Emacs inline functions (defsubst) are inlined
> as bytecode, they are usually not amenable to source optimisations. It
> is only when a defsubst is imported from a different .el file that has
> not yet been byte-compiled that it is integrated as source, and then
> the machinery in this patch will nicely propagate constant arguments
> into the body.

Well loops and setq without CFG IMO are likely to be difficult if not
impossible to optimize in a generic way.  But I agree that having a
simple optimization done earlier is always better given it can benefit
other ones.

  Andrea