Re: native compilation units

[Stefan Monnier <monnier@iro.umontreal.ca>]

> The fact that cl-flet (and cl-labels) are defined to provide immutable
> bindings is really a surprise to me.

Whether they are mutable or not is not directly relevant, tho: the
import part is that being lexically scoped, the compiler gets to see all
the places where it's used and can thus determine that it's
ever mutated.

> There is one kind of expression where Andrea isn't quite correct, and that
> is with respect to (eval-when-compile ...).

You don't need `eval-when-compile`.  It's already "not quite correct"
for lambda expressions.  What he meant is that the function associated
with a symbol can be changed in every moment.  But if you call
a function without going through such a globally-mutable indirection the
problem vanishes.

> Now the optimizer can treat ct-r1,ct-r2, and ct-r3 as constants for the
> purpose of propagation,

Same holds for

    (let* ((a (lambda (f) (lambda (x) (f (+ x 5)))))
           (b (lambda (y) (* y 3)))
           (f (funcall a b)))
      (lambda (z)
        (pow z (funcall f 6))))

>> It's also "modulo enough work on the compiler (and potentially some
>> primitive functions) to make the code fast".
> Absolutely, it just doesn't look to me like a very big lift compared to,
> say, what Andrea did.

It very depends on the specifics, but it's definitely not obviously true.
ELisp like Python has grown around a "slow language" so its code is
structured in such a way that most of the time the majority of the code
that's executed is actually not ELisp but C, over which the native
compiler has no impact.

> Does this mean the native compiled code can only produce closures in
> byte-code form?

Not directly, no.  But currently that's the case, yes.

> below with shared structure (the '(5)], but I don't see anything in
> the printed text to indicate it if read back in.

You need to print with `print-circle` bound to t, like the compiler does
when writing to a `.elc` file.

> I'm sure you're correct in terms of the current code base.  But isn't
> the history of these kinds of improvements in compilers for functional
> languages that coding styles that had been avoided in the past can be
> adopted and produce faster code than the original?

Right, but it's usually a slow co-evolution.

> In this case, it would be enabling the pervasive use of recursion and
> less reliance on side-effects.

Not everyone would agree that "pervasive use of recursion" is an improvement.

> Improvements in the gc wouldn't hurt, either.

Actually, nowadays lots of benchmarks are already bumping into the GC as
the main bottleneck.

>      ** The 'lexical-binding' local variable is always enabled.

Indeed, that's misleading.  Not sure how none of us noticed it before.


        Stefan