Re: Shrinking the C core

[Ihor Radchenko]

Emanuel Berg <incal@dataswamp.org> writes:

> Ihor Radchenko wrote:
>
>> AFAIK, users cannot specify type info manually, but types
>> are tracked when transforming Elisp byte code into
>> LIMP representation.
>
> What is LIMP?

AFAIU, native compilation code transforms Elisp into intermediate
representation called LIMPLE. I recommend reading
https://zenodo.org/record/3736363 where the native compilation process
is described in details. Because my own understanding is limited and I
may use terms not accurately.

>> The only problem (AFAIU) is that GCC JIT cannot reach inside
>> subr level, so all these information does not benefit Emacs
>> functions implemented in C.
>
> But surely that code is fast enough?

It is fast, but it could be faster if native compilation could cut off
some code branches inside the C code.

> Well, I guess optimally one would want to optimize everything
> including Emacs' C.
>
> Or do you mean it obstructs the optimization of our Elisp
> as well?

Let me demonstrate what I mean by example.

Consider a simple Elisp like

(let ((foo '(a b c))) (length foo))

`length' is defined in C like:

  EMACS_INT val;

  if (STRINGP (sequence))
    val = SCHARS (sequence);
....
  else if (CONSP (sequence))
    val = list_length (sequence);
....
  else
    wrong_type_argument (Qsequencep, sequence);

  return make_fixnum (val);

In theory, if we had full information, we could just optimize the
initial Elisp to

make_fixnum (list_length('(a b c)))

or even to just

3

as '(a b c) value is known at compile time.

However, despite knowing that the value of foo is a list constant at
compile time, native compilation code has no access to the
implementation details of `length' - it is a black box for native
compiler. So, we cannot perform the above optimization.

The workaround that is currently used is using special compile-time
expander for select C functions (like for `+' - `define_add1_sub1')
However, as we discussed earlier, this leads to multiple implementations
of the same function and makes maintenance more difficult.
So, only some very important C functions can be expanded like this.
For now, AFAIU, only `+' and `-' have native compiler expansion defined.
Other expansions are yet to be implemented.

-- 
Ihor Radchenko // yantar92,
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