RFC: INLINE defcfun and auto FTYPE declarations [with patch]

[Andrzej Walczak]

Hello Luis,

Indeed, I am guilty as well of writing a compiler-macro based Lisp-form
compiler - as hinted in the pro at c-l.net post :)
It was one of the motivations to add FTYPEs everywhere. Thank you for the
interesting pointer.

As stated in the quoted comment, double-float boxing is one of the reasons
why we inline CFFI stubs.
It's not like SBCL could chose a different call convention depending on
known type declarations.
Once inlined, there will be no difference for FABS-1 and FABS-2 - with or
without type of ftype declarations.
Thank you for the excellent example illustrating this classic issue.

The FTYPEs help SBCL produce better code if the double-float function stubs
are not inlined (for some reason).
Also, going back to the post you have mentioned, having FTYPEs on CFFI
could help compiler-macros
take informed decisions about code transformations.

E.g. (quickly typed - might not compile)

(declaim (ftype (function (double-float double-float) (values double-float
&optional)) pow) (inline pow))
(defcfun pow :double (base :double) (exp :double))

(defun test (&rest nums)
   (sum (bind #'pow 2) nums))

Could expand to:

(defun test (&rest nums)
  (flet ((#:f1 (#:e1)
           (declare (double-float #:e1))
           (pow 2 #:e1)))
    (declare (ftype (function (double-float) (values double-float
&optional)) #:f1)
                  (dynamic-extent #'#:f1))
    (let ((#:a1 (first nums)))
      (declare (double-float #:a1))
      (dolist (#:n1 (rest nums) #:a1)
        (setf #:a1 (+ #:a1 (funcall #'#:f1 #:n1)))))))

Which should allow the compiler to choose an unboxed representation for
passing arguments to the internal #'#:f1.
BTW: I am sure SBCL would do just fine with just 1/3rd of the declarations
above - but unsure which 1/3rd ... this time of year.

Cheers,

On Tue, Nov 7, 2017 at 7:12 PM, Luís Oliveira <luismbo at gmail.com> wrote:

> On Mon, Nov 6, 2017 at 5:10 PM, Andrzej Walczak
> <andrzejwalczak at google.com> wrote:
> > After some thoughts, I agree, that the inlining control is not a
> fully-baked
> > idea, yet.
>
> Incidentally, this recent pro at c-l.net message makes me a whole lot
> less skeptical about your suggested approach:
> <https://mailman.common-lisp.net/pipermail/pro/2017-November/001464.html>.
> The introspect-environment library seems to provide a nice portable
> API for inspecting optimization policies. Alas, there doesn't seem to
> be away to define your own declarations; I was hoping we might be able
> to something like (declaim (inline-cffi-functions <boolean>)) or
> something along those lines.
>
>
> > I'll split the patch into INLINE and FTYPE parts.
>
> Sounds like a good idea.
>
>
> > You may ask why we have decided to inline (almost) every of the DEFCFUNs?
> > CFFI interfaces to C - obviously - and a lot of code in C deals with
> > double-floats and 64-bit integers.
> > Without the stubs being inline, every call to C and back produces boxed
> > values, impacting the performance.
>
> Right. Float boxing; that's a classic. But, I (perhaps naively) didn't
> expect boxing to happen even without the declarations. Take the
> following example:
>
> (in-package :cffi)
>
> (declaim (optimize (speed 3) (space 0) (safety 0) (debug 0)))
>
> (declaim (inline fabs-1))
> (defun fabs-1 (x)
>   (foreign-funcall "fabs" :double x :double))
>
> (declaim (inline fabs-2))
> (defun fabs-2 (x)
>   (declare (double-float x))
>   (foreign-funcall "fabs" :double x :double))
>
> (defun foo-1 (x)
>   (floatp (fabs-1 x)))
>
> (defun foo-2 (x)
>   (floatp (fabs-2 x)))
>
> I expected FABS-1 and FABS-2 to be identical. (FOREIGN-FUNCALL
> eventually expands to ALIEN-FUNCALL and I was expecting that would
> provide all the type information SBCL's compiler might need.) The
> disassembly shows that FAB-2 is one instruction shorter. But
> disassembling FOO-1 and FOO-2 shows that they're identical.
>
> Perhaps this example is too contrived. Do you have a better one?
>
> Cheers,
>
> --
> Luís Oliveira
> http://kerno.org/~luis/
>



-- 
Andrzej Walczak
(Google/ITA Software Engineer)
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