constantfold


* Constantfold - User-defined constant folding facility 

This is a supporting library for NUMCL.
It provides a single macro, =(constantfold name &key commutative associative copier)=.

=constantfold= registers the function =NAME= as an additional form that is
considered as a candidate for a constant.  For example,

#+begin_src lisp
(defstruct point2d
  (x 0 :type 'real)
  (y 0 :type 'real))

(defun point2d-add/2 (p1 p2)
  (ematch* (p1 p2)
    (((point2d :x x1 :y y1)
      (point2d :x x2 :y y2))
     (make-point2d :x (+ x1 x2) :y (+ y1 y2)))))

(defun point2d-add (&rest args)
  (reduce #'point2d-add/2 args))

(constantfold make-point2d :copier copy-point2d)
(constantfold point2d-add  :copier copy-point2d :commutative t :associative t)
#+end_src

This allows the compiler to constant fold

#+begin_src lisp
(defun fn1 () (point2d-add (make-point2d :x 1 :y 2) (make-point2d :x 3 :y 4)))
#+end_src

and

#+begin_src lisp
(defun fn1 () (point2d-add #S(point2d :x 1 :y 2) #S(point2d :x 3 :y 4)))
#+end_src

into

#+begin_src lisp
(defun fn1 () (copy-point2d #S(point2d :x 4 :y 6))).
#+end_src

When =:associative= and/or =:commutative= is true, it performs the corresponding
code morphing and tries to fold constants as much as possible.  For example,

#+begin_src lisp
(defun fn2 (p1 p2 x y)
  (point2d-add (make-point2d :x 1 :y 2)
               p1
               (make-point2d :x 3 :y 4)
               p2
               (make-point2d :x x :y y)))
#+end_src

gets compiled into

#+begin_src lisp
(defun fn2 (p1 p2 x y)
  (point2d-add (copy-point2d #S(point2d :x 4 :y 6))
               p1
               p2
               (make-point2d :x x :y y)))
#+end_src

due to commutativity.

This library preserves the original semantics of the code. For example,

#+begin_src lisp
(defun fn3 ()
  (make-point2d :x 1 :y 2))
#+end_src

originally always instantiates a new object. This is compiled into

#+begin_src lisp
(defun fn3 ()
  (copy-point2d #S(point2d :x 1 :y 2)))
#+end_src

which also instantiates a new object.
The reason why =fn1/fn2= are allowed to remove some instance creations while
=fn3= does not perform the pruning is that the result of =make-point2d= in =fn1/fn2=
are so-called *rvalues* which are not visible to the user
while the result of =(make-point2d :x 1 :y 2)= is going to be returned to the user
and is thus an *lvalue*.

It might then be confusing why =fn2= does not completely remove the instantiation.
This is because =point2d-add= may perform a destructive operation on the argument,
and thus the value (e.g. slots) of the constant could be modified.
=copy-point2d= is called in order to avoid the effect of this destructive operation.

COPIER could be ommited when the object is known to be immutable
and there is no need for copying the object.
In this case the compilation result always returns the same object under =EQ=.

Note that if the result of the constant folding is an object of class
=CL:STANDARD-OBJECT= / =CL:STRUCTURE-OBJECT= / =CL:CONDITION= / =CL:CLASS=, the
corresponding =CL:MAKE-LOAD-FORM= method should be defined.

** Dependencies
This library is at least tested on implementation listed below:

+ SBCL 1.4.12 on X86-64 Linux 4.4.0-141-generic (author's environment)
+ SBCL 1.5.1  on X86-64 Linux 4.4.0-141-generic (author's environment)

Also, it depends on the following libraries:

+ trivia by *Masataro Asai* :
    NON-optimized pattern matcher compatible with OPTIMA, with extensible optimizer interface and clean codebase
+ alexandria by *Nikodemus Siivola <nikodemus@sb-studio.net>, and others.* :
    Alexandria is a collection of portable public domain utilities.
+ iterate by ** :
    Jonathan Amsterdam's iterator/gatherer/accumulator facility
+ lisp-namespace by *Masataro Asai* :
    Provides LISP-N --- extensible namespaces in Common Lisp.

[[./constantfold.png]]

** Author, License, Copyright

Masataro Asai (guicho2.71828@gmail.com)

Licensed under LGPL v3.

Copyright (c) 2019 IBM Corporation


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