Description of cleanup.scm

Purpose:
--------

CLEANUP is a general optimization phase, called several times.  It
primarly optimizes LETs and pseudo LETs (CALLs with LAMBDAs as their
operators).  This is a restricted form of static beta-substitution.

1. It substitutes bound variables by their values if the expressions
   for the values are sufficiently simple.  Currently only the
   following value expressions can be substituted:
    a) LOOKUP forms.
    b) QUOTE forms.
    c) CALLs to a simple magic cookie procedure (currently
       %STACK-CLOSURE-REF and %HEAP-CLOSURE-REF).  The essential
       property of such expressions is that they are side-effect
       insensitive and too cheap to pre-compute and bind to a
       name. (%HEAP-CLOSURE-REF is side-effect insensitive because
       %HEAP-CLOSURE-SET! is used only in limited ways to construct
       mutually referring closures).

   The substitution is, of course, careful to avoid name capture.

2. CALL to a LAMBDA is not quite identical to a LET, because there is
   a continuation to be considered.  If the continuation to the CALL
   is %MAKE-STACK-CLOSURE it is handled very specially (see code for
   details).

3. There is special handling for calls to explicit heap closures
   i.e. expressions of the form
      (CALL %INTERNAL-APPLY ...
        (CALL %MAKE-HEAP-CLOSURE '#F (LAMBDA (...) body) ...)
        ...)
   This can be converted to the simpler form
      (CALL (LAMBDA (...) body') ...)
   provided the body has no self-references.

4. There is special handling for calls to explicit trivial closures,
   similar to that for explicit heap closures.  This, too, replaces
   the closure with a LAMBDA expression.

5. Known operators with sufficiently constrained argument expressions
   are replaced by `simpler' expressions.  Most generic arithmetic
   operations are constant-folded.  Some are re-written, for example
      (fix:+ (fix:+ 1 x) 2)
   is rewritten to (fix:+ x 3) in two steps.

6. IF expressions are also constant-folded, and there are a couple of
   rewrites for improving combinations of IF and NOT.

Operators Introduced:
---------------------
none

Restrictions on Input:
----------------------
Special forms excluded: 
  ACCESS, DEFINE, DELAY, IN-PACKAGE, OR, SET!, THE-ENVIRONMENT
  UNASSIGNED?

Special forms introduced:
-------------------------
none

Magic Cookies handled specially:
--------------------------------
%HEAP-CLOSURE-REF for optimization 3 and as part of the restriction on
  optimization 1.
%STACK-CLOSURE-REF as part of the restriction on optimization 1.
%INTERNAL-APPLY for optimizations 3 and 4.
%MAKE-HEAP-CLOSURE for optimization 3.
%MAKE-TRIVIAL-CLOSURE for optimization 4.
%MAKE-STACK-CLOSURE for optimization 2.

Guarantees on Output:
---------------------
1. No empty binding forms like (LET () e)
2. No `chained' bindings of the form (LET ((x y)) ...)

[Original guarantee: `No unused bindings' is not true.]