From: Stephen Adams <edu/mit/csail/zurich/adams>
Date: Mon, 4 Mar 1996 04:52:25 +0000 (+0000)
Subject: Update.
X-Git-Tag: 20090517-FFI~5677
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=95a8e984ddc185c2c6d1746cce44a939a972bea0;p=mit-scheme.git

Update.
---

diff --git a/v8/src/compiler/documentation/midend/cleanup.txt b/v8/src/compiler/documentation/midend/cleanup.txt
index 74bef4a74..cbf645bab 100644
--- a/v8/src/compiler/documentation/midend/cleanup.txt
+++ b/v8/src/compiler/documentation/midend/cleanup.txt
@@ -7,17 +7,19 @@ 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 eliminates unused bound variables and substitutes bound
-variables by their values if the expressions for the values are
-sufficiently simple.  Currently only the following value expressions
-can be substituted:
+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
-      %VECTOR-INDEX, %STACK-CLOSURE-REF and %HEAP-CLOSURE-REF) in
-      which all of the operands trivial (QUOTE or LOOKUP forms, or
-      calls to trivial operators (%VECTOR-INDEX) with quoted operands)
-      and there is at most one LOOKUP argument.
+      %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
@@ -28,16 +30,22 @@ 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 ... (LAMBDA (...) body))
+    (CALL %MAKE-HEAP-CLOSURE '#F (LAMBDA (...) body) ...)
     ...)
 This can be converted to the simpler form
-  (CALL (LAMBDA (...') body'))
-if the body has no self-references.
+  (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.
+
 Operators Introduced:
 ---------------------
 none
@@ -50,14 +58,13 @@ Special forms excluded:
 
 Special forms introduced:
 -------------------------
-  none
+none
 
 Magic Cookies handled specially:
 --------------------------------
 %HEAP-CLOSURE-REF for optimization 3 and as part of the restriction on
   optimization 1.
-%VECTOR-INDEX and %STACK-CLOSURE-REF 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.
@@ -65,4 +72,7 @@ Magic Cookies handled specially:
 
 Guarantees on Output:
 ---------------------
- No unused bindings.
+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.]