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[Guile-commits] GNU Guile branch, master, updated. release_1-9-0-20-gb67
[Guile-commits] GNU Guile branch, master, updated. release_1-9-0-20-gb674d47
Mon, 22 Jun 2009 20:44:50 +0000
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- Log -----------------------------------------------------------------
Author: Andy Wingo <address@hidden>
Date: Mon Jun 22 22:44:34 2009 +0200
more docs to brainfuck->tree-il compiler
* module/language/brainfuck/compile-tree-il.scm (compile-tree-il): Wrap
the result in a ((lambda () ...)), so we can use toplevel-ref. Add
lots more comments.
Summary of changes:
module/language/brainfuck/compile-tree-il.scm | 63 ++++++++++++++++++++----
1 files changed, 52 insertions(+), 11 deletions(-)
diff --git a/module/language/brainfuck/compile-tree-il.scm
index c991631..62987fc 100644
@@ -49,32 +49,66 @@
;; This compiles a whole brainfuck program. This constructs a Tree-IL
;; code equivalent to Scheme code like this:
-;; (let ((pointer 0)
-;; (tape (make-vector tape-size 0)))
-;; (write-char #\newline)))
+;; ((lambda ()
+;; (let ((pointer 0)
+;; (tape (make-vector tape-size 0)))
+;; (write-char #\newline)))))
;; So first the pointer and tape variables are set up correctly, then the
;; program's body is executed in this context, and finally we output an
;; additional newline character in case the program does not output one.
+;; The fact that we are compiling to Guile primitives gives this
+;; implementation a number of interesting characteristics. First, the
+;; values of the tape cells do not underflow or overflow. We could make
+;; them do otherwise via compiling calls to "modulo" at certain points.
+;; In addition, tape overruns or underruns will be detected, and will
+;; throw an error, whereas a number of Brainfuck compilers do not detect
+;; We wrap the code in a lambda so that the body has a place to cache
+;; the looked-up locations of the primitive functions: vector-ref et al.
+;; This way we can use toplevel-ref instead of link-now + variable-ref.
+;; See the VM documentation for more info on those instructions.
+;; Normally when compiling you don't have to think about this at all,
+;; because the usual pattern is a bunch of definitions, then you call
+;; those definitions -- so the real work is in the functions anyway,
+;; which can use toplevel-ref. Here we just force that pattern into
;; Note that we're generating the S-expression representation of
;; Tree-IL, then using parse-tree-il to turn it into the actual Tree-IL
;; data structures. This makes the compiler more pleasant to look at,
;; but we do lose is the ability to propagate source information. Since
;; Brainfuck is so obtuse anyway, this shouldn't matter ;-)
-;; TODO: Find out and explain the details about env, the three return values
-;; how to use the options. Implement options to set the tape-size, maybe.
+;; `compile-tree-il' takes as its input the read expression, the
+;; environment, and some compile options. It returns the compiled
+;; expression, the environment appropriate for the next pass of the
+;; compiler -- in our case, just the environment unchanged -- and the
+;; continuation environment.
+;; The normal use of a continuation environment is if compiling one
+;; expression changes the environment, and that changed environment
+;; should be passed to the next compiled expression -- for example,
+;; changing the current module. But Brainfuck is incapable of that, so
+;; for us, the continuation environment is just the same environment we
+;; got in.
+;; FIXME: perhaps use options or the env to set the tape-size?
(define (compile-tree-il exp env opts)
- `(let (pointer tape) (pointer tape)
- ((const 0)
- (apply (primitive make-vector) (const ,tape-size) (const 0)))
- ,(compile-body exp)))
+ `(apply (lambda () ()
+ (let (pointer tape) (pointer tape)
+ ((const 0)
+ (apply (primitive make-vector) (const ,tape-size) (const
+ ,(compile-body exp)))))
@@ -137,6 +171,13 @@
+ ;; Indeed, letrec is the only way we have to loop in Tree-IL.
+ ;; Note that this does not mean that the closure must actually
+ ;; be created; later passes can compile tail-recursive letrec
+ ;; calls into inline code with gotos. Admittedly, that part of
+ ;; the compiler is not yet in place, but it will be, and in the
+ ;; meantime the code is still reasonably efficient.
((<bf-loop> . ,body)
(let ((iterate (gensym)))
(emit `(letrec (iterate) (,iterate)
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