> On Wed, Aug 28, 2019 at 01:41:22PM +0200, felix.winkelm...@bevuta.com wrote:
> > An attempt to fix #1645. See commit message for more details.
>
> Hi Felix,
>
> I've taken a look at it and the logic itself looks good, but I find the
> usage of fluid-let on a global variable unsettling. Why not pass the
> list in "walk" to itself and to "resolve-variable"?
>
> If that's too unwieldy, I would at least put the variable inside
> "canonicalize-expression" to clarify that it's only used there.
>
I moved the variable into c.-e. and also changed the error message
to be slightly more useful.
felix
From acfcb678de00be0f531c5ce9bab6b346ac70d6c7 Mon Sep 17 00:00:00 2001
From: felix <fe...@call-with-current-continuation.org>
Date: Wed, 28 Aug 2019 13:37:16 +0200
Subject: [PATCH] Catch self-referencing variables in `letrec' form
"(letrec ((x x)) ...)" results in and endless recursion in the optimizer, since
a letrec-bound
variable ends up being replacable by itself. The intermediate form generated by
expanding letrec
in the compiler binds "x" first to (##core#undefined) which the compiler uses
to assume
the variable has no value, leading to (in this context) somewhat questionable
conclusion.
Since this is a rather tricky part of the analysis framework of the compiler,
and due to the fact
that catching the situation earlier (during canonicalization) gives a more
useful error message,
"letrec" expands now into a use of the internal form
"##core#with-forbidden-refs", which marks
references to variables temporarily as invalid, until the code walk encounters
a lambda form.
"letrec" expands into a complex "let" form and is then walked again, during
which the information
about the original letrec is lost, so we need an intermediate form that also
takes care of
variable renaming. "##core#with-forbidden-refs" is dropped as soon as it is
walked.
A test from syntax-tests.scm that actually used such a self-reference has been
dropped.
No changes have been made to the interpreter. I think it's ok for the compiler
to be more
picky and give better error detection.
Signed-off-by: felix <fe...@call-with-current-continuation.org>
---
core.scm | 45 +++++++++++++++++++++++++++++++-----------
tests/syntax-tests.scm | 4 ----
2 files changed, 34 insertions(+), 15 deletions(-)
diff --git a/core.scm b/core.scm
index 5c49a683..9bb08b42 100644
--- a/core.scm
+++ b/core.scm
@@ -149,6 +149,7 @@
; (##core#the <type> <strict?> <exp>)
; (##core#typecase <info> <exp> (<type> <body>) ... [(else <body>)])
; (##core#debug-event {<event> <loc>})
+; (##core#with-forbidden-refs (<var> ...) <loc> <expr>)
; (<exp> {<exp>})
; - Core language:
@@ -512,7 +513,8 @@
;;; Expand macros and canonicalize expressions:
(define (canonicalize-expression exp)
- (let ((compiler-syntax '()))
+ (let ((compiler-syntax '())
+ (forbidden-refs '()))
(define (find-id id se) ; ignores macro bindings
(cond ((null? se) #f)
@@ -559,11 +561,9 @@
x) )
(define (resolve-variable x0 e dest ldest h)
-
(when (memq x0 unlikely-variables)
(warning
(sprintf "reference to variable `~s' possibly unintended" x0) ))
-
(let ((x (lookup x0)))
(d `(RESOLVE-VARIABLE: ,x0 ,x ,(map (lambda (x) (car x))
(##sys#current-environment))))
(cond ((not (symbol? x)) x0) ; syntax?
@@ -592,6 +592,13 @@
t)
e dest ldest h #f #f))))
((not (memq x e)) (##sys#alias-global-hook x #f h)) ; only if global
+ ((assq x forbidden-refs) =>
+ (lambda (a)
+ (let ((ln (cdr a)))
+ (quit-compiling
+ "~acyclical reference in LETREC binding for variable `~a'"
+ (if ln (sprintf "(~a) - " ln) "")
+ (get-real-name x)))))
(else x))))
(define (emit-import-lib name il)
@@ -766,12 +773,26 @@
(list (car b) '(##core#undefined)))
bindings)
(##core#let
- ,(map (lambda (t b) (list t (cadr b))) tmps
bindings)
+ ,(map (lambda (t b)
+ (list t `(##core#with-forbidden-refs
+ ,vars ,ln ,(cadr b))))
+ tmps bindings)
,@(map (lambda (v t)
`(##core#set! ,v ,t))
vars tmps)
(##core#let () ,@body) ) )
e dest ldest h ln #f)))
+
+ ((##core#with-forbidden-refs)
+ (let* ((loc (caddr x))
+ (vars (map (lambda (v)
+ (cons (resolve-variable v e dest
+ ldest h)
+ loc))
+ (cadr x))))
+ (fluid-let ((forbidden-refs
+ (append vars forbidden-refs)))
+ (walk (cadddr x) e dest ldest h ln #f))))
((##core#lambda)
(let ((llist (cadr x))
@@ -790,13 +811,15 @@
(body (parameterize
((##sys#current-environment se2))
(let ((body0 (canonicalize-body/ln
ln obody
compiler-syntax-enabled)))
- (walk
- (if emit-debug-info
- `(##core#begin
- (##core#debug-event
C_DEBUG_ENTRY (##core#quote ,dest))
- ,body0)
- body0)
- (append aliases e) #f #f dest
ln #f))))
+ (fluid-let ((forbidden-refs
'()))
+ (walk
+ (if emit-debug-info
+ `(##core#begin
+ (##core#debug-event
C_DEBUG_ENTRY (##core#quote ,dest))
+ ,body0)
+ body0)
+ (append aliases e)
+ #f #f dest ln #f)))))
(llist2
(build-lambda-list
aliases argc
diff --git a/tests/syntax-tests.scm b/tests/syntax-tests.scm
index 3637fde9..1c98d94c 100644
--- a/tests/syntax-tests.scm
+++ b/tests/syntax-tests.scm
@@ -1283,10 +1283,6 @@ other-eval
(bar foo))
bar))
-(t (void) (letrec ((foo (gc))
- (bar foo))
- bar))
-
;; Obscure letrec issue #1068
(t 1 (letrec ((foo (lambda () 1))
(bar (let ((tmp (lambda (x) (if x (foo) (bar #t)))))
--
2.19.1
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