From: David Kastrup <dak@gnu.org>
To: 17049@debbugs.gnu.org
Cc: David Kastrup <dak@gnu.org>
Subject: bug#17049: [PATCH v3] Make reverse! forego the cost of SCM_VALIDATE_LIST
Date: Tue, 1 Apr 2014 16:24:29 +0200 [thread overview]
Message-ID: <1396362269-11654-1-git-send-email-dak@gnu.org> (raw)
In-Reply-To: <87k3b9kuoz.fsf@yeeloong.lan>
* libguile/list.c (scm_reverse_x): Do not validate first argument to
reverse! in advance. Instead undo reversal in error case.
Signed-off-by: David Kastrup <dak@gnu.org>
---
libguile/list.c | 41 ++++++++++++++++++++++++++++++++++++-----
1 file changed, 36 insertions(+), 5 deletions(-)
diff --git a/libguile/list.c b/libguile/list.c
index 1f44ad0..41cc937 100644
--- a/libguile/list.c
+++ b/libguile/list.c
@@ -374,18 +374,49 @@ SCM_DEFINE (scm_reverse_x, "reverse!", 1, 1, 0,
"@code{reverse!}")
#define FUNC_NAME s_scm_reverse_x
{
- SCM_VALIDATE_LIST (1, lst);
+ SCM old_lst = lst;
+ SCM tail = SCM_BOOL_F;
+
if (SCM_UNBNDP (new_tail))
new_tail = SCM_EOL;
- while (!SCM_NULL_OR_NIL_P (lst))
+ if (SCM_NULL_OR_NIL_P (lst))
+ return new_tail;
+
+ /* SCM_VALIDATE_LIST would run through the whole list to make sure it
+ is not eventually circular. In contrast to most list operations,
+ reverse! cannot get stuck in an infinite loop but arrives back at
+ the start when given an eventually or fully circular list. Because
+ of that, we can save the cost of an upfront proper list check at
+ the price of having to do a double reversal in the error case.
+ */
+
+ while (scm_is_pair (lst))
{
SCM old_tail = SCM_CDR (lst);
- SCM_SETCDR (lst, new_tail);
- new_tail = lst;
+ SCM_SETCDR (lst, tail);
+ tail = lst;
lst = old_tail;
}
- return new_tail;
+
+ if (SCM_LIKELY (SCM_NULL_OR_NIL_P (lst)))
+ {
+ SCM_SETCDR (old_lst, new_tail);
+ return tail;
+ }
+
+ /* We did not start with a proper list. Undo the reversal. */
+
+ while (scm_is_pair (tail))
+ {
+ SCM old_tail = SCM_CDR (tail);
+ SCM_SETCDR (tail, lst);
+ lst = tail;
+ tail = old_tail;
+ }
+
+ SCM_WRONG_TYPE_ARG (1, lst);
+ return lst;
}
#undef FUNC_NAME
--
1.9.1
next prev parent reply other threads:[~2014-04-01 14:24 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-20 11:23 bug#17049: [PATCH] Make reverse! forego the cost of SCM_VALIDATE_LIST David Kastrup
2014-03-20 11:38 ` bug#17049: PostScriptum: David Kastrup
2014-03-20 14:19 ` bug#17049: [PATCH] Make reverse! forego the cost of SCM_VALIDATE_LIST David Kastrup
2014-03-20 20:50 ` Andy Wingo
2014-03-20 21:27 ` David Kastrup
2014-03-21 4:38 ` Mark H Weaver
2014-03-21 16:56 ` David Kastrup
2014-03-31 19:56 ` Mark H Weaver
2014-04-01 10:26 ` bug#17049: [PATCH v2] " David Kastrup
2014-04-01 10:40 ` David Kastrup
2014-04-01 14:09 ` Mark H Weaver
2014-04-01 14:24 ` David Kastrup [this message]
2014-04-01 16:35 ` bug#17049: [PATCH v3] " Mark H Weaver
2014-03-21 17:44 ` bug#17049: [PATCH] " David Kastrup
2014-03-30 16:36 ` bug#17049: Further change ideas David Kastrup
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