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Make reverse! forego the cost of SCM_VALIDATE_LIST

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* 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>
Signed-off-by: Mark H Weaver <mhw@netris.org>
This commit is contained in:
David Kastrup 2014-04-01 16:24:29 +02:00 • committed by Mark H Weaver
parent 8d124d2077
commit 0ece4850c5
1 changed files with 36 additions and 5 deletions
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41
libguile/list.c
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@ -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