tail: reduce calls to cdr.

* lib.c (tail): Don't call cdr on the same cell twice in the
loop body. tail is used in list_collect and friends, which are
used all over the place.

1 files changed, 3 insertions, 2 deletions

diff --git a/lib.c b/lib.c
index 48bf1807..a318aecc 100644
--- a/lib.c
+++ b/lib.c
@@ -655,8 +655,9 @@ val listref(val list, val ind)
 
 loc tail(val cons)
 {
-  while (consp(cdr(cons)))
-    cons = cdr(cons);
+  val d;
+  for (d = cdr(cons); consp(d); d = cdr(d))
+    cons = d;
   return cdr_l(cons);
 }