Fix bignum bugs with nth, elt, =

* src/bytecode.c (exec_byte_code): Support bignums
when implementing nth, elt, and =.
* src/lisp.h (SMALL_LIST_LEN_MAX): New constant.
* src/fns.c (Fnthcdr): Use it.
(Felt): Do not reject bignum indexes.

3 files changed, 20 insertions, 29 deletions

diff --git a/src/bytecode.c b/src/bytecode.c
index b27fa7c5c68..17457fc5742 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -832,13 +832,14 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
         CASE (Bnth):
           {
             Lisp_Object v2 = POP, v1 = TOP;
-            CHECK_FIXNUM (v1);
-            for (EMACS_INT n = XFIXNUM (v1); 0 < n && CONSP (v2); n--)
+            if (RANGED_FIXNUMP (0, v1, SMALL_LIST_LEN_MAX))
               {
-                v2 = XCDR (v2);
-                rarely_quit (n);
+                for (EMACS_INT n = XFIXNUM (v1); 0 < n && CONSP (v2); n--)
+                  v2 = XCDR (v2);
+                TOP = CAR (v2);
               }
-            TOP = CAR (v2);
+            else
+              TOP = Fnth (v1, v2);
             NEXT;
           }
 
@@ -985,15 +986,8 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
 
         CASE (Beqlsign):
           {
-            Lisp_Object v2 = POP, v1 = TOP;
-            if (FLOATP (v1) || FLOATP (v2))
-              TOP = arithcompare (v1, v2, ARITH_EQUAL);
-            else
-              {
-                CHECK_FIXNUM_OR_FLOAT_COERCE_MARKER (v1);
-                CHECK_FIXNUM_OR_FLOAT_COERCE_MARKER (v2);
-                TOP = EQ (v1, v2) ? Qt : Qnil;
-              }
+            Lisp_Object v1 = POP;
+            TOP = arithcompare (TOP, v1, ARITH_EQUAL);
             NEXT;
           }
 
@@ -1264,23 +1258,16 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
 
         CASE (Belt):
           {
-            if (CONSP (TOP))
+            Lisp_Object v2 = POP, v1 = TOP;
+            if (CONSP (v1) && RANGED_FIXNUMP (0, v2, SMALL_LIST_LEN_MAX))
               {
-                /* Exchange args and then do nth.  */
-                Lisp_Object v2 = POP, v1 = TOP;
-                CHECK_FIXNUM (v2);
+                /* Like the fast case for Bnth, but with args reversed.  */
                 for (EMACS_INT n = XFIXNUM (v2); 0 < n && CONSP (v1); n--)
-                  {
-                    v1 = XCDR (v1);
-                    rarely_quit (n);
-                  }
+                  v1 = XCDR (v1);
                 TOP = CAR (v1);
               }
             else
-              {
-                Lisp_Object v1 = POP;
-                TOP = Felt (TOP, v1);
-              }
+              TOP = Felt (v1, v2);
             NEXT;
           }
 
diff --git a/src/fns.c b/src/fns.c
index 9d681017c14..b368ffd58f2 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -1418,7 +1418,7 @@ DEFUN ("nthcdr", Fnthcdr, Snthcdr, 2, 2, 0,
       num = XFIXNUM (n);
 
       /* Speed up small lists by omitting circularity and quit checking.  */
-      if (num < 128)
+      if (num <= SMALL_LIST_LEN_MAX)
         {
           for (; 0 < num; num--, tail = XCDR (tail))
             if (! CONSP (tail))
@@ -1503,9 +1503,8 @@ N counts from zero.  If LIST is not that long, nil is returned.  */)
 
 DEFUN ("elt", Felt, Selt, 2, 2, 0,
        doc: /* Return element of SEQUENCE at index N.  */)
-  (register Lisp_Object sequence, Lisp_Object n)
+  (Lisp_Object sequence, Lisp_Object n)
 {
-  CHECK_FIXNUM (n);
   if (CONSP (sequence) || NILP (sequence))
     return Fcar (Fnthcdr (n, sequence));
 
diff --git a/src/lisp.h b/src/lisp.h
index 8f48a334844..c5593b21008 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4694,6 +4694,11 @@ enum
          Lisp_String))                                                  \
      : make_unibyte_string (str, len))
 
+/* The maximum length of "small" lists, as a heuristic.  These lists
+   are so short that code need not check for cycles or quits while
+   traversing.  */
+enum { SMALL_LIST_LEN_MAX = 127 };
+
 /* Loop over conses of the list TAIL, signaling if a cycle is found,
    and possibly quitting after each loop iteration.  In the loop body,
    set TAIL to the current cons.  If the loop exits normally,