From 1244713462569240bd25f0e4bbb3a2af9ac3c52d Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Tue, 21 Aug 2018 11:40:23 -0700
Subject: [PATCH] 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.
---
 src/bytecode.c | 39 +++++++++++++--------------------------
 src/fns.c      |  5 ++---
 src/lisp.h     |  5 +++++
 3 files changed, 20 insertions(+), 29 deletions(-)

diff --git a/src/bytecode.c b/src/bytecode.c
index b27fa7c5c6..17457fc574 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 9d681017c1..b368ffd58f 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 8f48a33484..c5593b2100 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,
-- 
2.17.1