--- numbers.c.~1.200.~	1970-01-01 10:00:01.000000000 +1000
+++ numbers.c	2003-09-23 16:33:30.000000000 +1000
@@ -4337,14 +4337,25 @@
     return z;
   else if (SCM_REALP (z))
     {
-      double u = floor (SCM_REAL_VALUE (z) + 0.5);
-      long lu = (long) u;
-      if (SCM_FIXABLE (lu))
-	return SCM_MAKINUM (lu);
-      else if (!xisinf (u) && !xisnan (u))
-	return scm_i_dbl2big (u);
-      else
+      /* The values SCM_MOST_POSITIVE_FIXNUM+1 and SCM_MOST_NEGATIVE_FIXNUM
+         are both powers of 2, so there's no rounding when making "double"
+         values from them.  If plain SCM_MOST_POSITIVE_FIXNUM was used it
+         could get rounded on a 64-bit machine, hence the "+1".
+
+         The use of floor to force to an integer value ensures we don't
+         depend on how a double->long cast will round or how mpz_set_d will
+         round.  For reference, double->long probably follows the hardware
+         rounding mode, but mpz_set_d truncates towards zero.  */
+
+      double u = SCM_REAL_VALUE (z);
+      if (xisinf (u) || xisnan (u))
 	scm_num_overflow (s_scm_inexact_to_exact);
+      u = floor (u + 0.5);
+      if (u < (double) (SCM_MOST_POSITIVE_FIXNUM+1)
+          && u >= (double) SCM_MOST_NEGATIVE_FIXNUM)
+	return SCM_MAKINUM ((long) u);
+      else
+	return scm_i_dbl2big (u);
     }
   else
     SCM_WRONG_TYPE_ARG (1, z);