[Emacs-diffs] scratch/raeburn-startup f48e12c 06/17: Reduce nested calls during substitution.

[Ken Raeburn]

branch: scratch/raeburn-startup
commit f48e12c8192156d5390e24a29aa52fa7455d25ec
Author: Ken Raeburn <address@hidden>
Commit: Ken Raeburn <address@hidden>

    Reduce nested calls during substitution.
    
    * src/lread.c (substitute_object_recurse): If the cons cell's cdr slot
    has another cons cell, process it iteratively instead of recursively.
---
 src/lread.c |   31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/src/lread.c b/src/lread.c
index f0fdc7d..5e04c70 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -3497,10 +3497,33 @@ substitute_object_recurse (Lisp_Object object, 
Lisp_Object placeholder, Lisp_Obj
 
     case Lisp_Cons:
       {
-       SUBSTITUTE (XCAR (subtree),
-                   XSETCAR (subtree, true_value));
-       SUBSTITUTE (XCDR (subtree),
-                   XSETCDR (subtree, true_value));
+       /* It'd be easy enough to just process the car and cdr slots
+          recursively, but since we can't do tail recursion, a
+          1000-element list produces 1000 recursive calls.  Instead,
+          process the car slot recursively, but iterate over cdr
+          slots as long as it looks like a proper list.  */
+       Lisp_Object pair = subtree;
+       while (XTYPE (pair) == Lisp_Cons)
+         {
+           Lisp_Object cdr;
+           SUBSTITUTE (XCAR (pair),
+                       XSETCAR (pair, true_value));
+           cdr = XCDR (pair);
+           if (XTYPE (cdr) != Lisp_Cons)
+             break;
+           /* Duplicate the top-of-function processing specific to a
+              cons cell.  */
+           if (EQ (placeholder, cdr))
+             break;
+           if (!EQ (Qnil, Fmemq (cdr, seen_list)))
+             break;
+           if (hash_lookup (XHASH_TABLE (read_objects_completed), cdr, NULL)
+               >= 0)
+             seen_list = Fcons (cdr, seen_list);
+           pair = cdr;
+         }
+       SUBSTITUTE (XCDR (pair),
+                   XSETCDR (pair, true_value));
        return subtree;
       }