Add an optimization for map-maps-a-list-element-to-more-list-elements

case, but add also notes explaining the relationship of this
patch and the earlier notes by Sarathy.

Subject: Map is still slow
Message-ID: <20000715212327.21656.qmail@hotmail.com>

p4raw-id: //depot/perl@6429

diff --git a/pp_ctl.c b/pp_ctl.c
index 75673dc..776754e 100644 (file)
--- a/pp_ctl.c
+++ b/pp_ctl.c
@@ -742,7 +742,7 @@ PP(pp_mapwhile)
             * is repeatedly extended for every set of items.  Is possible
             * to do this without any stack extension or copying at all
             * by maintaining a separate list over which the map iterates
-            * (like foreach does). */
+            * (like foreach does). --gsar */
 
            /* everything in the stack after the destination list moves
             * towards the end the stack by the amount of room needed */
@@ -750,6 +750,17 @@ PP(pp_mapwhile)
 
            /* items to shift up (accounting for the moved source pointer) */
            count = (SP - PL_stack_base) - (PL_markstack_ptr[-1] - 1);
+
+           /* This optimization is by Ben Tilly and it does
+            * things differently from what Sarathy (gsar)
+            * is describing.  The downside of this optimization is
+            * that leaves "holes" (uninitialized and hopefully unused areas)
+            * to the Perl stack, but on the other hand this
+            * shouldn't be a problem.  If Sarathy's idea gets
+            * implemented, this optimization should become
+            * irrelevant.  --jhi */
+            if (shift < count)
+                shift = count; /* Avoid shifting too often --Ben Tilly */
            
            EXTEND(SP,shift);
            src = SP;