Use null instead of undefined for deleted elements in code caches....

src/objects.cc

Index: src/objects.cc
===================================================================
--- src/objects.cc	(revision 338)
+++ src/objects.cc	(working copy)
@@ -2379,9 +2379,15 @@
   // First check whether we can update existing code cache without
   // extending it.
   int length = cache->length();
+  int deleted_index = -1;
   for (int i = 0; i < length; i += 2) {
     Object* key = cache->get(i);
+    if (key->IsNull()) {
+      if (deleted_index < 0) deleted_index = i;
+      continue;
+    }
     if (key->IsUndefined()) {
+      if (deleted_index > 0) i = deleted_index;

[Kasper Lund, 2008/09/18 10:56:22]

What if deleted_index is 0? Shouldn't you use that slot then? 

       cache->set(i + 0, name);
       cache->set(i + 1, code);
       return this;
@@ -2395,6 +2401,14 @@
     }
   }
 
+  // Reached the end of the code cache.  If there were deleted
+  // elements, reuse the space for the first of them.
+  if (deleted_index > 0) {

[Kasper Lund, 2008/09/18 10:56:22]

How about reusing if deleted_index is 0? Isn't that the right thing to do?

+    cache->set(deleted_index + 0, name);
+    cache->set(deleted_index + 1, code);
+    return this;
+  }
+
   // Extend the code cache with some new entries (at least one).
   int new_length = length + ((length >> 1) & ~1) + 2;
   ASSERT((new_length & 1) == 0);  // must be a multiple of two
@@ -2415,6 +2429,9 @@
   int length = cache->length();
   for (int i = 0; i < length; i += 2) {
     Object* key = cache->get(i);
+    if (key->IsNull()) {

[Kasper Lund, 2008/09/18 10:56:22]

Maybe add a comment here that explains that we're skipping deleted entries?

+      continue;
+    }
     if (key->IsUndefined()) {
       return key;
     }
@@ -2440,8 +2457,8 @@
 void Map::RemoveFromCodeCache(int index) {
   FixedArray* array = code_cache();
   ASSERT(array->length() >= index && array->get(index)->IsCode());
-  array->set_undefined(index - 1);  // key
-  array->set_undefined(index);  // code
+  array->set_null(index - 1);  // key

[Kasper Lund, 2008/09/18 10:56:22]

Add comment that explains that deleted entries are dealt with specially when
looking up in the cache so they can't be undefined (must be null).

+  array->set_null(index);  // code
 }