NewGC: Don't put tagged null pointers in garbage objects when updating pointers

src/mark-compact.cc

Index: src/mark-compact.cc
===================================================================
--- src/mark-compact.cc	(revision 8902)
+++ src/mark-compact.cc	(working copy)
@@ -2378,7 +2378,7 @@
     // We have to zap this pointer, because the store buffer may overflow later,
     // and then we have to scan the entire heap and we don't want to find
     // spurious newspace pointers in the old space.
-    *p = HeapObject::FromAddress(NULL);  // Fake heap object not in new space.
+    *p = reinterpret_cast<HeapObject*>(Smi::FromInt(0));

[Lasse Reichstein, 2011/08/12 08:47:13]

So we are not assuming that the value is HeapObject tagged anywhere? (Quite
possible, since we can't use it as such anyway).
It seems to end up in NewSpace::Contains which just uses address arithmetic, so
it should be safe (but that probably should take a HeapObject and filter out
smis earler). 
Maybe make StoreBuffer::Uniq and similar functions filter out the smis.

[Erik Corry, 2011/08/12 08:56:40]

This is the value in the slot that used to point to new space.  The slot is now
part of a dead object pointing to a dead object so we have to put something
there that neither the scan-on-scavenge page traversal nor the evacuation slot
buffer will choke on.  A Smi zero should be fine.  The signature of this
function gets a little in the way, but changing that is likely to cause more
pain.

[Lasse Reichstein, 2011/08/12 09:21:48]

Ah, so *p is always in the heap, and not in a store-buffer?

   }
 }