Rescan cells at the end of mark-sweep. This means they don't need a

src/ia32/code-stubs-ia32.cc

Index: src/ia32/code-stubs-ia32.cc
===================================================================
--- src/ia32/code-stubs-ia32.cc	(revision 10174)
+++ src/ia32/code-stubs-ia32.cc	(working copy)
@@ -4653,7 +4653,8 @@
     // megamorphic.
     __ cmp(ecx, Immediate(UninitializedSentinel(isolate)));
     __ j(equal, &initialize, Label::kNear);
-    // MegamorphicSentinel is a root so no write-barrier is needed.
+    // MegamorphicSentinel is an immortal immovable object (undefined) so no
+    // write-barrier is needed.
     __ mov(FieldOperand(ebx, JSGlobalPropertyCell::kValueOffset),
            Immediate(MegamorphicSentinel(isolate)));
     __ jmp(&call, Label::kNear);
@@ -4661,14 +4662,7 @@
     // An uninitialized cache is patched with the function.
     __ bind(&initialize);
     __ mov(FieldOperand(ebx, JSGlobalPropertyCell::kValueOffset), edi);
-    __ mov(ecx, edi);
-    __ RecordWriteField(ebx,
-                        JSGlobalPropertyCell::kValueOffset,
-                        ecx,
-                        edx,
-                        kDontSaveFPRegs,
-                        OMIT_REMEMBERED_SET,  // Cells are rescanned.
-                        OMIT_SMI_CHECK);
+    // No need for a write barrier here - cells are rescanned.
 
     __ bind(&call);
   }
@@ -4700,6 +4694,8 @@
     // non-function case.
     __ mov(ebx, Operand(esp, 0));
     __ mov(ebx, Operand(ebx, 1));
+    // MegamorphicSentinel is an immortal immovable object (undefined) so no
+    // write barrier is needed.
     __ mov(FieldOperand(ebx, JSGlobalPropertyCell::kValueOffset),
            Immediate(MegamorphicSentinel(isolate)));
   }