[Mac] Tighten up objc zombie dealloc implementation.

chrome/browser/cocoa/objc_zombie.mm

Index: chrome/browser/cocoa/objc_zombie.mm
diff --git a/chrome/browser/cocoa/objc_zombie.mm b/chrome/browser/cocoa/objc_zombie.mm
index a9d906395a1666fd33624abaa8de42fd457687d9..9a7c9630cc3b23b62f1e27d66f084a683d49f188 100644
--- a/chrome/browser/cocoa/objc_zombie.mm
+++ b/chrome/browser/cocoa/objc_zombie.mm
@@ -144,12 +144,17 @@ void ZombieDealloc(id self, SEL _cmd) {
   memset(self, '!', size);
 
   // If the instance is big enough, make it into a fat zombie and have
-  // it remember the old isa.  Otherwise make it a regular zombie.
+  // it remember the old |isa|.  Otherwise make it a regular zombie.
+  // Setting |isa| rather than using |object_setClass()| because that
+  // function is implemented with a memory barrier.  The runtime's
+  // |_internal_object_dispose()| (in objc-class.m) does this, so it
+  // should be safe (messaging free'd objects shouldn't be expected to
+  // be thread-safe in the first place).
   if (size >= g_fatZombieSize) {
-    object_setClass(self, g_fatZombieClass);
+    self->isa = g_fatZombieClass;
     static_cast<CrFatZombie*>(self)->wasa = wasa;
   } else {
-    object_setClass(self, g_zombieClass);
+    self->isa = g_zombieClass;

[Avi (use Gerrit), 2010/06/03 19:16:51]

Huh. isa is still public in the objc2 runtime? If this compiles on 64, go for
it. If not you might have to ifdef __LP64__ it.

[Scott Hess - ex-Googler, 2010/06/03 19:46:33]

Not sure what you mean.  struct objc_class has OBJC2_UNAVAILABLE annotations,
but struct objc_object just has Class isa with no annotations.

This code should never be called in 64-bit due to the nlist stuff not being
adapted yet.  I'm trying to figure out how to compile 64-bit, but:
   GYP_DEFINES="target_arch=x64" gclient sync
doesn't seem to change from i386.

   }
 
   // The new record to swap into |g_zombies|.  If |g_zombieCount| is