Atomic ops: sync with Chromium.

src/atomicops_internals_x86_msvc.h

Index: src/atomicops_internals_x86_msvc.h
diff --git a/src/atomicops_internals_x86_msvc.h b/src/atomicops_internals_x86_msvc.h
index fcf6a65107747bcdc5906118b7afe342f480a995..ad9cf9d80bbf2804f9e853876c7dbb1f619cba95 100644
--- a/src/atomicops_internals_x86_msvc.h
+++ b/src/atomicops_internals_x86_msvc.h
@@ -33,6 +33,15 @@
 #include "checks.h"
 #include "win32-headers.h"
 
+#if defined(V8_HOST_ARCH_64_BIT)
+// windows.h #defines this (only on x64). This causes problems because the
+// public API also uses MemoryBarrier at the public name for this fence. So, on
+// X64, undef it, and call its documented
+// (http://msdn.microsoft.com/en-us/library/windows/desktop/ms684208.aspx)
+// implementation directly.
+#undef MemoryBarrier
+#endif
+
 namespace v8 {
 namespace internal {
 
@@ -70,8 +79,13 @@ inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr,
 #error "We require at least vs2005 for MemoryBarrier"
 #endif
 inline void MemoryBarrier() {
+#if defined(V8_HOST_ARCH_64_BIT)
+  // See #undef and note at the top of this file.
+  __faststorefence();
+#else
   // We use MemoryBarrier from WinNT.h
   ::MemoryBarrier();
+#endif
 }
 
 inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,