Index: lib/sanitizer_common/sanitizer_atomic_msvc.h
===================================================================
--- lib/sanitizer_common/sanitizer_atomic_msvc.h
+++ lib/sanitizer_common/sanitizer_atomic_msvc.h
@@ -33,6 +33,10 @@
 extern "C" long _InterlockedExchangeAdd(  // NOLINT
     long volatile * Addend, long Value);  // NOLINT
 #pragma intrinsic(_InterlockedExchangeAdd)
+extern "C" char _InterlockedCompareExchange8(  // NOLINT
+    char volatile *Destination,                 // NOLINT
+    char Exchange, char Comparand);            // NOLINT
+#pragma intrinsic(_InterlockedCompareExchange8)
 extern "C" short _InterlockedCompareExchange16(  // NOLINT
     short volatile *Destination,                 // NOLINT
     short Exchange, short Comparand);            // NOLINT
@@ -176,9 +180,12 @@
                                            u8 xchgv,
                                            memory_order mo) {
 #ifdef _WIN64
-  // TODO(wwchrome): Implement same functionality without inline asm.
-  // Inline asm not supported in Win64.
-  __debugbreak();
+  u8 cmpv = *cmp;
+  u8 prev = (u8)_InterlockedCompareExchange8(
+      (volatile char*)&a->val_dont_use, (char)xchgv, (char)cmpv);
+  if (prev == cmpv)
+    return true;
+  *cmp = prev;
   return false;
 #else
   (void)mo;