Safe and efficient stack-limit based interrupt checking in C++.

runtime/vm/atomic_macos.h

Index: runtime/vm/atomic_macos.h
diff --git a/runtime/vm/atomic_macos.h b/runtime/vm/atomic_macos.h
index 288fe3cbcdf0afa637a98ef86ea9ec32d5014ebd..d9b89bc3924a8bed5fb7fed97f0995bdad4bcec1 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -21,6 +21,11 @@ inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
 }
 
 
+inline intptr_t AtomicOperations::FetchAndAdd(intptr_t* p, intptr_t delta) {
+  return __sync_fetch_and_add(p, delta);
+}
+
+
 #if !defined(USING_SIMULATOR)
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,
@@ -29,6 +34,17 @@ inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
 }
 #endif  // !defined(USING_SIMULATOR)
 
+
+inline uword AtomicOperations::LoadRelaxed(uword* ptr) {
+#if defined(__ATOMIC_RELAXED)
+  return __atomic_load_n(ptr, __ATOMIC_RELAXED);
+#else
+  // TODO(koda): Consider using C++11 <atomic> to avoid the barrier on more
+  // compilers/platforms.
+  return FetchAndAdd(reinterpret_cast<intptr_t*>(ptr), 0);
+#endif
+}
+
 }  // namespace dart
 
 #endif  // VM_ATOMIC_MACOS_H_