Revert "[vm] Implement more efficient CAS in simarm/simarm64 modes"

runtime/vm/atomic.h

Index: runtime/vm/atomic.h
diff --git a/runtime/vm/atomic.h b/runtime/vm/atomic.h
index e4dd4ab80d92df61d903ab5186608265de2a741f..41dfad18d4911376c83d602d268ffb523f364229 100644
--- a/runtime/vm/atomic.h
+++ b/runtime/vm/atomic.h
@@ -8,6 +8,7 @@
 #include "platform/globals.h"
 
 #include "vm/allocation.h"
+#include "vm/simulator.h"
 
 namespace dart {
 
@@ -15,23 +16,37 @@ class AtomicOperations : public AllStatic {
  public:
   // Atomically fetch the value at p and increment the value at p.
   // Returns the original value at p.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code.
   static uintptr_t FetchAndIncrement(uintptr_t* p);
   static intptr_t FetchAndIncrement(intptr_t* p);
 
   // Atomically increment the value at p by 'value'.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code.
   static void IncrementBy(intptr_t* p, intptr_t value);
   static void IncrementInt64By(int64_t* p, int64_t value);
 
   // Atomically fetch the value at p and decrement the value at p.
   // Returns the original value at p.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code.
   static uintptr_t FetchAndDecrement(uintptr_t* p);
   static intptr_t FetchAndDecrement(intptr_t* p);
 
   // Atomically decrement the value at p by 'value'.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code.
   static void DecrementBy(intptr_t* p, intptr_t value);
 
   // Atomically compare *ptr to old_value, and if equal, store new_value.
   // Returns the original value at ptr.
+  //
+  // NOTE: OK to use with memory locations that are accessed by generated code
   static uword CompareAndSwapWord(uword* ptr, uword old_value, uword new_value);
   static uint32_t CompareAndSwapUint32(uint32_t* ptr,
                                        uint32_t old_value,
@@ -47,6 +62,16 @@ class AtomicOperations : public AllStatic {
 
 }  // namespace dart
 
+#if defined(USING_SIMULATOR) && !defined(TARGET_ARCH_DBC)
+#define USING_SIMULATOR_ATOMICS
+#endif
+
+#if defined(USING_SIMULATOR_ATOMICS)
+// We need to use the simulator to ensure that atomic operations are observed
+// both in C++ and in generated code if the simulator is active.
+#include "vm/atomic_simulator.h"
+#endif
+
 #if defined(HOST_OS_ANDROID)
 #include "vm/atomic_android.h"
 #elif defined(HOST_OS_FUCHSIA)