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

runtime/vm/simulator_arm64.h

Index: runtime/vm/simulator_arm64.h
diff --git a/runtime/vm/simulator_arm64.h b/runtime/vm/simulator_arm64.h
index a1a36699e393b688ee2820b8ee9b4055a3f8ffa9..78059ea176ae5f7b450e033a9bb4574d94129732 100644
--- a/runtime/vm/simulator_arm64.h
+++ b/runtime/vm/simulator_arm64.h
@@ -101,15 +101,6 @@ class Simulator {
                bool fp_return = false,
                bool fp_args = false);
 
-  // Implementation of atomic compare and exchange in the same synchronization
-  // domain as other synchronization primitive instructions (e.g. ldrex, strex).
-  static uword CompareExchange(uword* address,
-                               uword compare_value,
-                               uword new_value);
-  static uint32_t CompareExchangeUint32(uint32_t* address,
-                                        uint32_t compare_value,
-                                        uint32_t new_value);
-
   // Runtime and native call support.
   enum CallKind {
     kRuntimeCall,
@@ -187,22 +178,6 @@ class Simulator {
   inline intptr_t ReadX(uword addr, Instr* instr);
   inline void WriteX(uword addr, intptr_t value, Instr* instr);
 
-  // We keep track of 16 exclusive access address tags across all threads.
-  // Since we cannot simulate a native context switch, which clears
-  // the exclusive access state of the local monitor (using the CLREX
-  // instruction), we associate the thread requesting exclusive access to the
-  // address tag. Multiple threads requesting exclusive access (using the LDREX
-  // instruction) to the same address will result in multiple address tags being
-  // created for the same address, one per thread.
-  // At any given time, each thread is associated to at most one address tag.
-  static Mutex* exclusive_access_lock_;
-  static const int kNumAddressTags = 16;
-  static struct AddressTag {
-    Thread* thread;
-    uword addr;
-  } exclusive_access_state_[kNumAddressTags];
-  static int next_address_tag_;
-
   // Synchronization primitives support.
   void ClearExclusive();
   intptr_t ReadExclusiveX(uword addr, Instr* instr);
@@ -211,15 +186,13 @@ class Simulator {
   intptr_t ReadExclusiveW(uword addr, Instr* instr);
   intptr_t WriteExclusiveW(uword addr, intptr_t value, Instr* instr);
 
-  // Set access to given address to 'exclusive state' for current thread.
-  static void SetExclusiveAccess(uword addr);
-
-  // Returns true if the current thread has exclusive access to given address,
-  // returns false otherwise. In either case, set access to given address to
-  // 'open state' for all threads.
-  // If given addr is NULL, set access to 'open state' for current
-  // thread (CLREX).
-  static bool HasExclusiveAccessAndOpen(uword addr);
+  // Exclusive access reservation: address and value observed during
+  // load-exclusive. Store-exclusive verifies that address is the same and
+  // performs atomic compare-and-swap with remembered value to observe value
+  // changes. This implementation of ldxr/stxr instructions does not detect
+  // ABA situation and our uses of ldxr/stxr don't need this detection.
+  uword exclusive_access_addr_;
+  uword exclusive_access_value_;
 
   // Helper functions to set the conditional flags in the architecture state.
   void SetNZFlagsW(int32_t val);