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

runtime/vm/simulator_arm.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Declares a Simulator for ARM instructions if we are not generating a native
 // ARM binary. This Simulator allows us to run and debug ARM code generation on
 // regular desktop machines.
 // Dart calls into generated code by "calling" the InvokeDartCode stub,
 // which will start execution in the Simulator or forwards to the real entry
 // on a ARM HW platform.
@@ skipping 86 matching lines @@
   // If fp_args is true, the parameters0-3 are placed in S0-3. Otherwise, they
   // are placed in R0-3.
   int64_t Call(int32_t entry,
                int32_t parameter0,
                int32_t parameter1,
                int32_t parameter2,
                int32_t parameter3,
                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,
     kLeafRuntimeCall,
     kLeafFloatRuntimeCall,
     kBootstrapNativeCall,
     kNativeCall
   };
   static uword RedirectExternalReference(uword function,
                                          CallKind call_kind,
@@ skipping 85 matching lines @@
   inline void WriteH(uword addr, uint16_t value, Instr* instr);
 
   inline intptr_t ReadW(uword addr, Instr* instr);
   inline void WriteW(uword addr, intptr_t value, Instr* instr);
 
   // Synchronization primitives support.
   void ClearExclusive();
   intptr_t ReadExclusiveW(uword addr, Instr* instr);
   intptr_t WriteExclusiveW(uword addr, intptr_t value, Instr* instr);
 
-  // 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 ldrex/strex instructions does not detect
-  // ABA situation and our uses of ldrex/strex don't need this detection.
-  uword exclusive_access_addr_;
-  uword exclusive_access_value_;
+  // 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_;
+
+  // 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);
 
   // Executing is handled based on the instruction type.
   void DecodeType01(Instr* instr);  // Both type 0 and type 1 rolled into one.
   void DecodeType2(Instr* instr);
   void DecodeType3(Instr* instr);
   void DecodeType4(Instr* instr);
   void DecodeType5(Instr* instr);
   void DecodeType6(Instr* instr);
   void DecodeType7(Instr* instr);
   void DecodeSIMDDataProcessing(Instr* instr);
@@ skipping 14 matching lines @@
   }
 
   friend class SimulatorDebugger;
   friend class SimulatorSetjmpBuffer;
   DISALLOW_COPY_AND_ASSIGN(Simulator);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_SIMULATOR_ARM_H_