- Use the simulator to do atomic operations that could also

runtime/vm/simulator_mips.h

Index: runtime/vm/simulator_mips.h
===================================================================
--- runtime/vm/simulator_mips.h	(revision 41298)
+++ runtime/vm/simulator_mips.h	(working copy)
@@ -17,11 +17,12 @@
 #endif
 
 #include "vm/constants_mips.h"
-#include "vm/object.h"
 
 namespace dart {
 
 class Isolate;
+class Mutex;
+class RawObject;
 class SimulatorSetjmpBuffer;
 
 class Simulator {
@@ -110,6 +111,12 @@
                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);
+
   // Runtime and native call support.
   enum CallKind {
     kRuntimeCall,
@@ -190,6 +197,38 @@
   inline double ReadD(uword addr, Instr* instr);
   inline void WriteD(uword addr, double value, Instr* instr);
 
+  // In Dart, there is at most one thread per isolate.
+  // We keep track of 16 exclusive access address tags across all isolates.
+  // 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 isolate requesting exclusive access to the
+  // address tag. Multiple isolates 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 isolate.
+  // At any given time, each isolate is associated to at most one address tag.
+  static Mutex* exclusive_access_lock_;
+  static const int kNumAddressTags = 16;
+  static struct AddressTag {
+    Isolate* isolate;
+    uword addr;
+  } exclusive_access_state_[kNumAddressTags];
+  static int next_address_tag_;
+
+  // Synchronization primitives support.

[koda, 2014/10/27 14:05:11]

This code seems currently unused on MIPS, right?

If you think all three copies of this code will remain the same, then I strongly
urge you to consider sharing it in a utility class.

[zra, 2014/10/27 14:53:12]

Agree.

[Ivan Posva, 2014/10/27 17:58:24]

How about atomic_simulator.h?

+  void ClearExclusive();
+  intptr_t ReadExclusiveW(uword addr, Instr* instr);
+  intptr_t WriteExclusiveW(uword addr, intptr_t value, Instr* instr);

[koda, 2014/10/27 14:05:11]

Document return value.

+
+  // Set access to given address to 'exclusive state' for current isolate.
+  static void SetExclusiveAccess(uword addr);
+
+  // Returns true if the current isolate has exclusive access to given address,
+  // returns false otherwise. In either case, set access to given address to
+  // 'open state' for all isolates.
+  // If given addr is NULL, set access to 'open state' for current
+  // isolate (CLREX).
+  static bool HasExclusiveAccessAndOpen(uword addr);
+
   void DoBranch(Instr* instr, bool taken, bool likely);
   void DoBreak(Instr *instr);