More precise break points and stepping when debugging...

src/arm/assembler-arm.h

Index: src/arm/assembler-arm.h
===================================================================
--- src/arm/assembler-arm.h	(revision 4816)
+++ src/arm/assembler-arm.h	(working copy)
@@ -629,22 +629,39 @@
   // Distance between start of patched return sequence and the emitted address
   // to jump to.
 #ifdef USE_BLX
-  // Return sequence is:
+  // Patched return sequence is:
   //  ldr  ip, [pc, #0]   @ emited address and start
   //  blx  ip
   static const int kPatchReturnSequenceAddressOffset =  0 * kInstrSize;
 #else
-  // Return sequence is:
+  // Patched return sequence is:
   //  mov  lr, pc         @ start of sequence
   //  ldr  pc, [pc, #-4]  @ emited address
   static const int kPatchReturnSequenceAddressOffset =  kInstrSize;
 #endif
 
+  // Distance between start of patched debug break slot and the emitted address
+  // to jump to.
+#ifdef USE_BLX
+  // Patched debug break slot code is:
+  //  ldr  ip, [pc, #0]   @ emited address and start
+  //  blx  ip
+  static const int kPatchDebugBreakSlotAddressOffset =  0 * kInstrSize;
+#else
+  // Patched debug break slot code is:
+  //  mov  lr, pc         @ start of sequence
+  //  ldr  pc, [pc, #-4]  @ emited address
+  static const int kPatchDebugBreakSlotAddressOffset =  kInstrSize;
+#endif
+
   // Difference between address of current opcode and value read from pc
   // register.
   static const int kPcLoadDelta = 8;
 
-  static const int kJSReturnSequenceLength = 4;
+  static const int kJSReturnSequenceInstructions = 4;
+  static const int kDebugBreakSlotInstructions = 3;
+  static const int kDebugBreakSlotLength =
+      kDebugBreakSlotInstructions * kInstrSize;
 
   // ---------------------------------------------------------------------------
   // Code generation
@@ -981,13 +998,16 @@
   // Mark address of the ExitJSFrame code.
   void RecordJSReturn();
 
+  // Mark address of a debug break slot.
+  void RecordDebugBreakSlot();
+
   // Record a comment relocation entry that can be used by a disassembler.
   // Use --debug_code to enable.
   void RecordComment(const char* msg);
 
   void RecordPosition(int pos);
   void RecordStatementPosition(int pos);
-  void WriteRecordedPositions();
+  bool WriteRecordedPositions();
 
   int pc_offset() const { return pc_ - buffer_; }
   int current_position() const { return current_position_; }