DBC: Fix not-stopping/crashing at fast Smi op breakpoints.

runtime/vm/debugger_dbc.cc

Index: runtime/vm/debugger_dbc.cc
diff --git a/runtime/vm/debugger_dbc.cc b/runtime/vm/debugger_dbc.cc
index 5f82cd5368a10a3456def0225c9684c30ca2cadf..b4b6c1f059f85e3019f9aa94e08208906cef6e96 100644
--- a/runtime/vm/debugger_dbc.cc
+++ b/runtime/vm/debugger_dbc.cc
@@ -25,6 +25,11 @@ static Instr* CallInstructionFromReturnAddress(uword pc) {
 }
 
 
+static Instr* FastSmiInstructionFromReturnAddress(uword pc) {
+  return reinterpret_cast<Instr*>(pc) - 2;
+}
+
+
 void CodeBreakpoint::PatchCode() {
   ASSERT(!is_enabled_);
   const Code& code = Code::Handle(code_);
@@ -54,6 +59,17 @@ void CodeBreakpoint::PatchCode() {
       default:
         UNREACHABLE();
     }
+
+    // If this call is the fall-through for a fast Smi op, also disable the fast
+    // Smi op.
+    if ((Bytecode::DecodeOpcode(saved_value_) == Bytecode::kInstanceCall2) &&
+        Bytecode::IsFastSmiOpcode(*FastSmiInstructionFromReturnAddress(pc_))) {
+      saved_value_fastsmi_ = *FastSmiInstructionFromReturnAddress(pc_);
+      *FastSmiInstructionFromReturnAddress(pc_) =
+          Bytecode::Encode(Bytecode::kNop, 0, 0, 0);
+    } else {
+      saved_value_fastsmi_ = Bytecode::kTrap;
+    }
   }
   is_enabled_ = true;
 }
@@ -75,6 +91,12 @@ void CodeBreakpoint::RestoreCode() {
       default:
         UNREACHABLE();
     }
+
+    if (saved_value_fastsmi_ != Bytecode::kTrap) {
+      Instr current_instr = *FastSmiInstructionFromReturnAddress(pc_);
+      ASSERT(Bytecode::DecodeOpcode(current_instr) == Bytecode::kNop);
+      *FastSmiInstructionFromReturnAddress(pc_) = saved_value_fastsmi_;
+    }
   }
   is_enabled_ = false;
 }