Pattern match on generated code to find edge counters.

runtime/vm/code_patcher_ia32.cc

Index: runtime/vm/code_patcher_ia32.cc
diff --git a/runtime/vm/code_patcher_ia32.cc b/runtime/vm/code_patcher_ia32.cc
index 9cccd56c19639dbc612561ff512f2b1b8cb77ddd..ce36f00d4bdc914b101c59341e3a9681ca971f8c 100644
--- a/runtime/vm/code_patcher_ia32.cc
+++ b/runtime/vm/code_patcher_ia32.cc
@@ -149,7 +149,7 @@ class StaticCall : public ValueObject {
 };
 
 
-// The expected pattern of a dart closure call:
+// The expected pattern of a Dart closure call:
 //   mov EDX, arguments_descriptor_array
 //   call target_address
 //   <- return address
@@ -259,6 +259,34 @@ intptr_t CodePatcher::InstanceCallSizeInBytes() {
   return InstanceCall::kNumInstructions * InstanceCall::kInstructionSize;
 }
 
+
+// The expected code pattern of an edge counter in unoptimized code:
+//  b8 imm32    mov EAX, immediate
+class EdgeCounter : public ValueObject {
+ public:
+  EdgeCounter(uword start, const Code& ignored) : start_(start) {
+    ASSERT(IsValid(start));
+  }
+
+  static bool IsValid(uword start) {
+    return (*reinterpret_cast<uint8_t*>(start) == 0xb8);
+  }
+
+  RawObject* edge_counter() const {
+    return *reinterpret_cast<RawObject**>(start_ + 1);
+  }
+
+ private:
+  uword start_;
+};
+
+
+RawObject* CodePatcher::GetEdgeCounterAt(uword pc, const Code& code) {
+  ASSERT(code.ContainsInstructionAt(pc));
+  EdgeCounter counter(pc, code);
+  return counter.edge_counter();
+}
+
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32