Pattern match on generated code to find edge counters.

runtime/vm/code_patcher_mips.cc

Index: runtime/vm/code_patcher_mips.cc
diff --git a/runtime/vm/code_patcher_mips.cc b/runtime/vm/code_patcher_mips.cc
index f25b4602ef81f41ca737795854b7d634991490de..20df6cfc1e3c48f436dd538534e2db745185b879 100644
--- a/runtime/vm/code_patcher_mips.cc
+++ b/runtime/vm/code_patcher_mips.cc
@@ -84,6 +84,45 @@ RawFunction* CodePatcher::GetUnoptimizedStaticCallAt(
   return ic_data.GetTargetAt(0);
 }
 
+
+// The expected code pattern of an edge counter in unoptimized code:
+//     lw t0, imm(pp)
+class EdgeCounter : public ValueObject {
+ public:
+  EdgeCounter(uword start, const Code& code)
+      : start_(start), object_pool_(Array::Handle(code.ObjectPool())) {
+    ASSERT(IsValid(start));
+  }
+
+  static bool IsValid(uword start) {
+    Instr* instr = Instr::At(start);
+    return (instr->OpcodeField() == LW)
+        && (instr->RtField() == T0)
+        && (instr->RsField() == PP);
+  }
+
+  RawObject* edge_counter() const {
+    return object_pool_.At(IndexFromPPLoad(start_));
+  }
+
+ private:
+  static intptr_t IndexFromPPLoad(uword start) {
+    ASSERT(IsValid(start));
+    int offset = Instr::At(start)->SImmField() + kHeapObjectTag;

[Florian Schneider, 2013/09/30 08:55:38]

What about indices in the constant pool greater than the maximum immediate size
(kImmbits=16)?

I think the assember will emit a multiple instruction (like on ARM) in that
case.

+    return (offset - Array::data_offset()) / kWordSize;
+  }
+
+  uword start_;
+  const Array& object_pool_;
+};
+
+
+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_MIPS