Heuristically predict interference on the back edge and use it to minimize number of register reshu…

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 23af66fac10c51e340667bf61730d765f98254f9..c33e9ea364bdc03eceb25797e2105ee0d03d2768 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -1162,7 +1162,8 @@ class PhiInstr : public Definition {
     : block_(block),
       inputs_(num_inputs),
       is_alive_(false),
-      representation_(kTagged) {
+      representation_(kTagged),
+      reaching_defs_(NULL) {
     for (intptr_t i = 0; i < num_inputs; ++i) {
       inputs_.Add(NULL);
     }
@@ -1223,6 +1224,14 @@ class PhiInstr : public Definition {
 
   virtual void InferRange();
 
+  BitVector* reaching_defs() const {
+    return reaching_defs_;
+  }
+
+  void set_reaching_defs(BitVector* reaching_defs) {
+    reaching_defs_ = reaching_defs;
+  }
+
  private:
   friend class ConstantPropagator;  // Direct access to inputs_.
 
@@ -1231,6 +1240,8 @@ class PhiInstr : public Definition {
   bool is_alive_;
   Representation representation_;
 
+  BitVector* reaching_defs_;
+
   DISALLOW_COPY_AND_ASSIGN(PhiInstr);
 };