Remove half of the checks for comparing two Obejcts

src/hydrogen-check-elimination.cc

Index: src/hydrogen-check-elimination.cc
diff --git a/src/hydrogen-check-elimination.cc b/src/hydrogen-check-elimination.cc
index bbd3042fb7af6659f4107fffb7127e1298000593..ca37fa1c9d9fb3c151aa88b645b1a482b3767cc1 100644
--- a/src/hydrogen-check-elimination.cc
+++ b/src/hydrogen-check-elimination.cc
@@ -101,6 +101,10 @@ class HCheckTable : public ZoneObject {
         ReduceCheckHeapObject(HCheckHeapObject::cast(instr));
         break;
       }
+      case HValue::kCompareObjectEqAndBranch: {
+        ReduceCompareObjectEqAndBranch(HCompareObjectEqAndBranch::cast(instr));
+        break;
+      }
       default: {
         // If the instruction changes maps uncontrollably, drop everything.
         if (instr->CheckGVNFlag(kChangesMaps) ||
@@ -266,6 +270,50 @@ class HCheckTable : public ZoneObject {
     }
   }
 
+  void ReduceCompareObjectEqAndBranch(HCompareObjectEqAndBranch* instr) {
+    HCheckMaps* removed = NULL;
+    HValue* left = instr->left();
+    HValue* right = instr->right();
+    if (left->IsCheckMaps() && right->IsCheckMaps()) {
+      HCheckMaps* left_check = HCheckMaps::cast(left);
+      HCheckMaps* right_check = HCheckMaps::cast(right);
+      MapSet left_maps = left_check->map_set().Copy(phase_->zone());
+      MapSet right_maps = right_check->map_set().Copy(phase_->zone());
+      if (left_maps->Equals(right_maps)) {
+        // Object equality comparison guarantees one check suffices, so
+        // eliminate the other. After LICM, we could easily find the correct
+        // one (loop invariant object's map check could be hoisted out of loop).
+        removed = left->block()->block_id() > right->block()->block_id()
+            ? left_check : right_check;
+      }
+    } else if (left->IsCheckMaps() || right->IsCheckMaps()) {
+      // For the case one side map check is eliminated by previous optimization.
+      HValue* obj = left->IsCheckMaps() ? right : left;
+      HCheckMaps* check = (obj == left) ? HCheckMaps::cast(right)
+                                        : HCheckMaps::cast(left);
+      MapSet obj_maps = FindMaps(obj);
+      MapSet check_maps = check->map_set().Copy(phase_->zone());
+      if (obj_maps->IsSubset(check_maps)) {
+        // Obj check is more strict; the remaining check is redundant.
+        removed = check;
+      }
+    }
+
+    if (removed != NULL) {
+      // Check whether there is prior CheckHeapObject refering to the same
+      // object as the removed CheckMaps.
+      if (removed->previous()->IsCheckHeapObject()) {
+        HCheckHeapObject* cho = HCheckHeapObject::cast(removed->previous());
+        if (cho->value() == removed->ActualValue()) {
+          cho->DeleteAndReplaceWith(cho->value());
+          INC_STAT(removed_cho_);
+        }
+      }
+      removed->DeleteAndReplaceWith(removed->ActualValue());
+      INC_STAT(removed_);
+    }
+  }
+
   void ReduceStoreNamedField(HStoreNamedField* instr) {
     HValue* object = instr->object()->ActualValue();
     if (instr->has_transition()) {