[turbofan] Utilize String comparison feedback.

Make use of the previously introduced String feedback for compare
operations in TurboFan.

R=jarin@chromium.org
BUG=v8:5267,v8:5400

Committed: https://crrev.com/5d4253ecfb6ddcbbd7eb5654e728efa9559284a2
Cr-Original-Commit-Position: refs/heads/master@{#41163}
Review-Url: https://codereview.chromium.org/2523463002
Cr-Commit-Position: refs/heads/master@{#43211}

src/compiler/js-typed-lowering.cc

@@ -98,6 +98,16 @@ class JSBinopReduction final {
     return false;
   }
 
+  bool IsStringCompareOperation() {
+    if (lowering_->flags() & JSTypedLowering::kDeoptimizationEnabled) {
+      DCHECK_EQ(1, node_->op()->EffectOutputCount());
+      return (CompareOperationHintOf(node_->op()) ==
+              CompareOperationHint::kString) &&
+             BothInputsMaybe(Type::String());
+    }
+    return false;
+  }
+
   // Check if a string addition will definitely result in creating a ConsString,
   // i.e. if the combined length of the resulting string exceeds the ConsString
   // minimum length.
@@ -149,6 +159,24 @@ class JSBinopReduction final {
     }
   }
 
+  // Checks that both inputs are String, and if we don't know
+  // statically that one side is already a String, insert a
+  // CheckString node.
+  void CheckInputsToString() {
+    if (!left_type()->Is(Type::String())) {
+      Node* left_input = graph()->NewNode(simplified()->CheckString(), left(),
+                                          effect(), control());
+      node_->ReplaceInput(0, left_input);
+      update_effect(left_input);
+    }
+    if (!right_type()->Is(Type::String())) {
+      Node* right_input = graph()->NewNode(simplified()->CheckString(), right(),
+                                           effect(), control());
+      node_->ReplaceInput(1, right_input);
+      update_effect(right_input);
+    }
+  }
+
   // Checks that both inputs are InternalizedString, and if we don't know
   // statically that one side is already an InternalizedString, insert a
   // CheckInternalizedString node.
@@ -816,6 +844,10 @@ Reduction JSTypedLowering::ReduceJSComparison(Node* node) {
     r.ConvertInputsToNumber();
     less_than = simplified()->NumberLessThan();
     less_than_or_equal = simplified()->NumberLessThanOrEqual();
+  } else if (r.IsStringCompareOperation()) {
+    r.CheckInputsToString();
+    less_than = simplified()->StringLessThan();
+    less_than_or_equal = simplified()->StringLessThanOrEqual();
   } else {
     return NoChange();
   }
@@ -975,6 +1007,9 @@ Reduction JSTypedLowering::ReduceJSEqual(Node* node, bool invert) {
   } else if (r.IsReceiverCompareOperation()) {
     r.CheckInputsToReceiver();
     return r.ChangeToPureOperator(simplified()->ReferenceEqual(), invert);
+  } else if (r.IsStringCompareOperation()) {
+    r.CheckInputsToString();
+    return r.ChangeToPureOperator(simplified()->StringEqual(), invert);
   }
   return NoChange();
 }
@@ -1032,6 +1067,9 @@ Reduction JSTypedLowering::ReduceJSStrictEqual(Node* node, bool invert) {
     // both sides refer to the same Receiver than.
     r.CheckLeftInputToReceiver();
     return r.ChangeToPureOperator(simplified()->ReferenceEqual(), invert);
+  } else if (r.IsStringCompareOperation()) {
+    r.CheckInputsToString();
+    return r.ChangeToPureOperator(simplified()->StringEqual(), invert);
   }
   return NoChange();
 }