Constant-fold strings in HGraphBuilder::BuildBinaryOperation.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 50e3b2124df60ddc29331f0d9d4feb8f753c6a54..8bbabe4f48accce2e79640f9a505b354dc10bc32 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -10499,6 +10499,12 @@ HValue* HGraphBuilder::BuildBinaryOperation(
         HConstant::cast(right)->StringValue()->length() == 0) {
       return left;
     }
+    if (left->IsConstant() && HConstant::cast(left)->HasStringValue() &&

[Vyacheslav Egorov (Google), 2015/02/19 15:14:10]

Maybe have 

Handle<String> left_string = left->IsConstant() &&
HConstant::cast(left)->HasStringValue() ? HConstant::cast(right)->StringValue()
: Handle<String>();
Handle<String> right_string = left->IsConstant() &&
HConstant::cast(left)->HasStringValue() ? HConstant::cast(right)->StringValue()
: Handle<String>();

then all three ifs can use it:

if (!left_string.is_null() && left_string->length() == 0) {
  ...
}

if (!right_string.is_null() && right_string->length() == 0) {
  ...
}

if (!left_string.is_null() && !right_string.is_null()) {
  ..
}

+        right->IsConstant() && HConstant::cast(right)->HasStringValue()) {
+      return AddUncasted<HStringAdd>(
+          left, right, allocation_mode.GetPretenureMode(),
+          STRING_ADD_CHECK_NONE, allocation_mode.feedback_site());
+    }
 
     // Register the dependent code with the allocation site.
     if (!allocation_mode.feedback_site().is_null()) {