Constant-fold strings in HGraphBuilder::BuildBinaryOperation.

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/hydrogen.h"
 
 #include <sstream>
 
 #include "src/v8.h"
 
@@ skipping 10481 matching lines @@
     if (left->IsConstant() &&
         HConstant::cast(left)->HasStringValue() &&
         HConstant::cast(left)->StringValue()->length() == 0) {
       return right;
     }
     if (right->IsConstant() &&
         HConstant::cast(right)->HasStringValue() &&
         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()) {
       DCHECK(!graph()->info()->IsStub());
       Handle<AllocationSite> site(allocation_mode.feedback_site());
       AllocationSite::RegisterForDeoptOnTenureChange(site, top_info());
     }
 
     // Inline the string addition into the stub when creating allocation
     // mementos to gather allocation site feedback, or if we can statically
@@ skipping 2922 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal