Refactor fast path for empty constant strings in BinaryOp.

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 8936 matching lines @@
       ASSERT(left_type->Is(Type::String()));
       right = BuildNumberToString(right, right_type);
     } else if (!right_type->Is(Type::String())) {
       ASSERT(left_type->Is(Type::String()));
       HValue* function = AddLoadJSBuiltin(Builtins::STRING_ADD_LEFT);
       Add<HPushArgument>(left);
       Add<HPushArgument>(right);
       return AddUncasted<HInvokeFunction>(function, 2);
     }
 
-    // Inline the string addition into the stub when creating allocation
-    // mementos to gather allocation site feedback.
-    if (graph()->info()->IsStub() &&
-        allocation_mode.CreateAllocationMementos()) {
-      return BuildStringAdd(left, right, allocation_mode);
+    // Fast path for empty constant strings.
+    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;
     }
 
     // Register the dependent code with the allocation site.
     if (!allocation_mode.feedback_site().is_null()) {
       ASSERT(!graph()->info()->IsStub());
       Handle<AllocationSite> site(allocation_mode.feedback_site());
       AllocationSite::AddDependentCompilationInfo(
           site, AllocationSite::TENURING, top_info());
     }
 
-    // Inline string addition if we know that we'll create a cons string.
-    if (left->IsConstant()) {
-      HConstant* c_left = HConstant::cast(left);
-      if (c_left->HasStringValue()) {
-        int c_left_length = c_left->StringValue()->length();
-        if (c_left_length == 0) {
-          return right;
-        } else if (c_left_length + 1 >= ConsString::kMinLength) {
-          return BuildStringAdd(left, right, allocation_mode);
-        }
-      }
-    }
-    if (right->IsConstant()) {
-      HConstant* c_right = HConstant::cast(right);
-      if (c_right->HasStringValue()) {
-        int c_right_length = c_right->StringValue()->length();
-        if (c_right_length == 0) {
-          return left;
-        } else if (c_right_length + 1 >= ConsString::kMinLength) {
-          return BuildStringAdd(left, right, allocation_mode);
-        }
-      }
+    // Inline the string addition into the stub when creating allocation
+    // mementos to gather allocation site feedback, or if we can statically
+    // infer that we're going to create a cons string.
+    if ((graph()->info()->IsStub() &&
+         allocation_mode.CreateAllocationMementos()) ||
+        (left->IsConstant() &&
+         HConstant::cast(left)->HasStringValue() &&
+         HConstant::cast(left)->StringValue()->length() + 1 >=
+         ConsString::kMinLength) ||

[mvstanton, 2014/01/21 15:52:26]

Can ConsString::kMinLength be indented 2 spaces? It would better attach it to
the expression on the line above.

[Benedikt Meurer, 2014/01/22 12:29:54]

Done.

+        (right->IsConstant() &&
+         HConstant::cast(right)->HasStringValue() &&
+         HConstant::cast(right)->StringValue()->length() + 1 >=
+         ConsString::kMinLength)) {

[mvstanton, 2014/01/21 15:52:26]

Same.

[Benedikt Meurer, 2014/01/22 12:29:54]

Done.

+      return BuildStringAdd(left, right, allocation_mode);
     }
 
     // Fallback to using the string add stub.
     return AddUncasted<HStringAdd>(
         left, right, allocation_mode.GetPretenureMode(),
         STRING_ADD_CHECK_NONE, allocation_mode.feedback_site());
   }
 
   if (graph()->info()->IsStub()) {
     left = EnforceNumberType(left, left_type);
@@ skipping 2002 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal