[crankshaft] Re-add fast-case for string add left/right.

src/code-stubs-hydrogen.cc

Index: src/code-stubs-hydrogen.cc
diff --git a/src/code-stubs-hydrogen.cc b/src/code-stubs-hydrogen.cc
index 9f9b14e309d975f15c77020d72a6e38b8e3e0a28..43cf265e97f94af41800587d3ac5831674df74f1 100644
--- a/src/code-stubs-hydrogen.cc
+++ b/src/code-stubs-hydrogen.cc
@@ -113,6 +113,8 @@ class CodeStubGraphBuilderBase : public HGraphBuilder {
                                         HValue* shared_info,
                                         HValue* native_context);
 
+  HValue* CheckString(HValue* input, bool convert);
+
  private:
   HValue* BuildArraySingleArgumentConstructor(JSArrayBuilder* builder);
   HValue* BuildArrayNArgumentsConstructor(JSArrayBuilder* builder,
@@ -1456,6 +1458,66 @@ Handle<Code> BinaryOpWithAllocationSiteStub::GenerateCode() {
 }
 
 
+HValue* CodeStubGraphBuilderBase::CheckString(HValue* input, bool convert) {
+  if (!convert) return BuildCheckString(input);
+  IfBuilder if_inputissmi(this);
+  HValue* inputissmi = if_inputissmi.If<HIsSmiAndBranch>(input);
+  if_inputissmi.Then();
+  {
+    // Convert the input smi to a string.
+    Push(BuildNumberToString(input, Type::SignedSmall()));
+  }
+  if_inputissmi.Else();
+  {
+    HValue* input_map =
+        Add<HLoadNamedField>(input, inputissmi, HObjectAccess::ForMap());
+    HValue* input_instance_type = Add<HLoadNamedField>(
+        input_map, inputissmi, HObjectAccess::ForMapInstanceType());
+    IfBuilder if_inputisstring(this);
+    if_inputisstring.If<HCompareNumericAndBranch>(
+        input_instance_type, Add<HConstant>(FIRST_NONSTRING_TYPE), Token::LT);
+    if_inputisstring.Then();
+    {
+      // The input is already a string.
+      Push(input);
+    }
+    if_inputisstring.Else();
+    {
+      // Convert to primitive first (if necessary), see
+      // ES6 section 12.7.3 The Addition operator.
+      IfBuilder if_inputisprimitive(this);
+      STATIC_ASSERT(FIRST_PRIMITIVE_TYPE == FIRST_TYPE);
+      if_inputisprimitive.If<HCompareNumericAndBranch>(
+          input_instance_type, Add<HConstant>(LAST_PRIMITIVE_TYPE), Token::LTE);
+      if_inputisprimitive.Then();
+      {
+        // The input is already a primitive.
+        Push(input);
+      }
+      if_inputisprimitive.Else();
+      {
+        // TODO(bmeurer): Add support for fast ToPrimitive conversion using
+        // a dedicated ToPrimitiveStub.
+        Add<HPushArguments>(input);
+        Push(Add<HCallRuntime>(Runtime::FunctionForId(Runtime::kToPrimitive),
+                               1));
+      }
+      if_inputisprimitive.End();
+      // Convert the primitive to a string value.
+      ToStringDescriptor descriptor(isolate());
+      ToStringStub stub(isolate());
+      HValue* values[] = {context(), Pop()};
+      Push(AddUncasted<HCallWithDescriptor>(
+          Add<HConstant>(stub.GetCode()), 0, descriptor,
+          Vector<HValue*>(values, arraysize(values))));
+    }
+    if_inputisstring.End();
+  }
+  if_inputissmi.End();
+  return Pop();
+}
+
+
 template <>
 HValue* CodeStubGraphBuilder<StringAddStub>::BuildCodeInitializedStub() {
   StringAddStub* stub = casted_stub();
@@ -1467,10 +1529,12 @@ HValue* CodeStubGraphBuilder<StringAddStub>::BuildCodeInitializedStub() {
 
   // Make sure that both arguments are strings if not known in advance.
   if ((flags & STRING_ADD_CHECK_LEFT) == STRING_ADD_CHECK_LEFT) {
-    left = BuildCheckString(left);
+    left =
+        CheckString(left, (flags & STRING_ADD_CONVERT) == STRING_ADD_CONVERT);
   }
   if ((flags & STRING_ADD_CHECK_RIGHT) == STRING_ADD_CHECK_RIGHT) {
-    right = BuildCheckString(right);
+    right =
+        CheckString(right, (flags & STRING_ADD_CONVERT) == STRING_ADD_CONVERT);
   }
 
   return BuildStringAdd(left, right, HAllocationMode(pretenure_flag));