Revert "Clean up after r16292 (disable optimization for StringWrappers)."

src/arm/code-stubs-arm.cc

Index: src/arm/code-stubs-arm.cc
diff --git a/src/arm/code-stubs-arm.cc b/src/arm/code-stubs-arm.cc
index 8ec05910a53ba541b50f494210625966e60632d4..3a1d1282a6bc65d5fc0a77d648d9a4be2c2491e4 100644
--- a/src/arm/code-stubs-arm.cc
+++ b/src/arm/code-stubs-arm.cc
@@ -5790,6 +5790,7 @@ void StringAddStub::GenerateConvertArgument(MacroAssembler* masm,
   __ b(lt, &done);
 
   // Check the number to string cache.
+  Label not_cached;
   __ bind(&not_string);
   // Puts the cached result into scratch1.
   NumberToStringStub::GenerateLookupNumberStringCache(masm,
@@ -5798,9 +5799,26 @@ void StringAddStub::GenerateConvertArgument(MacroAssembler* masm,
                                                       scratch2,
                                                       scratch3,
                                                       scratch4,
-                                                      slow);
+                                                      &not_cached);
   __ mov(arg, scratch1);
   __ str(arg, MemOperand(sp, stack_offset));
+  __ jmp(&done);
+
+  // Check if the argument is a safe string wrapper.
+  __ bind(&not_cached);
+  __ JumpIfSmi(arg, slow);
+  __ CompareObjectType(
+      arg, scratch1, scratch2, JS_VALUE_TYPE);  // map -> scratch1.
+  __ b(ne, slow);
+  __ ldrb(scratch2, FieldMemOperand(scratch1, Map::kBitField2Offset));
+  __ and_(scratch2,
+          scratch2, Operand(1 << Map::kStringWrapperSafeForDefaultValueOf));
+  __ cmp(scratch2,
+         Operand(1 << Map::kStringWrapperSafeForDefaultValueOf));
+  __ b(ne, slow);
+  __ ldr(arg, FieldMemOperand(arg, JSValue::kValueOffset));
+  __ str(arg, MemOperand(sp, stack_offset));
+
   __ bind(&done);
 }