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

src/mips/code-stubs-mips.cc

Index: src/mips/code-stubs-mips.cc
diff --git a/src/mips/code-stubs-mips.cc b/src/mips/code-stubs-mips.cc
index 68b0c5fa679c63f14429725434568e354739510b..b536209c1952fcaf02594f4269bb7105de5f27c9 100644
--- a/src/mips/code-stubs-mips.cc
+++ b/src/mips/code-stubs-mips.cc
@@ -5861,6 +5861,7 @@ void StringAddStub::GenerateConvertArgument(MacroAssembler* masm,
   __ Branch(&done, lt, scratch1, Operand(FIRST_NONSTRING_TYPE));
 
   // Check the number to string cache.
+  Label not_cached;
   __ bind(&not_string);
   // Puts the cached result into scratch1.
   NumberToStringStub::GenerateLookupNumberStringCache(masm,
@@ -5869,9 +5870,23 @@ void StringAddStub::GenerateConvertArgument(MacroAssembler* masm,
                                                       scratch2,
                                                       scratch3,
                                                       scratch4,
-                                                      slow);
+                                                      &not_cached);
   __ mov(arg, scratch1);
   __ sw(arg, MemOperand(sp, stack_offset));
+  __ jmp(&done);
+
+  // Check if the argument is a safe string wrapper.
+  __ bind(&not_cached);
+  __ JumpIfSmi(arg, slow);
+  __ GetObjectType(arg, scratch1, scratch2);  // map -> scratch1.
+  __ Branch(slow, ne, scratch2, Operand(JS_VALUE_TYPE));
+  __ lbu(scratch2, FieldMemOperand(scratch1, Map::kBitField2Offset));
+  __ li(scratch4, 1 << Map::kStringWrapperSafeForDefaultValueOf);
+  __ And(scratch2, scratch2, scratch4);
+  __ Branch(slow, ne, scratch2, Operand(scratch4));
+  __ lw(arg, FieldMemOperand(arg, JSValue::kValueOffset));
+  __ sw(arg, MemOperand(sp, stack_offset));
+
   __ bind(&done);
 }