[stubs] Properly handle length overflow in StringAddStub.

src/crankshaft/hydrogen.cc

Index: src/crankshaft/hydrogen.cc
diff --git a/src/crankshaft/hydrogen.cc b/src/crankshaft/hydrogen.cc
index 7fce48961f3a2c8d78d23b43b7e3a08fa9e7c77e..db60d6f3523d029b45b2cbd1cf3393b62c619b44 100644
--- a/src/crankshaft/hydrogen.cc
+++ b/src/crankshaft/hydrogen.cc
@@ -2413,7 +2413,20 @@ HValue* HGraphBuilder::BuildAddStringLengths(HValue* left_length,
   HValue* length = AddUncasted<HAdd>(left_length, right_length);
   // Check that length <= kMaxLength <=> length < MaxLength + 1.
   HValue* max_length = Add<HConstant>(String::kMaxLength + 1);
-  Add<HBoundsCheck>(length, max_length);
+  if (top_info()->IsStub()) {
+    // This is a mitigation for crbug.com/627934; the real fix
+    // will be to migrate the StringAddStub to TurboFan one day.

[jgruber, 2017/01/31 15:13:47]

FYI: We now have a TF StringAddStub. Not sure from the description if we can
simply remove this branch? The test from this CL passes.

[Benedikt Meurer, 2017/01/31 17:57:20]

I think we still inline this into the BinaryOpIC for String addition, so for now
just keep it.

+    IfBuilder if_invalid(this);
+    if_invalid.If<HCompareNumericAndBranch>(length, max_length, Token::GT);
+    if_invalid.Then();
+    {
+      Add<HCallRuntime>(
+          Runtime::FunctionForId(Runtime::kThrowInvalidStringLength), 0);
+    }
+    if_invalid.End();
+  } else {
+    Add<HBoundsCheck>(length, max_length);
+  }
   return length;
 }