v8_types.py contains special cases like this:

    'SerializedScriptValue': '{cpp_value} ? {cpp_value}->deserialize() : v8::Local<v8::Value>(v8::Null({isolate}))',

{cpp_value} isn't necessarily just a local variable; it can be a function call that isn't free. In these cases it gets evaluated twice in the success case.

For example, in V8Internals.cpp:

    v8SetReturnValue(info, impl->deserializeBuffer(buffer) ? impl->deserializeBuffer(buffer)->deserialize() : v8::Local<v8::Value>(v8::Null(info.GetIsolate())));

Here we deserialize the buffer, producing a PassRefPtr<SerializedScriptValue>, and then if if that succeeded, we deserialize it again in order to invoke SerializedScriptValue::deserialize.

We should only do this once, either by using a local variable, or possibly by being clever with a lambda, like:

([](PassRefPtr<SerializedScriptValue> x) { return x ? x->deserialize() : ...; })(impl->deserializeBuffer(buffer));

It looks like there may be other cases where {cpp_value} is evaluated twice, and I'm not sure how the bindings team would prefer  to deal with this.