Optimize RepeatedPtrField to use copy constructors where possible #14198
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The most common MergeFrom() scenario is that items are newly appended to an existing repeated field that has never been shrunk. For these cases copy constructing new entries is the cheapest option. This change overloads MergeFrom<> for `std::string`, `StringPiece`, `Message` and `MessageLite`, leaving the remaining templated implementation for all concrete message types supporting arena enabled copy construction, which is implemented as an out of line factory functor implementation. Going forward we should change `Arena::CreateMaybeMessage()` to return MessageLite* and accept const MessageLite& for copy construction. This will allow us to directly pass Arena::CreateMaybeMessage<T>() into these out of line functions. PiperOrigin-RevId: 568220325
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Optimize RepeatedPtrField to use copy constructors where possible
The most common MergeFrom() scenario is that items are newly appended to an existing repeated field that has never been shrunk. For these cases copy constructing new entries is the cheapest option. This change overloads MergeFrom<> for
std::string,StringPiece,MessageandMessageLite, leaving the remaining templated implementation for all concrete message types supporting arena enabled copy construction, which is implemented as an out of line factory functor implementation.Going forward we should change
Arena::CreateMaybeMessage()to return MessageLite* and accept const MessageLite& for copy construction. This will allow us to directly pass Arena::CreateMaybeMessage() into these out of line functions.