- not using extensions, as 3 does not support it - not using map, as 2 does not support it
and we basically landed on restricting ourselves to use the common denominator among all post-2.5 versions. Wondering if this sounds reasonable to you - always great to hear the original author's advice.
Plus, I am wondering if there are recommended ways of reducing protobuf runtime size, we use protobuf-lite but if there are any further wins it would be definitely nice for memory and space constrained problems.
You are only using proto3 schemas if you start your .proto file with 'syntax = "proto3;"': https://developers.google.com/protocol-buffers/docs/proto3 But if you do this, your .proto file will no longer work with Protobuf <3.0.
We have been working on some optimizations to help the linker strip generated code where possible. I recommend compiling with -ffunction-sections/-fdata-sections if you aren't already, and -gc-sections on your linker invocation.
proto3 was designed to be a simplification of proto2 schemas. It removed several features such as extensions and field presence. Each .proto file specifies whether it is a proto2 or proto3 schema.
Protobuf 3.0 added several features such as maps, but these features were added to both proto2 and proto3 schemas.
Sorry this is kind of confusing.