Do Google's Protocol Buffers automatically align data efficiently?

Question

In a typical C or C++ struct the developer must explicitly order data members in a way that provides efficient memory alignment and padding, if that is an issue.

Google's Protocol Buffers behave a lot like structs and it is not clear how the compilation of these affects memory layout. Does anyone know if this tendency to organize data in a specific order for the sake of efficient memory layout is automatically handled by the protocol buffer compiler? I have been unable to find any information on this.

I.E. the buffer might actually internally order the data differently than it is specified in the message object of the protobuf.

Answer 1

In a typical C or C++ struct the developer must explicitly order data members in a way that provides efficient memory alignment and padding, if that is an issue.

Actually this is not entirely true.

It's true that most compilers (actually all I know of) tend to align struct elements to machine word addresses. They do this, due to performance reasons because it's usually cheaper to read from a word address and just mask away some bits than to read from the word address, shift the word, so the value you are looking for is right aligned and the mask away the bits not needed. (Of course this depends on the architecture you are compiling for)

So why is your statement I quoted above not true? - Because of the fact that compilers are arranging elements as described above, they also offer the programmer the opportunity to influnece this behavior. Usually this is done using a compiler specific pragma.

For example GCC and MS C Compilers provide a pragma called "pack" which allows the programmer to change the alignment behavior of the compiler for specific structs. Of course, if you choose to set pack to '1', the memory usage is improvide, but this will possibly impact your runtime behavior.

What never happens to my knowledge is a reordering of the members in a struct by the compiler.