Basically all common CPUs are LE.
(And basically all BE CPUs have dedicated instructions for reading LE data. It's true I haven't yet added the inline assembly to use those instructions in Cap'n Proto's reference implementation, but that's only because no one actually cares about these architectures.)
So in basically all real use, there's no machine-instruction-level difference between accessing a field of a capnp struct and accessing a field of a C struct. If the instructions are identical, then how can you say one is "deferred parsing" and the other isn't? What meaning does any such distinction have?
> There's a performance cost, but hopefully you're only doing serialization/deserialization when you intend to hit the disk or wire to read/write into/out of your struct.
Disk is usually cached, meaning it's already in physical memory and you're wasting time making a copy rather than using the data in-place.
Over the network, within a datacenter, bandwidth is basically infinite (in that your CPU probably can't process bytes as fast as your network interface can). Time spent serializing and parsing is very real and wasteful. I've seen servers spending 30% or more of their CPU time parsing protobufs.
Over the long-haul internet, perhaps the CPU time spent parsing/serializing is not as relevant compared to the time spent transmitting. Still, I'd rather spend my CPU cycles elsewhere -- like in a dedicated compression algorithm -- rather than twiddling bytes needlessly in a parser.
> handling arbitrary binary formats
Yes, we all agree that some binary formats can't be handled any other way. But if you're in control of the format you use, then you can design it in a way that doesn't require a "bag of bytes" model, and your code can be much simpler and more adaptable as a result.
This. Optimizing performance on BE is a waste of everyone's time and resources.
(But the status quo on BE is that it does a load followed by a byte swap, which is probably pretty cheap anyway. The compiler might even already know how to optimize that into the appropriate LE-load instruction.)
Assuming little-endian CPU arch. It's followed by a byte reorder on big-endian architectures. (And you assume all Windows instances are little-endian, which probably-is-but-may-not-be the case.) You made the decision to optimize for what you consider the common case, but it does not generalize without added translation code to all cases. You may have hidden the translation code behind CapnProto's generated accessors, but CapnProto's structs are translation-free the way AWS Lambda is "serverless".
> To call that "deferring the parsing to access time" does not make sense.
Except you are deferring translation work (like byte reordering) to access time. Either that or you're hiding it in the serialization APIs. Again, it's like "serverless" computing: just because you've hidden it doesn't mean it's gone away.
> Byte-by-byte parsing is a valid way to do parsing but not the only way. Byte-by-byte parsers tend to be slow and -- arguably, more importantly -- overly complex and rigid.
There's a performance cost, but hopefully you're only doing serialization/deserialization when you intend to hit the disk or wire to read/write into/out of your struct. All in-memory processing happens in whatever endianness and alignment makes your CPU and compiler happy.
There's nothing "overly complex and rigid" about understanding a binary format as a bag of bytes and fetching scalar values (including offsets into the data structure) from it accordingly. This is how shit gets done when it comes to portably handling arbitrary binary formats. CapnProto can score a few wins by assuming things about the target CPU/compiler, restricting the binary format to conform to some of those assumptions, and papering over the rest with code hidden behind some of its APIs. But it's not a general solution to the problem of extracting meaning from an arbitrary hunk of bytes that may or may not have come from a CapnProto-conformant application.