"Say you have a loop that goes over a million structs, but only accesses one field. Drop the extras, do the loop, then add them back." Why? Why would you copy

"Say you have a loop that goes over a million structs, but only accesses one field. Drop the extras, do the loop, then add them back." Why? Why would you copy at all? Are you saying that with small structs you'll be able to fit more of your data into caches? I haven't timed it (anyone have 20 minutes to spare?) but then you have to make full copy, very expensive not to mention having major implications on memory usage. If my compiler would do that under the hood, even when set to 'optimize for speed', I'd be pretty annoyed. The only way to know if such a thing is faster, is by looking at each use specifically - at which point we're not talking about a compiler anymore. Well ok we'd be talking about "runtime pgo", and approaching JIT territory. Furthermore, such an 'optimization' would also have to, e.g., detect offsetof() and make sure that that works correctly, and account for packing and people relying on that, and unions, etc etc. I'm not going to spend much time here further on examining and deconstructing this whole concept, especially as I still fail to comprehend the goal.