[dangoldin]

Really cool stuff and a nice introduction but curious how much modern compilers do for you already. Especially if you shift to the JIT world - what ends up being the difference between code where people optimize for this vs write in a style optimized around code readability/reuse/etc.

[tux1968]

JIT compilers can't compensate for poorly organized data. Ultimately, understanding these low-level concepts, affect high-level algorithm design and selection.

Watching the Andrew Kelly video mentioned above, really drives home the point that even if your compiler automatically optimizes structure ordering, to minimize padding and alignment issues, it can't fix other higher-level decisions. An example being, using two separate lists of structs to maintain their state data, rather than a single list with each struct having an enum to record its state.

[kllrnohj]

JIT languages tend to have the worst language-provided locality as they are often accompanied by GCs and lack of value types (there are exceptions to this, but it's broadly the case). And a JIT cannot re-arrange heap memory layout of objects as it must be hot-swappable. This is why despite incredibly huge investments in them such languages just never reach aot performance despite how much theoretical advantage a jit could have.

AOT'd languages could re-arrange a struct for better locality however the majority (if not all) languages rigidly require the fields are laid out in the order defined for various reasons.

[gpderetta]

> the majority (if not all) languages rigidly require the fields are laid out in the order defined for various reasons.

The as-if rule gives an escape hatch, although in practice it is hard to take advantage of, especially without whole program optimization.

[almostgotcaught]

I wish people would stop saying this. It's like CS-woo the idea that some magical solution exists that saves you from having to think real hard about the hardware, that magical abstractions save the day.

All of these things boil down to combinatorial optimization problems (bin packing ring a bell?). And there are no widely available compilers or JITs or whatever that bundle ILP solvers. Thus, what you're really getting with every compiler is a heuristic/approximate solution (to many many combinatorial optimization problems). Decide for yourself whether you're comfortable with your code just being approximately good or if you need to actually understand how your system works.