4 ms·

Fair comment. If these flags are hardly used then I guess it is a bit surprising that this is necessary.

• toast0 4y ago
  The flags are hardly read, but computed often. Computing them while doing operations is nearly free in hardware, so it makes sense to add them in hardware if you can. It's not nearly free in software, but it's important to do it where not doing it might be obervable in normal flow (tricky things with interrupts are out of luck, even if you always do the software calculation, you could interrupt in the middle of it). In many cases, it's easy to determine the flags aren't observable and you can skip software computation.
• delusional 4y ago
  I have no real information on the topic, but to me it looks like a "why not" optimization. It was probably pretty cheap to put into the hardware (seeing as it was already an ARM extension), the engineers probably figured there was some risk that it would be really important for some workload, and once they had included it they might as well use it. In other words, it's probably not necessary but was included early on out of an abundance of caution. They knew that if it turned out to be used in some binary somewhere, it would be a major performance killer. I'd be interested in seeing a benchmark with the hardware flag turned off and the translation/optimization setup used for linux enabled. I bet the difference would be negligible.