improve GC performance by 5% with builtin_prefetch
The mark phase of non-incremental major GC is (I believe) dominated by pointer chasing. One way we can improve that is by prefetching cachelines from memory before they are accessed, to reduce stalls. I did some experimenting, and the following patch reduces the time spent on a full GC from ~ 950 milliseconds to ~ 900 milliseconds, a small but stable improvement. I would love if additional folks have other benchmarks (or could point me at them) to see if this holds up beyond the web service I tested, and whether something like this could be considered for inclusion.
I also attempted a more "principled" approach based on an optimization described in the GC handbook: putting a FIFO queue in front of the mark stack, and prefetching addresses as they enter the queue. However, I wasn't able to see any performance improvement there despite testing a number of queue sizes from 4 to 64. Its possible I implemented this wrong, or misjudged the access patterns (if e.g. the memory of a VALUE is accessed before push_mark_stack is called, it would invalidate this approach). The code for that alternative is here: https://github.com/bpowers/ruby/commit/d790d0c15047c36c23850a112093fe0e32fd3262