Comment by pizlonator
1 year ago
I get that and that’s also been my experience, just not for interpreters.
In interpreters, my experience is that fallback paths are well behaved if you just make them noinline and then ensure that the amount of interpreter state is small enough to fit in callee save regs.
Mike Pall makes an argument that interpreters are especially susceptible to this problem, and I find it convincing, since it matches my experience: https://web.archive.org/web/20180331141631/http://article.gm...
There are a bunch of arguments in there that don't match my experience, which includes the JSC interpreter. JSC had an interpreter written in C++ and one written in assembly, and the main reason for using the assembly one was not raw perf - it was so the interpreter knows the JIT's ABI for fast JIT<->interpreter calls.
Mike's argument about control flow diamonds being bad for optimization is especially questionable. It's only bad if one branch of the diamond uses a lot more registers than the other, which as I said, can be fixed by using noinline.