> The problem is 95% about laying out the instruction dispatching code for the branch predictor to work optimally.
A fun fact I learned while writing this post is that that's no longer true! Modern branch predictors can pretty much accurately predict through a single indirect jump, if the run is long enough and the interpreted code itself has stable behavior!
My experiments on this project anecdotally agree; they didn't make it into the post but I also explored a few of the interpreters through hardware CPU counters and `perf stat`, and branch misprediction never showed up as a dominant factor.
(post author here) Oh, this is something I could have called out explicitly: The tail-calling interpreter relies on a feature (the `preserve_none` calling convention) that only landed in clang-19. That means you can only test it on that version. That coincidence (that 19 added both this feature, and the regression) is part of why this was so easy to miss at first, and why I had to "triangulate" with so many different benchmarks to be confident I understood what was going on.
> The problem is 95% about laying out the instruction dispatching code for the branch predictor to work optimally.
A fun fact I learned while writing this post is that that's no longer true! Modern branch predictors can pretty much accurately predict through a single indirect jump, if the run is long enough and the interpreted code itself has stable behavior!
Here's a paper that studied this (for both real hardware and a certain simulated branch predictor): https://inria.hal.science/hal-01100647/document
My experiments on this project anecdotally agree; they didn't make it into the post but I also explored a few of the interpreters through hardware CPU counters and `perf stat`, and branch misprediction never showed up as a dominant factor.