I was optimizing the Lua transpiler for my modding language grug and ran into a really weird LuaJIT performance bug. The same benchmark could randomly run 20× slower, and it turned out a LuaJIT NYI (Not Yet Implemented) could silently blacklist an unrelated hot loop.
I wrote up the investigation here.
It goes from the benchmark mystery through LuaJIT's trace recorder internals, and ends with a PR to get unpack off LuaJIT's NYI list. Feedback is very welcome! :)
Link: https://streamhpc.com/blog/2026-08-05/the-luajit-nyi-that-silently-poisoned-an-unrelated-hot-loop/
Subreddit: r/programming