Performance of the Python 3.14 tail-call interpreter

March 10, 2025 · Nelson Elhage

URL: https://blog.nelhage.com/post/cpython-tail-call/

“Unfortunately, as I will document in this post, these impressive performance gains turned out to be primarily due to inadvertently working around a regression in LLVM 19.”

Python 3.14’s tail-call interpreter shipped with a 10 to 15% speedup claim. Most of that came from an LLVM 19 bug that slowed the baseline. Against a sound baseline the gain is 1 to 5%. The benchmark measured a compiler regression.