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gh-128563: Add correction note to tail call in whats new #130908

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Mar 6, 2025
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gh-128563: Add correction note to tail call in whats new #130908

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Add correction note to tail call in whats new
Fidget-Spinner Mar 6, 2025
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Update 3.14.rst
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15 changes: 14 additions & 1 deletion Doc/whatsnew/3.14.rst
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Expand Up @@ -270,7 +270,7 @@ It uses tail calls between small C functions that implement individual
Python opcodes, rather than one large C case statement.
For certain newer compilers, this interpreter provides
significantly better performance. Preliminary numbers on our machines suggest
anywhere from -3% to 30% faster Python code, and a geometric mean of 9-15%
anywhere up to 30% faster Python code, and a geometric mean of 3-5%
faster on ``pyperformance`` depending on platform and architecture. The
baseline is Python 3.14 built with Clang 19 without this new interpreter.

Expand All @@ -295,6 +295,19 @@ For further information on how to build Python, see

__ https://en.wikipedia.org/wiki/Tail_call

.. attention::

This section previously reported a 9-15% geomean speedup. This number has since been
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Can we say "geometric mean" here? I think geomean is too jargonny.

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🤦 thanks, but I automerged too quickly.

cautiously revised down to 3-5%. While we expect performance results to be better
than what we report, our estimates are more conservative due to a
`compiler bug <https://github.com/llvm/llvm-project/issues/106846>`_ found in
Clang/LLVM 19. We were unaware of this bug, and it artifically boosted
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Instead of saying "artificially boosted our numbers" (which seems to imply it magically made tail calling even better), maybe say "which negatively impacted the performance of the traditional non-tail calling interpreter loop"

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Yeah Stefan Marr pointed out that the artificially boosting comment makes no sense too. So I addressed that here #130911

our numbers, resulting in inaccurate results. We sincerely apologize for
communicating results that were only accurate for certain versions of LLVM 19
and 20. At the time of writing, this bug has not yet been fixed in LLVM 19-21. Thus
any benchmarks with those versions of LLVM may produce artifically inflated numbers.
(Thanks to Nelson Elhage for bringing this to light.)

(Contributed by Ken Jin in :gh:`128563`, with ideas on how to implement this
in CPython by Mark Shannon, Garrett Gu, Haoran Xu, and Josh Haberman.)

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