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Write about Tier 2 and JIT in "what's new 3.13" (#114826)
(This will soon be superseded by Ken Jin's much more detailed version.)
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@ -81,6 +81,13 @@ Important deprecations, removals or restrictions:
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* Python 3.13 and later have two years of full support,
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followed by three years of security fixes.
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Interpreter improvements:
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* A basic :ref:`JIT compiler <whatsnew313-jit-compiler>` was added.
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It is currently disabled by default (though we may turn it on later).
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Performance improvements are modest -- we expect to be improving this
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over the next few releases.
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New Features
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============
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@ -477,6 +484,46 @@ Optimizations
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FreeBSD and Solaris. See the ``subprocess`` section above for details.
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(Contributed by Jakub Kulik in :gh:`113117`.)
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.. _whatsnew313-jit-compiler:
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Experimental JIT Compiler
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=========================
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When CPython is configured using the ``--enable-experimental-jit`` option,
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a just-in-time compiler is added which can speed up some Python programs.
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The internal architecture is roughly as follows.
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* We start with specialized *Tier 1 bytecode*.
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See :ref:`What's new in 3.11 <whatsnew311-pep659>` for details.
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* When the Tier 1 bytecode gets hot enough, it gets translated
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to a new, purely internal *Tier 2 IR*, a.k.a. micro-ops ("uops").
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* The Tier 2 IR uses the same stack-based VM as Tier 1, but the
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instruction format is better suited to translation to machine code.
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* We have several optimization passes for Tier 2 IR, which are applied
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before it is interpreted or translated to machine code.
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* There is a Tier 2 interpreter, but it is mostly intended for debugging
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the earlier stages of the optimization pipeline. If the JIT is not
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enabled, the Tier 2 interpreter can be invoked by passing Python the
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``-X uops`` option or by setting the ``PYTHON_UOPS`` environment
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variable to ``1``.
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* When the ``--enable-experimental-jit`` option is used, the optimized
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Tier 2 IR is translated to machine code, which is then executed.
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This does not require additional runtime options.
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* The machine code translation process uses an architecture called
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*copy-and-patch*. It has no runtime dependencies, but there is a new
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build-time dependency on LLVM.
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(JIT by Brandt Bucher, inspired by a paper by Haoran Xu and Fredrik Kjolstad.
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Tier 2 IR by Mark Shannon and Guido van Rossum.
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Tier 2 optimizer by Ken Jin.)
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Deprecated
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==========
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