gh-139038: Add JIT What's New for 3.15

Doc/whatsnew/3.15.rst

@@ -73,6 +73,7 @@ Summary -- Release highlights
   <whatsnew315-utf8-default>`
 * :pep:`782`: :ref:`A new PyBytesWriter C API to create a Python bytes object
   <whatsnew315-pep782>`
+* :ref:`The JIT compiler has been significantly upgraded <whatsnew315-jit>`
 * :ref:`Improved error messages <whatsnew315-improved-error-messages>`
 
 
@@ -850,6 +851,91 @@ csv
   (Contributed by Maurycy Pawłowski-Wieroński in :gh:`137628`.)
 
 
+.. _whatsnew315-jit:
+
+Upgraded JIT compiler
+=====================
+
+Results from the `pyperformance <https://github.com/python/pyperformance>`__
+benchmark suite report
+`3-4% <https://github.com/facebookexperimental/free-threading-benchmarking/blob/main/results/bm-20251214-3.15.0a2%2B-6cddf04-JIT/bm-20251214-vultr-x86_64-python-6cddf04344a1e8ca9df5-3.15.0a2%2B-6cddf04-vs-base.svg>`__
+geometric mean performance improvement for the JIT over the standard CPython
+interpreter built with all optimizations enabled. The speedups for JIT
+builds versus no JIT builds range from roughly 20% slowdown to over
+100% speedup (ignoring the ``unpack_sequence`` microbenchmark) on
+x86-64 Linux and AArch64 macOS systems.
+
+.. attention::
+    These results are not yet final.
+
+The major upgrades to the JIT are:
+
+* LLVM 21 build-time dependency
+* New tracing frontend
+* Basic register allocation in the JIT
+* More JIT optimizations
+* Better machine code generation
+
+.. rubric:: LLVM 21 build-time dependency
+
+The JIT compiler now uses LLVM 21 for build-time stencil generation. As
+always, LLVM is only needed when building CPython with the JIT enabled;
+end users running Python do not need LLVM installed. Instructions for
+installing LLVM can be found in the `JIT compiler documentation
+<https://github.com/python/cpython/blob/main/Tools/jit/README.md>`__
+for all supported platforms.
+
+(Contributed by Savannah Ostrowski in :gh:`140973`.)
+
+.. rubric:: A new tracing frontend
+
+The JIT compiler now supports significantly more bytecode operations and
+control flow than in Python 3.14, enabling speedups on a wider variety of
+code. For example, simple Python object creation is now understood by the
+3.15 JIT compiler. Overloaded operations and generators are also partially
+supported. This was made possible by an overhauled JIT tracing frontend
+that records actual execution paths through code, rather than estimating
+them as the previous implementation did.
+
+(Contributed by Ken Jin in :gh:`139109`. Support for Windows added by
+Mark Shannon in :gh:`141703`.)
+
+.. rubric:: Basic register allocation in the JIT
+
+A basic form of register allocation has been added to the JIT compiler's
+optimizer. This allows the JIT compiler to avoid certain stack operations
+altogether and instead operate on registers. This allows the JIT to produce
+more efficient traces by avoiding reads and writes to memory.
+
+(Contributed by Mark Shannon in :gh:`135379`.)
+
+.. rubric:: More JIT optimizations
+
+More `constant-propagation <https://en.wikipedia.org/wiki/Constant_folding>`__
+is now performed. This means when the JIT compiler detects that certain user
+code results in constants, the code can be simplified by the JIT.
+
+(Contributed by Ken Jin and Savannah Ostrowski in :gh:`132732`.)
+
+The JIT avoids :term:`reference count`\ s where possible. This generally
+reduces the cost of most operations in Python.
+
+(Contributed by Ken Jin, Donghee Na, Nadeshiko Manju, Savannah Ostrowski,
+Noam Cohen, Tomas Roun, PuQing in :gh:`134584`.)
+
+.. rubric:: Better machine code generation
+
+The JIT compiler's machine code generator now produces better machine code
+for x86-64 and AArch64 macOS and Linux targets. In general, users should
+experience lower memory usage for generated machine code and more efficient
+machine code versus the old JIT.
+
+(Contributed by Brandt Bucher in :gh:`136528` and :gh:`136528`.
+Implementation for AArch64 contributed by Mark Shannon in :gh:`139855`.
+Additional optimizations for AArch64 contributed by Mark Shannon and
+Diego Russo in :gh:`140683` and :gh:`142305`.)
+
+
 Removed
 =======