JIT Planning for 3.15 and 3.16

[savannahostrowski]

Following a conversation at the CPython Core Dev Sprint, this is an uber issue tracking JIT performance goals and planning for 3.15 and 3.16.

The goal here is to capture our goals and the work that we believe is necessary to enable the JIT (on by default).

3.15 (May 2026)

Goals & "features"

Must Haves

• 5% faster (geometric mean)
  • Best case: 50% faster
  • Worst case: -10% faster
• JIT coverage (i.e., how much code is being JIT'ted): 80% JIT coverage can be improved considerably #142982
• Free-threading supported (pre-beta) Make the JIT thread-safe #133171 gh-141594: A free-threaded JIT (Part 1) #141595
• Support for out-of-process profilers and debuggers Supporting stack unwinding in the JIT compiler #126910

Nice to haves

• 10% faster (geometric mean)
  • Best case: 100% faster
  • Worst case 1% slower
• JIT coverage (i.e., how much code is being JIT'ted): 90%
• Robust documentation

How will we do this?

• Register allocation GH-135379: Top of stack caching for the JIT. #135465
• More optimizations Better uop coverage in the JIT optimizer #131798 Eliminate redundant refcounting in the JIT #134584
• Trace recording Trace recording JIT #139109
• Code gen
• LLVM 21 Update JIT builds to use LLVM 21 #140479
• Warm up faster
• More specialization Broader specialization in the Specializing Adaptive Interpreter for better JIT performance #143732

3.16

Goals & "features"

Must Haves

• ~ similar debuggability and performance across Tier 1 platforms
• 2+ maintainers in front-end, middle-end, and back-end
• ...Anything listed in 3.15 nice to have that was not achieved (e.g. docs, better coverage, faster)

How will we do this?

• Unboxing (int/float/other types)
• LLVM 22

Linked PRs

• gh-139038: Add JIT What's New for 3.15 #142845
• gh-139038: Add macOS performance numbers for the JIT #143479
• gh-139038: Update JIT perf figures for 3.14a4 #143705
• gh-139038: Link to Savannah's webpage for JIT results #146013
• gh-139038: Update JIT results in preparation for 3.15a8 #148018

[corona10]

Here is link the issue related JITxFree-threadng: #133171