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GH-137959: Replace shim code in jitted code with a single trampoline function.#137961

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markshannon merged 7 commits into
python:mainfrom
faster-cpython:jit-trampoline
Aug 21, 2025
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GH-137959: Replace shim code in jitted code with a single trampoline function.#137961
markshannon merged 7 commits into
python:mainfrom
faster-cpython:jit-trampoline

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@markshannon

@markshannon markshannon commented Aug 19, 2025 •
edited by bedevere-app Bot
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@bedevere-app bedevere-app Bot mentioned this pull request Aug 19, 2025
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@markshannon markshannon marked this pull request as ready for review August 19, 2025 16:24
@markshannon

markshannon commented Aug 20, 2025

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Benchmark results show a hint of a speedup, but in the noise. No slowdown, though.

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Some things I observed. Correct me if I'm wrong:

  1. This actually splits out the tier 2 interpreter from the tier 1 interpreter. Which is likely a significant saving for our tier 2 debug build time and tier 2 interpreter. Nice!
  2. The shim/trampoline is only compiled once lazily and reused across the whole runtime. Then reused across all JIT functions. We currently do two jumps (one into the shim/trampoline, one into the exec->jit_code). You need to lock this with a mutex in case multiple threads try to compile the shim at the same time.

If my understanding above is correct, then this is alright to me.

@markshannon

markshannon commented Aug 21, 2025

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Some things I observed. Correct me if I'm wrong:

  1. This actually splits out the tier 2 interpreter from the tier 1 interpreter. Which is likely a significant saving for our tier 2 debug build time and tier 2 interpreter. Nice!
  2. The shim/trampoline is only compiled once lazily and reused across the whole runtime. Then reused across all JIT functions. We currently do two jumps (one into the shim/trampoline, one into the exec->jit_code). You need to lock this with a mutex in case multiple threads try to compile the shim at the same time.

If my understanding above is correct, then this is alright to me.

That is all correct. I'll add a comment explaining it.

Hide details View details @markshannon markshannon merged commit a8d9d94 into python:main Aug 21, 2025
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@brandtbucher

brandtbucher commented Aug 21, 2025

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Nice, sorry I wasn't able to review in time, but it looks mostly good (love the unification of the two JIT execution engines).

Two notes:

  • I prefer the old name "shim" throughout to the new name "trampoline". Trampoline already has another meaning in the JIT compiler, and we renamed this to "shim" a while back to make the distinction clearer.
  • I don't love that we leak the shim code when the runtime is finalized. We should free it in Py_Finalize or whatever.

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