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GeezerVison is showing github.com — gh-143158: Hot cold code splitting for JIT compiler by waterlens · Pull Request #149292 · python/cpython

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@waterlens

@waterlens waterlens commented May 2, 2026 •
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Issue: gh-143158

This PR implements trace-level hot/cold code splitting for the JIT.

Previously, each uop stencil was emitted as one contiguous code block, so hot fast-path code and rarely executed cold/error-path code were interleaved
throughout the compiled trace:

[uop0 hot+cold][uop1 hot+cold][uop2 hot+cold]...

This change separates each stencil into hot and cold code and emits the trace as:

[uop0 hot][uop1 hot][uop2 hot]...[uop0 cold][uop1 cold][uop2 cold]...

Plan:

  • Move cold code after _JIT_CONTINUE and introduce _JIT_COLD_START for positioning.
  • Split cold / hot code when emitting stencils

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@bedevere-app

bedevere-app Bot commented May 2, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@python-cla-bot

python-cla-bot Bot commented May 2, 2026 •
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All commit authors signed the Contributor License Agreement.

CLA signed
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@bedevere-app bedevere-app Bot mentioned this pull request May 2, 2026
Hot-cold splitting for JIT stencils #143158
Open
@read-the-docs-community

read-the-docs-community Bot commented May 2, 2026 •
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Documentation build overview

📚 cpython-previews | 🛠️ Build #32638145 | 📁 Comparing d6ce2a1 against main (3a1df78)

  🔍 Preview build  

79 files changed · ± 78 modified · - 1 deleted

± Modified

  • c-api/buffer.html
  • c-api/code.html
  • c-api/concrete.html
  • c-api/descriptor.html
  • c-api/exceptions.html
  • c-api/gen.html
  • c-api/intro.html
  • c-api/perfmaps.html
  • c-api/set.html
  • c-api/typeobj.html
  • and 68 more...

- Deleted

  • c-api/sentinel.html
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@bedevere-app

bedevere-app Bot commented May 11, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@waterlens
waterlens force-pushed the hot-cold-splitting branch from d6ce2a1 to c180cc0 Compare May 11, 2026 15:31
@bedevere-app

bedevere-app Bot commented May 11, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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waterlens added 2 commits May 18, 2026 22:40
@waterlens
Move cold code after _JIT_CONTINUE
d2da9a5
@waterlens
Split JIT stencils into hot and cold code
f56e969
@waterlens
waterlens force-pushed the hot-cold-splitting branch from c180cc0 to f56e969 Compare May 18, 2026 14:40
@bedevere-app

bedevere-app Bot commented May 18, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@waterlens
Fix some issues
a20783f 
`replace` didn't preserve relocation;
root block should be at the start of the block list;
fix the pc-relative addressing of branching between hot and cold sections
@bedevere-app

bedevere-app Bot commented May 18, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@waterlens
Preserve block order before _JIT_ENTRY
b618909
@bedevere-app

bedevere-app Bot commented May 19, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@waterlens
Reformat changed files
5a395d2
@bedevere-app

bedevere-app Bot commented May 19, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@waterlens

waterlens commented May 19, 2026

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@Fidget-Spinner would you mind helping me mark this PR with skip news? I think there is no news for JIT internals changes.
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@eendebakpt eendebakpt added the skip news label May 19, 2026
@waterlens
Clarify hot/cold JIT relocation handling
93c2179
@waterlens
waterlens marked this pull request as ready for review May 23, 2026 04:44
@waterlens
waterlens requested review from brandtbucher, diegorusso and savannahostrowski as code owners May 23, 2026 04:44
@bedevere-app bedevere-app Bot added the awaiting review label May 23, 2026
@markshannon

markshannon commented May 26, 2026

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I think we need to do hot/cold splitting in the code generator.
#143158 (comment)
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@github-actions

github-actions Bot commented Aug 25, 2026

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This PR is stale because it has been open for 90 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label Aug 25, 2026
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@brandtbucher brandtbucher Awaiting requested review from brandtbucher brandtbucher is a code owner
@savannahostrowski savannahostrowski Awaiting requested review from savannahostrowski
@diegorusso diegorusso Awaiting requested review from diegorusso diegorusso is a code owner

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3 participants

@waterlens @markshannon @eendebakpt
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