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gh-143158: Hot cold code splitting for JIT compiler - #149292

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waterlens:hot-cold-splitting
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gh-143158: Hot cold code splitting for JIT compiler#149292
waterlens wants to merge 6 commits into
python:mainfrom
waterlens:hot-cold-splitting

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@waterlens

@waterlenswaterlens 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-appBot 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-botBot commented May 2, 2026•
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All commit authors signed the Contributor License Agreement.

CLA signed

@bedevere-appbedevere-appBot mentioned this pull request May 2, 2026
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read-the-docs-communityBot 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

-Deleted

@bedevere-app

bedevere-appBot 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.

@waterlens
waterlensforce-pushed the hot-cold-splitting branch from d6ce2a1 to c180cc0CompareMay 11, 2026 15:31
@bedevere-app

bedevere-appBot 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.

@waterlens
waterlensforce-pushed the hot-cold-splitting branch from c180cc0 to f56e969CompareMay 18, 2026 14:40
@bedevere-app

bedevere-appBot 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.

@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-appBot 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.

@bedevere-app

bedevere-appBot 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.

@bedevere-app

bedevere-appBot 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.

@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.

@waterlens
waterlens marked this pull request as ready for review May 23, 2026 04:44
@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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@brandtbucherbrandtbucherAwaiting requested review from brandtbucherbrandtbucher is a code owner
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