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Deoptimization:
How YJIT Speeds Up
Ruby by Slowing Down
@k0kubun / RubyKaigi 2025
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@k0kubun
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@k0kubun
Z
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We’re hiring
https://candidate.shopify.com/form/ruby-kaigi-2025
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YJIT
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https://railsatscale.com/2025-01-10-yjit-3-4-even-faster-and-more-memory-ef
fi
cient/
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Deoptimization
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Deoptimization
What if you can “slow down” Ruby at any time?
You can speculatively optimize Ruby and invalidate it later
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Playing with Deoptimization
Build Ruby with:
./con
fi
gure --enable-yjit=dev
Run Ruby with:
ruby --yjit-call-threshold=1 --yjit-dump-disasm
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Playing with Deoptimization
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YJIT Deoptimization
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Code Patching
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Rewrite
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Rewrite
Jump
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Global Invalidation
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Ruby 3.4’s New
YJIT Deoptimization
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Invalidation on
Escaped Locals
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?
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Invalidation on
Singleton Classes
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Singleton Class Invalidation
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Lazy Frame Push
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Lazy Frame Push
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YJIT-only Ruby Methods
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Array#each
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Primitive.attr! :c_trace
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Conclusion
Deoptimization enables speculative optimizations with
lazy invalidation
Ruby 3.4 YJIT optimizes method calls and local variables
using deoptimization