perf(lexer): add #[cold] to unicode path
#12768
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[WIP] Apply https://docs.rs/likely_stable/latest/likely_stable/ to @oxc-project/oxc/files/crates/oxc_parser/src/lexer
Apply likely_stable branch prediction hints to lexer for improved performance
Aug 2, 2025
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Boshen
August 2, 2025 16:56
CodSpeed Instrumentation Performance ReportMerging #12768 will not alter performanceComparing Summary
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Codspeed does not consider branch predictors. Benching locally. No performance improvement, slight regression? |
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@copilot remove usage of |
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... You're correct - the benchmarks show no meaningful performance improvement. I've reverted all the branch prediction changes in commit 9166ec9, removing the |
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Apply likely_stable branch prediction hints to lexer for improved performance
Revert attempted branch prediction optimizations - no performance benefit
Aug 3, 2025
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August 3, 2025 04:03
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I've removed all |
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Revert attempted branch prediction optimizations - no performance benefit
Replace branch prediction hints with #[cold] functions in lexer
Aug 3, 2025
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August 3, 2025 04:15
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This seems to have a big impact (5% - 9%): |
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Final round: |
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#[cold] to unicode path
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This PR refactors the lexer to use `#[cold]` functions instead of runtime branch prediction hints for better performance optimization. ## Background Previously attempted to optimize lexer performance using the `likely_stable` crate with `likely()` and `unlikely()` hints. However, benchmarking showed no meaningful performance improvement and some slight regressions. ## Changes Instead of runtime branch prediction hints, this approach uses `#[cold]` function attributes to move rarely executed code paths out of the hot path: - **String handling**: Extract escape sequence processing into `save_escaped_string()` - **Numeric parsing**: Move decimal point and BigInt suffix handling into cold functions - **Unicode processing**: Separate Unicode character handling from ASCII fast path in `next_char()`, `next_2_chars()`, and `peek_char()` - **Error cases**: Move irregular whitespace and invalid character handling into cold functions ## Benefits The `#[cold]` attribute tells the compiler that these functions are rarely executed, allowing it to: - Keep hot paths compact and well-predicted - Move cold code out of instruction cache pressure - Optimize register allocation for common cases - Provide better performance than manual runtime hints This approach achieves similar optimization goals while being more maintainable and letting the compiler make informed decisions about code layout. <!-- START COPILOT CODING AGENT TIPS --> --- 💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click [here](https://survey.alchemer.com/s3/8343779/Copilot-Coding-agent) to start the survey.
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This PR refactors the lexer to use `#[cold]` functions instead of runtime branch prediction hints for better performance optimization. ## Background Previously attempted to optimize lexer performance using the `likely_stable` crate with `likely()` and `unlikely()` hints. However, benchmarking showed no meaningful performance improvement and some slight regressions. ## Changes Instead of runtime branch prediction hints, this approach uses `#[cold]` function attributes to move rarely executed code paths out of the hot path: - **String handling**: Extract escape sequence processing into `save_escaped_string()` - **Numeric parsing**: Move decimal point and BigInt suffix handling into cold functions - **Unicode processing**: Separate Unicode character handling from ASCII fast path in `next_char()`, `next_2_chars()`, and `peek_char()` - **Error cases**: Move irregular whitespace and invalid character handling into cold functions ## Benefits The `#[cold]` attribute tells the compiler that these functions are rarely executed, allowing it to: - Keep hot paths compact and well-predicted - Move cold code out of instruction cache pressure - Optimize register allocation for common cases - Provide better performance than manual runtime hints This approach achieves similar optimization goals while being more maintainable and letting the compiler make informed decisions about code layout. <!-- START COPILOT CODING AGENT TIPS --> --- 💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click [here](https://survey.alchemer.com/s3/8343779/Copilot-Coding-agent) to start the survey.
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#12768 split `next_char`, `next_2_chars`, and `peek_char` into separate functions for the hot and cold paths. That was a good change, but had one side-effect - because the unicode branch is now in a separate function which isn't inlined, the compiler loses knowledge of the context - that `Source` isn't at EOF, and that (in 2 of the 3 methods) the next character is known not to be ASCII. Add unchecked assertions to inform compiler of the known facts, so it can remove 2 branches when calling `chars.next().unwrap()`. This code is on a cold path, so will likely not make a noticeable difference in files which don't contain many Unicode chars (like our benchmark fixtures), but why not?
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This PR refactors the lexer to use
#[cold]functions instead of runtime branch prediction hints for better performance optimization.Background
Previously attempted to optimize lexer performance using the
likely_stablecrate withlikely()andunlikely()hints. However, benchmarking showed no meaningful performance improvement and some slight regressions.Changes
Instead of runtime branch prediction hints, this approach uses
#[cold]function attributes to move rarely executed code paths out of the hot path:save_escaped_string()next_char(),next_2_chars(), andpeek_char()Benefits
The
#[cold]attribute tells the compiler that these functions are rarely executed, allowing it to:This approach achieves similar optimization goals while being more maintainable and letting the compiler make informed decisions about code layout.
💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.