perf(transformer/tagged-template): add #[cold] hint to unlikely path

[overlookmotel]

Follow-on after #15834. Tiny perf optimization. Tagged template literals are pretty rare. Add #[cold] attr to transform_tagged_template to hint to compiler that the branch for "yes, it's a tagged template" in enter_expression is unlikely to be taken. This should make the compiler make "not a tagged template" the default path.

[overlookmotel]

• perf(transformer/tagged-template): add #[cold] hint to unlikely path #18034 👈 (View in Graphite)
• refactor(transformer/tagged-template): reformat comment #18033
• refactor(transformer/tagged-template): reorder functions #18032
• refactor(transformer/tagged-template): rename var #18031
• refactor(transformer/tagged-template): merge functions #18030
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing om/01-15-perf_transformer_tagged-template_add_cold_hint_to_unlikely_path (d6236f7) with om/01-15-refactor_transformer_tagged-template_reformat_comment (70541d6)}

Summary

✅ 38 untouched benchmarks
⏩ 7 skipped benchmarks¹

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Copilot]

Pull request overview

This PR adds a #[cold] attribute to the transform_tagged_template method to optimize performance by hinting to the compiler that tagged template expressions are rare. This allows the compiler to optimize the common path (non-tagged templates) in the enter_expression method.

Changes:

• Added #[cold] attribute to transform_tagged_template method with an explanatory comment

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[overlookmotel]

@Dunqing Apologies for the stack of nits. I just thought I'd split it into multiple PRs so it's easy to see what each change does. Hopefully we won't have to touch this transform again for a long time, so I thought might as well make these changes now, and get it done with!

[Dunqing]

Is it okay in a CSS-in-JS project? That would be common rather than rare, I think, but still less in terms of Expression.

[Dunqing]

@Dunqing Apologies for the stack of nits. I just thought I'd split it into multiple PRs so it's easy to see what each change does. Hopefully we won't have to touch this transform again for a long time, so I thought might as well make these changes now, and get it done with!

No worries!

[Dunqing]

Feel free to merge if you think it is good!

[overlookmotel]

Is it okay in a CSS-in-JS project? That would be common rather than rare, I think, but still less in terms of Expression.

I suspect that even in a CSS-in-JS project, other expressions like x = y, x == y, x || y, () => x will outnumber the tagged template literals. But anyway this PR only adds #[cold] attr - meaning "this branch is less likely than the other one". I didn't also add #[inline(never)] which is the one which says "this branch is really rare".

#[cold] attr is really unergonomic. I wish Rust would get on with making likely() and unlikely() stable. They're much easier to use and the meaning is clearer. But only on nightly! https://doc.rust-lang.org/nightly/std/hint/fn.unlikely.html

[overlookmotel]

Merge activity

• Jan 15, 1:44 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jan 15, 1:44 PM UTC: overlookmotel added this pull request to the Graphite merge queue.
• Jan 15, 1:55 PM UTC: Merged by the Graphite merge queue.

[overlookmotel]

Feel free to merge if you think it is good!

Well to be honest I don't think it makes much difference either way. But I don't think it's bad, so merging.

[Dunqing]

#[cold] attr is really unergonomic. I wish Rust would get on with making likely() and unlikely() stable. They're much easier to use and the meaning is clearer. But only on nightly! https://doc.rust-lang.org/nightly/std/hint/fn.unlikely.html

TIL, thank you!