perf(codegen): further reduce memory allocations in generate_line_offset_tables

[overlookmotel]

#13054 added a nice optimization to SourcemapBuilder. During generation of line/offset tables, it reuses a single Vec for column indexes for each line, rather than creating a new Vec on each turn of the inner loop. This reduces the number of times that Vec may have to grow as column indexes get added to it.

Take this optimization a step further by re-using the same Vec across all lines.

columns Vec is not consumed on each line, but each time the contents are copied into a boxed slice - except when reaching EOF, where we can consume columns, as its work is done.

This memory-copying was likely happening anyway, as Vec<u32> -> Box<[u32]> conversion has to drop the spare capacity of the Vec, which will likely cause a reallocation.

Also, avoid using iterators to create the boxed slices. Vec::clone followed by Vec::into_boxed_slice is a bit more explicit and so may help compiler to see that it only needs to allocate exactly columns.len() slots for the Box<[u32]>.

Note: I also tried columns.drain(..).collect() instead of columns.clone().into_boxed_slice() + columns.clear(). But it looks like the Drain abstraction doesn't get completely removed by compiler. https://godbolt.org/z/Trv47j4hP So I think into_boxed_slice is probably preferable.

[overlookmotel]

• perf(codegen): further reduce memory allocations in generate_line_offset_tables #13056 👈 (View in Graphite)
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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #13056 will not alter performance

_{Comparing 08-13-perf_codegen_further_reduce_memory_allocations_in_generate_line_offset_tables_ (1385c71) with main (b0558a4)¹}

Summary

✅ 34 untouched benchmarks

Footnotes

1. No successful run was found on main (1385c71) during the generation of this report, so b0558a4 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

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[Boshen]

Merge activity

• Aug 13, 3:35 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Aug 13, 3:35 PM UTC: Boshen added this pull request to the Graphite merge queue.
• Aug 13, 3:44 PM UTC: Merged by the Graphite merge queue.