perf(codegen): reduce allocations in SourcemapBuilder#13677
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CodSpeed Instrumentation Performance ReportMerging #13677 will not alter performanceComparing Summary
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Pull Request Overview
This PR optimizes memory allocation patterns in the SourcemapBuilder by reverting from mem::take back to clone with adaptive capacity management.
Key changes:
- Replaces
mem::takewithcloneto avoid unnecessary reallocations - Removes fixed capacity reservation in favor of adaptive growth
- Adds detailed comments explaining the memory allocation optimization rationale
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Follow-on after #13670. That PR made 2 changes: 1. Pre-allocating capacity in `columns` `Vec`. 2. `mem::take`-ing `columns` for each line, rather than re-using it. In my opinion, the 1st change is good, but the 2nd is not. Revert the usage of `mem::take`, and add a comment explaining why the `.clone()` is not as bad as it looks! Before this PR: `columns` will likely have spare capacity, so `mem::take(&mut columns).into_boxed_slice()` will perform a reallocation to drop the excess capacity, and then `columns.reserve(256)` performs a 2nd allocation. Approach after this PR: 1. `columns.clone().into_boxed_slice()` performs 1 allocation, and copies the data from `columns` into this new allocation. `columns.clear()` does not perform any reallocation, and is a very cheap operation. 2. `columns` `Vec` is reused over and over, and will grow adaptively depending on how heavy the file's use of unicode chars is, rather than always going back to estimated max capacity of 256. Benchmarks show a very small positive difference.
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#13670 optimized `SourcemapBuilder` to pre-reserve capacity in `columns` `Vec`. However, after #13677, `columns` will resize adaptively depending on how many unicode chars in source text. So now initial capacity of 256 (1 KiB) is probably excessive for most cases. Reduce it to 16 (64 bytes, which is 1 x CPU cache line). Codspeed shows little change in perf (max +0.1%, min -0.1%), and memory usage will definitely be reduced.
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Follow-on after #13670.
That PR made 2 changes:
columnsVec.mem::take-ingcolumnsfor each line, rather than re-using it.In my opinion, the 1st change is good, but the 2nd is not.
Revert the usage of
mem::take, and add a comment explaining why the.clone()is not as bad as it looks!Before this PR:
columnswill likely have spare capacity, somem::take(&mut columns).into_boxed_slice()will perform a reallocation to drop the excess capacity, and thencolumns.reserve(256)performs a 2nd allocation.Approach after this PR:
columns.clone().into_boxed_slice()performs 1 allocation, and copies the data fromcolumnsinto this new allocation.columns.clear()does not perform any reallocation, and is a very cheap operation.columnsVecis reused over and over, and will grow adaptively depending on how heavy the file's use of unicode chars is, rather than always going back to estimated max capacity of 256.Benchmarks show a very small positive difference.