pjfanning opened a new pull request, #3430:
URL: https://github.com/apache/pekko/pull/3430
The ByteString2 class (2-fragment ByteString) had cross-boundary read
methods that used 8 individual byteAt calls for Long reads. Each byteAt call
has a branch (check which fragment). The optimization:
- Long reads (BE/LE): Build two sub-long values from byteAtUnchecked calls
(which skip the bounds check branch) and combine them with bitwise OR. This
reduces the branch overhead from 8 per Long to 8 virtual calls (one per byte,
no extra branching per call) plus pure arithmetic assembly.
- Short/Int reads: Kept using byteAt via SWARUtil helper methods. With only
2-4 bytes needed, the branch overhead is minimal and not worth adding
complexity.
Why not direct array access: The first/second fields are typed as ByteString
(trait), and .bytes/.startIndex are private[pekko] on the concrete subclasses
but not accessible from the nested ByteString2 class context. byteAtUnchecked
provides the same bounds-check-skipping benefit through virtual dispatch.
Files changed:
- ByteStrings.iterator: LazyList → List (eliminates per-element thunk
allocation)
- ByteStrings.slice: Direct fragment collection instead of drop+dropRight
- ByteStrings.apply: Binary search instead of linear scan
- ByteStrings.map: Uses byteAtUnchecked
- ByteString2.readLong{BE,LE}Unchecked: byteAtUnchecked-based assembly
instead of individual byteAt calls
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