Motivation:
I recently discovered that UnsafeRawBufferPointer.init(rebasing:) is
surprisingly expensive, with 7 traps and 11 branches. A simple
replacement can make it a lot cheaper, down to two traps and four
branches. This ends up having pretty drastic effects on
ByteBuffer-heavy NIO code, which often outlines the call to that
initializer and loses the ability to make a bunch of site-local
optimisations.
While this has been potentially fixed upstream with
https://github.com/apple/swift/pull/34879, there is no good reason to
wait until Swift 5.4 for this improvement.
Due to the niche use-case, I didn't bother doing this for _every_
rebasing in the program. In particular, there is at least one
UnsafeBufferPointer(rebasing:) that I didn't do this with, and there are
uses in both NIOTLS and NIOHTTP1 that I didn't change. While we can fix
those if we really need to, it would be nice to avoid this helper
proliferating too far through our codebase.
Modifications:
- Replaced the use of URBP.init(rebasing:) with a custom hand-rolled
version that avoids Slice.count.
Result:
Cheaper code. One NIOHTTP2 benchmark sees a 2.9% speedup from this
change alone.
Cory Benfield