Commits on Nov 27, 2020

1. Implement faster pointer rebasing. …

   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
   apple/swift#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.

   

   Lukasa committed Nov 27, 2020
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