[Bug 1720832] Re: Optimized memcmp for arm64

[dann frazier]

** Changed in: glibc (Ubuntu)
     Assignee: (unassigned) => Adam Conrad (adconrad)

** Changed in: glibc (Ubuntu)
       Status: New => Confirmed

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Title:
  Optimized memcmp for arm64

Status in glibc package in Ubuntu:
  Confirmed

Bug description:
  A patch has recently landed upstream to optimize memcmp for AArch64:

  commit 922369032c604b4dcfd535e1bcddd4687e7126a5
  Author: Wilco Dijkstra <wdijkstr at arm.com>
  Date:   Thu Aug 10 17:00:38 2017 +0100

      [AArch64] Optimized memcmp.
      
      This is an optimized memcmp for AArch64.  This is a complete rewrite
      using a different algorithm.  The previous version split into cases
      where both inputs were aligned, the inputs were mutually aligned and
      unaligned using a byte loop.  The new version combines all these cases,
      while small inputs of less than 8 bytes are handled separately.
      
      This allows the main code to be sped up using unaligned loads since
      there are now at least 8 bytes to be compared.  After the first 8 bytes,
      align the first input.  This ensures each iteration does at most one
      unaligned access and mutually aligned inputs behave as aligned.
      After the main loop, process the last 8 bytes using unaligned accesses.
      
      This improves performance of (mutually) aligned cases by 25% and
      unaligned by >500% (yes >6 times faster) on large inputs.
      
              * sysdeps/aarch64/memcmp.S (memcmp):
              Rewrite of optimized memcmp.

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