arm64: Use optimized memcmp

Patch written by Wilco Dijkstra submitted for review to newlib:
https://sourceware.org/ml/newlib/2017/msg00524.html



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.

2017-06-28  Wilco Dijkstra  <wdijkstr@arm.com>

        * bionic/libc/arch-arm64/generic/bionic/memcmp.S (memcmp):
                Rewrite of optimized memcmp.

GLIBC benchtests/bench-memcmp.c performance comparison for Cortex-A53:

Length    1, alignment  1/ 1:        153%
Length    1, alignment  1/ 1:        119%
Length    1, alignment  1/ 1:        154%
Length    2, alignment  2/ 2:        121%
Length    2, alignment  2/ 2:        140%
Length    2, alignment  2/ 2:        121%
Length    3, alignment  3/ 3:        105%
Length    3, alignment  3/ 3:        105%
Length    3, alignment  3/ 3:        105%
Length    4, alignment  4/ 4:        155%
Length    4, alignment  4/ 4:        154%
Length    4, alignment  4/ 4:        161%
Length    5, alignment  5/ 5:        173%
Length    5, alignment  5/ 5:        173%
Length    5, alignment  5/ 5:        173%
Length    6, alignment  6/ 6:        145%
Length    6, alignment  6/ 6:        145%
Length    6, alignment  6/ 6:        145%
Length    7, alignment  7/ 7:        125%
Length    7, alignment  7/ 7:        125%
Length    7, alignment  7/ 7:        125%
Length    8, alignment  8/ 8:        111%
Length    8, alignment  8/ 8:        130%
Length    8, alignment  8/ 8:        124%
Length    9, alignment  9/ 9:        160%
Length    9, alignment  9/ 9:        160%
Length    9, alignment  9/ 9:        150%
Length   10, alignment 10/10:        170%
Length   10, alignment 10/10:        137%
Length   10, alignment 10/10:        150%
Length   11, alignment 11/11:        160%
Length   11, alignment 11/11:        160%
Length   11, alignment 11/11:        160%
Length   12, alignment 12/12:        146%
Length   12, alignment 12/12:        168%
Length   12, alignment 12/12:        156%
Length   13, alignment 13/13:        167%
Length   13, alignment 13/13:        167%
Length   13, alignment 13/13:        173%
Length   14, alignment 14/14:        167%
Length   14, alignment 14/14:        168%
Length   14, alignment 14/14:        168%
Length   15, alignment 15/15:        168%
Length   15, alignment 15/15:        173%
Length   15, alignment 15/15:        173%
Length    1, alignment  0/ 0:        134%
Length    1, alignment  0/ 0:        127%
Length    1, alignment  0/ 0:        119%
Length    2, alignment  0/ 0:        94%
Length    2, alignment  0/ 0:        94%
Length    2, alignment  0/ 0:        106%
Length    3, alignment  0/ 0:        82%
Length    3, alignment  0/ 0:        87%
Length    3, alignment  0/ 0:        82%
Length    4, alignment  0/ 0:        115%
Length    4, alignment  0/ 0:        115%
Length    4, alignment  0/ 0:        122%
Length    5, alignment  0/ 0:        127%
Length    5, alignment  0/ 0:        119%
Length    5, alignment  0/ 0:        127%
Length    6, alignment  0/ 0:        103%
Length    6, alignment  0/ 0:        100%
Length    6, alignment  0/ 0:        100%
Length    7, alignment  0/ 0:        82%
Length    7, alignment  0/ 0:        91%
Length    7, alignment  0/ 0:        87%
Length    8, alignment  0/ 0:        111%
Length    8, alignment  0/ 0:        124%
Length    8, alignment  0/ 0:        124%
Length    9, alignment  0/ 0:        136%
Length    9, alignment  0/ 0:        136%
Length    9, alignment  0/ 0:        136%
Length   10, alignment  0/ 0:        136%
Length   10, alignment  0/ 0:        135%
Length   10, alignment  0/ 0:        136%
Length   11, alignment  0/ 0:        136%
Length   11, alignment  0/ 0:        136%
Length   11, alignment  0/ 0:        135%
Length   12, alignment  0/ 0:        136%
Length   12, alignment  0/ 0:        136%
Length   12, alignment  0/ 0:        136%
Length   13, alignment  0/ 0:        135%
Length   13, alignment  0/ 0:        136%
Length   13, alignment  0/ 0:        136%
Length   14, alignment  0/ 0:        136%
Length   14, alignment  0/ 0:        136%
Length   14, alignment  0/ 0:        136%
Length   15, alignment  0/ 0:        136%
Length   15, alignment  0/ 0:        136%
Length   15, alignment  0/ 0:        136%
Length    4, alignment  0/ 0:        115%
Length    4, alignment  0/ 0:        115%
Length    4, alignment  0/ 0:        115%
Length   32, alignment  0/ 0:        127%
Length   32, alignment  7/ 2:        395%
Length   32, alignment  0/ 0:        127%
Length   32, alignment  0/ 0:        127%
Length    8, alignment  0/ 0:        111%
Length    8, alignment  0/ 0:        124%
Length    8, alignment  0/ 0:        124%
Length   64, alignment  0/ 0:        128%
Length   64, alignment  6/ 4:        475%
Length   64, alignment  0/ 0:        131%
Length   64, alignment  0/ 0:        134%
Length   16, alignment  0/ 0:        128%
Length   16, alignment  0/ 0:        119%
Length   16, alignment  0/ 0:        128%
Length  128, alignment  0/ 0:        129%
Length  128, alignment  5/ 6:        475%
Length  128, alignment  0/ 0:        130%
Length  128, alignment  0/ 0:        129%
Length   32, alignment  0/ 0:        126%
Length   32, alignment  0/ 0:        126%
Length   32, alignment  0/ 0:        126%
Length  256, alignment  0/ 0:        127%
Length  256, alignment  4/ 8:        545%
Length  256, alignment  0/ 0:        126%
Length  256, alignment  0/ 0:        128%
Length   64, alignment  0/ 0:        171%
Length   64, alignment  0/ 0:        171%
Length   64, alignment  0/ 0:        174%
Length  512, alignment  0/ 0:        126%
Length  512, alignment  3/10:        585%
Length  512, alignment  0/ 0:        126%
Length  512, alignment  0/ 0:        127%
Length  128, alignment  0/ 0:        129%
Length  128, alignment  0/ 0:        128%
Length  128, alignment  0/ 0:        129%
Length 1024, alignment  0/ 0:        125%
Length 1024, alignment  2/12:        611%
Length 1024, alignment  0/ 0:        126%
Length 1024, alignment  0/ 0:        126%
Length  256, alignment  0/ 0:        128%
Length  256, alignment  0/ 0:        127%
Length  256, alignment  0/ 0:        128%
Length 2048, alignment  0/ 0:        125%
Length 2048, alignment  1/14:        625%
Length 2048, alignment  0/ 0:        125%
Length 2048, alignment  0/ 0:        125%
Length  512, alignment  0/ 0:        126%
Length  512, alignment  0/ 0:        127%
Length  512, alignment  0/ 0:        127%
Length 4096, alignment  0/ 0:        125%
Length 4096, alignment  0/16:        125%
Length 4096, alignment  0/ 0:        125%
Length 4096, alignment  0/ 0:        125%
Length 1024, alignment  0/ 0:        126%
Length 1024, alignment  0/ 0:        126%
Length 1024, alignment  0/ 0:        126%
Length 8192, alignment  0/ 0:        125%
Length 8192, alignment 63/18:        636%
Length 8192, alignment  0/ 0:        125%
Length 8192, alignment  0/ 0:        125%
Length   16, alignment  1/ 2:        317%
Length   16, alignment  1/ 2:        317%
Length   16, alignment  1/ 2:        317%
Length   32, alignment  2/ 4:        395%
Length   32, alignment  2/ 4:        395%
Length   32, alignment  2/ 4:        398%
Length   64, alignment  3/ 6:        475%
Length   64, alignment  3/ 6:        475%
Length   64, alignment  3/ 6:        477%
Length  128, alignment  4/ 8:        479%
Length  128, alignment  4/ 8:        479%
Length  128, alignment  4/ 8:        479%
Length  256, alignment  5/10:        543%
Length  256, alignment  5/10:        539%
Length  256, alignment  5/10:        543%
Length  512, alignment  6/12:        585%
Length  512, alignment  6/12:        585%
Length  512, alignment  6/12:        585%
Length 1024, alignment  7/14:        611%
Length 1024, alignment  7/14:        611%
Length 1024, alignment  7/14:        611%

Signed-off-by: Joe Maples <joe@frap129.org>