internal/bytealg: fix alignment code in compare_riscv64.s

internal/bytealg: fix alignment code in compare_riscv64.s

The riscv64 implementation of compare has an optimization that is
applied when both pointers share the same alignment but that alignment
is not 8 bytes.  In this case it tries to align both pointers to an 8 byte boundaries,
by individually comparing the first few bytes of each buffer.  Unfortunately,
the existing code is incorrect.  It adjusts the pointers by the wrong number
of bytes resulting, in most cases, in pointers that are not 8 byte aligned.

This commit fixes the issue by individually comparing the first
(8 - (pointer & 7)) bytes of each buffer rather than the first
(pointer & 7) bytes.

We also remove an unnecessary immediate MOV instruction.

This particular optimization is not covered by any of the existing
benchmarks so a new benchmark, benchmarkCompareBytesBigBothUnaligned,
is provided.  The benchmark tests the case where both pointers have
the same alignment but may not be 8 byte aligned.  Results of the
new benchmark along with some of the existing benchmarks generated on
a SiFive HiFive Unmatched A00 with 16GB of RAM running Ubuntu 23.04
are presented below.

CompareBytesEqual-4                           70.00n ±  6%   68.32n ±  0%   -2.40% (p=0.020 n=10)
CompareBytesToNil-4                           19.31n ±  0%   18.47n ±  0%   -4.35% (p=0.000 n=10)
CompareBytesEmpty-4                           16.79n ±  0%   15.95n ±  0%   -4.97% (p=0.000 n=10)
CompareBytesIdentical-4                       19.94n ± 15%   18.32n ± 13%   -8.15% (p=0.040 n=10)
CompareBytesSameLength-4                      37.93n ±  0%   42.44n ±  1%  +11.91% (p=0.000 n=10)
CompareBytesDifferentLength-4                 37.93n ±  0%   42.44n ±  0%  +11.89% (p=0.000 n=10)
CompareBytesBigUnaligned/offset=1-4           3.881m ± 14%   3.880m ± 15%        ~ (p=0.436 n=10)
CompareBytesBigUnaligned/offset=2-4           3.884m ±  0%   3.875m ±  0%        ~ (p=0.190 n=10)
CompareBytesBigUnaligned/offset=3-4           3.858m ±  1%   3.868m ±  1%        ~ (p=0.105 n=10)
CompareBytesBigUnaligned/offset=4-4           3.877m ±  1%   3.876m ±  0%        ~ (p=0.529 n=10)
CompareBytesBigUnaligned/offset=5-4           3.859m ±  0%   3.874m ±  0%   +0.39% (p=0.009 n=10)
CompareBytesBigUnaligned/offset=6-4           3.878m ±  1%   3.876m ±  0%        ~ (p=0.353 n=10)
CompareBytesBigUnaligned/offset=7-4           3.868m ±  1%   3.877m ±  0%        ~ (p=0.190 n=10)
CompareBytesBigBothUnaligned/offset=0-4       1.586m ±  0%   1.765m ±  0%  +11.30% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=1-4     153.132m ±  1%   1.765m ±  1%  -98.85% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=2-4     152.930m ±  1%   1.765m ±  1%  -98.85% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=3-4     152.093m ±  1%   1.769m ±  0%  -98.84% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=4-4       1.602m ±  0%   1.764m ±  0%  +10.11% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=5-4     152.314m ±  1%   1.768m ±  0%  -98.84% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=6-4     152.905m ±  1%   1.764m ±  1%  -98.85% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=7-4     152.951m ±  1%   1.804m ±  2%  -98.82% (p=0.000 n=10)
CompareBytesBig-4                             1.441m ± 21%   1.373m ± 55%        ~ (p=0.481 n=10)
CompareBytesBigIdentical-4                    19.94n ±  1%   19.10n ±  0%   -4.21% (p=0.001 n=10)
geomean                                       243.7µ         76.65µ        -68.54%

CompareBytesBigUnaligned/offset=1-4          257.7Mi ± 12%     257.7Mi ± 13%          ~ (p=0.424 n=10)
CompareBytesBigUnaligned/offset=2-4          257.5Mi ±  0%     258.1Mi ±  0%          ~ (p=0.190 n=10)
CompareBytesBigUnaligned/offset=3-4          259.2Mi ±  1%     258.5Mi ±  1%          ~ (p=0.105 n=10)
CompareBytesBigUnaligned/offset=4-4          257.9Mi ±  1%     258.0Mi ±  0%          ~ (p=0.529 n=10)
CompareBytesBigUnaligned/offset=5-4          259.1Mi ±  0%     258.1Mi ±  0%     -0.39% (p=0.008 n=10)
CompareBytesBigUnaligned/offset=6-4          257.9Mi ±  1%     258.0Mi ±  0%          ~ (p=0.353 n=10)
CompareBytesBigUnaligned/offset=7-4          258.5Mi ±  1%     257.9Mi ±  0%          ~ (p=0.190 n=10)
CompareBytesBigBothUnaligned/offset=0-4      630.6Mi ±  0%     566.6Mi ±  0%    -10.15% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=1-4      6.533Mi ±  1%   566.545Mi ±  1%  +8572.48% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=2-4      6.537Mi ±  1%   566.683Mi ±  1%  +8568.27% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=3-4      6.576Mi ±  1%   565.200Mi ±  0%  +8495.43% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=4-4      624.2Mi ±  0%     566.9Mi ±  0%     -9.18% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=5-4      6.566Mi ±  1%   565.758Mi ±  0%  +8516.41% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=6-4      6.542Mi ±  1%   567.036Mi ±  1%  +8567.35% (p=0.000 n=10)
CompareBytesBigBothUnaligned/offset=7-4      6.542Mi ±  1%   554.390Mi ±  2%  +8374.05% (p=0.000 n=10)
CompareBytesBig-4                            694.2Mi ± 18%     728.1Mi ± 35%          ~ (p=0.481 n=10)
CompareBytesBigIdentical-4                   47.83Ti ±  1%     49.92Ti ±  0%     +4.39% (p=0.002 n=10)
geomean                                      170.0Mi           813.8Mi         +378.66%

Change-Id: I0a2d0386d5ca1ffa249682a12ebd1533508e31e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/497838
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: M Zhuo <mzh@golangcn.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: M Zhuo <mzh@golangcn.org>