internal/bytealg: optimize IndexByte for riscv64

internal/bytealg: optimize IndexByte for riscv64

The existing implementations of IndexByte and IndexByteString for
riscv64 are very simplistic.  They load and compare a single byte at
a time in a tight loop.  It's possible to improve performance in the
general case by loading and checking 8 bytes at a time.  This is
achieved using the 'Determine if a word has a byte equal to n' bit
hack from https://graphics.stanford.edu/~seander/bithacks.html.

We broadcast the byte we're looking for across a 64 bit register,
let v be the result of xoring that register with 8 bytes loaded
from the buffer and then use the formula,

(((v) - 0x0101010101010101UL) & ~(v) & 0x8080808080808080UL)

which evaluates to true if any one of the bytes in v is 0, i.e,
matches the byte we're looking for.  We then just need to figure
out which byte out of the 8 it is to return the correct index.

This change generally improves performance when the byte we're
looking for is not in the first 24 bytes of the buffer and degrades
performance slightly when it is.

Some example benchmarks results from the bytes and strings package
are presented below.  These were generated on a VisionFive2 running
Ubuntu 24.04.

Subset of bytes Index benchmarks

IndexByte/10                   46.49n ± 0%   44.08n ± 0%   -5.19% (p=0.000 n=10)
IndexByte/32                   75.98n ± 0%   67.90n ± 0%  -10.63% (p=0.000 n=10)
IndexByte/4K                   5.512µ ± 0%   2.113µ ± 0%  -61.67% (p=0.000 n=10)
IndexByte/4M                   7.354m ± 0%   3.218m ± 0%  -56.24% (p=0.000 n=10)
IndexByte/64M                  90.15m ± 0%   33.86m ± 0%  -62.44% (p=0.000 n=10)
IndexBytePortable/10           50.41n ± 0%   54.92n ± 1%   +8.94% (p=0.000 n=10)
IndexBytePortable/32           111.9n ± 0%   115.5n ± 0%   +3.22% (p=0.000 n=10)
IndexBytePortable/4K           10.99µ ± 0%   10.99µ ± 0%   +0.04% (p=0.000 n=10)
IndexBytePortable/4M           11.24m ± 0%   11.24m ± 0%        ~ (p=0.218 n=10)
IndexBytePortable/64M          179.8m ± 0%   179.8m ± 0%   +0.01% (p=0.001 n=10)
IndexRune/10                   104.2n ± 0%   104.4n ± 0%   +0.19% (p=0.000 n=10)
IndexRune/32                   133.7n ± 0%   139.3n ± 0%   +4.23% (p=0.000 n=10)
IndexRune/4K                   5.573µ ± 0%   2.184µ ± 0%  -60.81% (p=0.000 n=10)
IndexRune/4M                   5.634m ± 0%   2.112m ± 0%  -62.51% (p=0.000 n=10)
IndexRune/64M                  90.19m ± 0%   33.87m ± 0%  -62.45% (p=0.000 n=10)
IndexRuneASCII/10              50.42n ± 2%   47.14n ± 0%   -6.52% (p=0.000 n=10)
IndexRuneASCII/32              79.64n ± 1%   70.39n ± 0%  -11.61% (p=0.000 n=10)
IndexRuneASCII/4K              5.516µ ± 0%   2.115µ ± 0%  -61.66% (p=0.000 n=10)
IndexRuneASCII/4M              5.634m ± 0%   2.112m ± 0%  -62.51% (p=0.000 n=10)
IndexRuneASCII/64M             90.16m ± 0%   33.86m ± 0%  -62.44% (p=0.000 n=10)
IndexRuneUnicode/Latin/10      82.14n ± 0%   82.07n ± 0%   -0.09% (p=0.000 n=10)
IndexRuneUnicode/Latin/32      111.6n ± 0%   117.1n ± 0%   +4.93% (p=0.000 n=10)
IndexRuneUnicode/Latin/4K      6.222µ ± 0%   3.429µ ± 0%  -44.89% (p=0.000 n=10)
IndexRuneUnicode/Latin/4M      8.189m ± 0%   4.706m ± 0%  -42.53% (p=0.000 n=10)
IndexRuneUnicode/Latin/64M     171.8m ± 2%   105.8m ± 0%  -38.44% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/10   89.69n ± 0%   89.67n ± 0%   -0.02% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/32   119.1n ± 0%   124.1n ± 0%   +4.20% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4K   8.002µ ± 0%   6.232µ ± 0%  -22.12% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4M   9.501m ± 0%   7.510m ± 0%  -20.95% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/64M  186.5m ± 0%   150.3m ± 0%  -19.41% (p=0.000 n=10)
IndexRuneUnicode/Han/10        117.8n ± 0%   118.1n ± 0%   +0.25% (p=0.000 n=10)
IndexRuneUnicode/Han/32        151.5n ± 0%   154.0n ± 0%   +1.65% (p=0.000 n=10)
IndexRuneUnicode/Han/4K        6.664µ ± 0%   4.125µ ± 0%  -38.11% (p=0.000 n=10)
IndexRuneUnicode/Han/4M        8.526m ± 0%   5.502m ± 0%  -35.46% (p=0.000 n=10)
IndexRuneUnicode/Han/64M       171.8m ± 1%   112.2m ± 0%  -34.68% (p=0.000 n=10)
Index/10                       199.3n ± 1%   199.4n ± 0%        ~ (p=1.000 n=10)
Index/32                       547.7n ± 0%   547.3n ± 0%   -0.08% (p=0.001 n=10)
Index/4K                       38.62µ ± 0%   38.62µ ± 0%   -0.01% (p=0.023 n=10)
Index/4M                       40.46m ± 0%   40.45m ± 0%        ~ (p=0.105 n=10)
Index/64M                      648.5m ± 0%   648.4m ± 0%        ~ (p=1.000 n=10)
IndexEasy/10                   70.25n ± 0%   70.92n ± 0%   +0.95% (p=0.000 n=10)
IndexEasy/32                  104.60n ± 0%   95.67n ± 0%   -8.54% (p=0.000 n=10)
IndexEasy/4K                   5.544µ ± 0%   2.142µ ± 0%  -61.36% (p=0.000 n=10)
IndexEasy/4M                   7.354m ± 0%   3.213m ± 0%  -56.32% (p=0.000 n=10)
IndexEasy/64M                 114.93m ± 2%   52.61m ± 0%  -54.22% (p=0.000 n=10)
IndexHard1                     10.09m ± 0%   10.09m ± 0%        ~ (p=0.393 n=10)
IndexHard2                     10.09m ± 0%   10.09m ± 0%        ~ (p=0.481 n=10)
IndexHard3                     10.09m ± 0%   10.09m ± 0%        ~ (p=1.000 n=10)
IndexHard4                     10.09m ± 0%   10.09m ± 0%        ~ (p=0.739 n=10)
LastIndexHard1                 10.71m ± 0%   10.71m ± 0%        ~ (p=0.052 n=10)
LastIndexHard2                 10.71m ± 0%   10.71m ± 0%        ~ (p=0.218 n=10)
LastIndexHard3                 10.71m ± 0%   10.71m ± 0%        ~ (p=0.739 n=10)
IndexAnyASCII/1:1              30.13n ± 0%   30.79n ± 0%   +2.19% (p=0.000 n=10)
IndexAnyASCII/1:2              31.49n ± 0%   32.16n ± 0%   +2.13% (p=0.000 n=10)
IndexAnyASCII/1:4              34.16n ± 0%   34.82n ± 0%   +1.93% (p=0.000 n=10)
IndexAnyASCII/1:8              39.50n ± 0%   40.16n ± 0%   +1.67% (p=0.000 n=10)
IndexAnyASCII/1:16             50.20n ± 0%   50.87n ± 0%   +1.33% (p=0.000 n=10)
IndexAnyASCII/1:32             81.04n ± 0%   50.29n ± 0%  -37.94% (p=0.000 n=10)
IndexAnyASCII/1:64            119.80n ± 0%   66.94n ± 0%  -44.13% (p=0.000 n=10)
IndexAnyASCII/16:1             54.86n ± 0%   55.53n ± 0%   +1.22% (p=0.000 n=10)
IndexAnyASCII/16:2             268.2n ± 0%   268.2n ± 0%        ~ (p=1.000 n=10)
IndexAnyASCII/16:4             288.1n ± 0%   288.1n ± 0%        ~ (p=1.000 n=10) ¹
IndexAnyASCII/16:8             328.3n ± 0%   328.2n ± 0%        ~ (p=0.370 n=10)
IndexAnyASCII/16:16            413.4n ± 0%   413.4n ± 0%        ~ (p=0.628 n=10)
IndexAnyASCII/16:32            574.0n ± 0%   573.9n ± 0%        ~ (p=0.141 n=10)
IndexAnyASCII/16:64            895.1n ± 0%   895.1n ± 0%        ~ (p=0.548 n=10)
IndexAnyASCII/256:1            381.4n ± 0%   175.4n ± 0%  -53.99% (p=0.000 n=10)
IndexAnyASCII/256:2            2.998µ ± 0%   2.998µ ± 0%        ~ (p=0.365 n=10)
IndexAnyASCII/256:4            3.018µ ± 0%   3.018µ ± 0%        ~ (p=0.650 n=10)
IndexAnyASCII/256:8            3.058µ ± 0%   3.064µ ± 0%   +0.20% (p=0.011 n=10)
IndexAnyASCII/256:16           3.143µ ± 0%   3.150µ ± 0%   +0.21% (p=0.000 n=10)
IndexAnyASCII/256:32           3.303µ ± 0%   3.307µ ± 0%   +0.12% (p=0.000 n=10)
IndexAnyASCII/256:64           3.625µ ± 0%   3.638µ ± 0%   +0.36% (p=0.000 n=10)
IndexAnyUTF8/1:1               30.13n ± 0%   30.94n ± 0%   +2.69% (p=0.000 n=10)
IndexAnyUTF8/1:2               31.49n ± 0%   32.30n ± 0%   +2.59% (p=0.000 n=10)
IndexAnyUTF8/1:4               34.16n ± 0%   35.03n ± 0%   +2.55% (p=0.000 n=10)
IndexAnyUTF8/1:8               39.50n ± 0%   40.16n ± 0%   +1.67% (p=0.000 n=10)
IndexAnyUTF8/1:16              50.20n ± 0%   50.84n ± 0%   +1.27% (p=0.000 n=10)
IndexAnyUTF8/1:32              81.02n ± 0%   61.55n ± 0%  -24.03% (p=0.000 n=10)
IndexAnyUTF8/1:64             119.80n ± 0%   80.04n ± 0%  -33.19% (p=0.000 n=10)
IndexAnyUTF8/16:1              489.0n ± 0%   489.0n ± 0%        ~ (p=1.000 n=10)
IndexAnyUTF8/16:2              361.9n ± 0%   372.6n ± 0%   +2.96% (p=0.000 n=10)
IndexAnyUTF8/16:4              404.7n ± 0%   415.4n ± 0%   +2.64% (p=0.000 n=10)
IndexAnyUTF8/16:8              489.9n ± 0%   500.7n ± 0%   +2.20% (p=0.000 n=10)
IndexAnyUTF8/16:16             661.2n ± 0%   671.9n ± 0%   +1.62% (p=0.000 n=10)
IndexAnyUTF8/16:32            1004.0n ± 0%   881.6n ± 0%  -12.19% (p=0.000 n=10)
IndexAnyUTF8/16:64             1.767µ ± 0%   1.129µ ± 0%  -36.11% (p=0.000 n=10)
IndexAnyUTF8/256:1             7.072µ ± 0%   7.072µ ± 0%        ~ (p=0.387 n=10)
IndexAnyUTF8/256:2             4.700µ ± 0%   4.872µ ± 0%   +3.66% (p=0.000 n=10)
IndexAnyUTF8/256:4             5.386µ ± 0%   5.557µ ± 0%   +3.18% (p=0.000 n=10)
IndexAnyUTF8/256:8             6.752µ ± 0%   6.923µ ± 0%   +2.53% (p=0.000 n=10)
IndexAnyUTF8/256:16            9.493µ ± 0%   9.664µ ± 0%   +1.80% (p=0.000 n=10)
IndexAnyUTF8/256:32            14.97µ ± 0%   12.93µ ± 0%  -13.64% (p=0.000 n=10)
IndexAnyUTF8/256:64            27.15µ ± 0%   16.89µ ± 0%  -37.80% (p=0.000 n=10)
LastIndexAnyASCII/1:1          30.78n ± 0%   31.45n ± 0%   +2.18% (p=0.000 n=10)
LastIndexAnyASCII/1:2          32.13n ± 0%   32.80n ± 0%   +2.07% (p=0.000 n=10)
LastIndexAnyASCII/1:4          34.81n ± 0%   35.48n ± 0%   +1.92% (p=0.000 n=10)
LastIndexAnyASCII/1:8          40.14n ± 0%   40.81n ± 0%   +1.67% (p=0.000 n=10)
LastIndexAnyASCII/1:16         50.85n ± 0%   51.51n ± 0%   +1.30% (p=0.000 n=10)
LastIndexAnyASCII/1:32         84.03n ± 0%   50.85n ± 0%  -39.49% (p=0.000 n=10)
LastIndexAnyASCII/1:64        121.50n ± 0%   68.16n ± 0%  -43.90% (p=0.000 n=10)
LastIndexAnyASCII/16:1         249.7n ± 0%   249.7n ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/16:2         255.2n ± 0%   255.2n ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/16:4         274.0n ± 0%   274.0n ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/16:8         314.1n ± 0%   314.1n ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/16:16        403.8n ± 0%   403.8n ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/16:32        564.4n ± 0%   564.4n ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/16:64        885.5n ± 0%   885.5n ± 0%        ~ (p=0.474 n=10)
LastIndexAnyASCII/256:1        2.819µ ± 0%   2.819µ ± 0%        ~ (p=0.211 n=10)
LastIndexAnyASCII/256:2        2.824µ ± 0%   2.824µ ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/256:4        2.843µ ± 0%   2.843µ ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/256:8        2.883µ ± 0%   2.883µ ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/256:16       2.973µ ± 0%   2.973µ ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/256:32       3.133µ ± 0%   3.133µ ± 0%        ~ (p=0.628 n=10)
LastIndexAnyASCII/256:64       3.454µ ± 0%   3.454µ ± 0%        ~ (p=1.000 n=10)
LastIndexAnyUTF8/1:1           30.78n ± 0%   31.45n ± 0%   +2.18% (p=0.000 n=10)
LastIndexAnyUTF8/1:2           32.13n ± 0%   32.80n ± 0%   +2.07% (p=0.000 n=10)
LastIndexAnyUTF8/1:4           34.81n ± 0%   35.48n ± 0%   +1.92% (p=0.000 n=10)
LastIndexAnyUTF8/1:8           40.14n ± 0%   40.81n ± 0%   +1.67% (p=0.000 n=10)
LastIndexAnyUTF8/1:16          50.84n ± 0%   51.52n ± 0%   +1.33% (p=0.000 n=10)
LastIndexAnyUTF8/1:32          83.87n ± 0%   62.90n ± 0%  -25.00% (p=0.000 n=10)
LastIndexAnyUTF8/1:64         121.50n ± 0%   81.67n ± 0%  -32.78% (p=0.000 n=10)
LastIndexAnyUTF8/16:1          330.0n ± 0%   330.0n ± 0%        ~ (p=1.000 n=10)
LastIndexAnyUTF8/16:2          365.4n ± 1%   376.1n ± 0%   +2.93% (p=0.000 n=10)
LastIndexAnyUTF8/16:4          399.9n ± 0%   410.6n ± 0%   +2.68% (p=0.000 n=10)
LastIndexAnyUTF8/16:8          485.5n ± 0%   496.2n ± 0%   +2.20% (p=0.000 n=10)
LastIndexAnyUTF8/16:16         656.8n ± 0%   667.5n ± 0%   +1.63% (p=0.000 n=10)
LastIndexAnyUTF8/16:32         999.3n ± 0%   882.6n ± 0%  -11.68% (p=0.000 n=10)
LastIndexAnyUTF8/16:64         1.744µ ± 0%   1.129µ ± 0%  -35.26% (p=0.000 n=10)
LastIndexAnyUTF8/256:1         4.023µ ± 0%   4.023µ ± 0%    0.00% (p=0.033 n=10)
LastIndexAnyUTF8/256:2         4.645µ ± 0%   4.816µ ± 0%   +3.68% (p=0.000 n=10)
LastIndexAnyUTF8/256:4         5.217µ ± 0%   5.388µ ± 0%   +3.28% (p=0.000 n=10)
LastIndexAnyUTF8/256:8         6.587µ ± 0%   6.758µ ± 0%   +2.60% (p=0.000 n=10)
LastIndexAnyUTF8/256:16        9.327µ ± 0%   9.498µ ± 0%   +1.83% (p=0.000 n=10)
LastIndexAnyUTF8/256:32        14.81µ ± 0%   12.92µ ± 0%  -12.73% (p=0.000 n=10)
LastIndexAnyUTF8/256:64        26.69µ ± 0%   16.84µ ± 0%  -36.92% (p=0.000 n=10)
IndexPeriodic/IndexPeriodic2   625.6µ ± 0%   625.6µ ± 0%        ~ (p=0.529 n=10)
IndexPeriodic/IndexPeriodic4   625.5µ ± 0%   625.6µ ± 0%   +0.01% (p=0.002 n=10)
IndexPeriodic/IndexPeriodic8   625.4µ ± 0%   625.4µ ± 0%   +0.01% (p=0.001 n=10)
IndexPeriodic/IndexPeriodic16  236.5µ ± 0%   225.4µ ± 0%   -4.69% (p=0.000 n=10)
IndexPeriodic/IndexPeriodic32  171.1µ ± 3%   133.4µ ± 0%  -22.05% (p=0.000 n=10)
IndexPeriodic/IndexPeriodic64 139.10µ ± 3%   89.28µ ± 0%  -35.82% (p=0.000 n=10)
geomean                        4.222µ        3.628µ       -14.0

Subset of strings Index benchmarks

IndexRune                      110.7n ± 0%   117.7n ± 0%   +6.32% (p=0.000 n=10)
IndexRuneLongString            246.6n ± 0%   187.4n ± 3%  -24.01% (p=0.000 n=10)
IndexRuneFastPath              46.82n ± 0%   46.06n ± 0%   -1.62% (p=0.000 n=10)
Index                          48.28n ± 0%   47.61n ± 0%   -1.39% (p=0.000 n=10)
LastIndex                      34.50n ± 0%   34.50n ± 0%        ~ (p=1.000 n=10) ¹
IndexByte                      41.72n ± 0%   40.83n ± 0%   -2.13% (p=0.000 n=10)
IndexHard1                     10.01m ± 0%   10.01m ± 0%   +0.02% (p=0.000 n=10)
IndexHard2                     10.01m ± 0%   10.01m ± 0%   +0.02% (p=0.000 n=10)
IndexHard3                     10.01m ± 0%   10.01m ± 0%   +0.02% (p=0.000 n=10)
IndexHard4                     10.01m ± 0%   10.01m ± 0%   +0.02% (p=0.000 n=10)
LastIndexHard1                 10.71m ± 0%   10.71m ± 0%   +0.03% (p=0.000 n=10)
LastIndexHard2                 10.71m ± 0%   10.71m ± 0%   +0.03% (p=0.000 n=10)
LastIndexHard3                 10.71m ± 0%   10.71m ± 0%   +0.03% (p=0.000 n=10)
IndexTorture                   71.33µ ± 0%   71.37µ ± 0%   +0.05% (p=0.000 n=10)
IndexAnyASCII/1:1              34.40n ± 0%   35.07n ± 0%   +1.95% (p=0.000 n=10)
IndexAnyASCII/1:2              46.87n ± 0%   47.54n ± 0%   +1.43% (p=0.000 n=10)
IndexAnyASCII/1:4              49.53n ± 0%   50.20n ± 0%   +1.35% (p=0.000 n=10)
IndexAnyASCII/1:8              54.86n ± 0%   55.53n ± 0%   +1.22% (p=0.000 n=10)
IndexAnyASCII/1:16             65.56n ± 0%   66.24n ± 0%   +1.04% (p=0.000 n=10)
IndexAnyASCII/1:32             86.97n ± 0%   77.82n ± 0%  -10.52% (p=0.000 n=10)
IndexAnyASCII/1:64            134.50n ± 0%   98.57n ± 0%  -26.71% (p=0.000 n=10)
IndexAnyASCII/16:1             54.19n ± 0%   54.86n ± 0%   +1.24% (p=0.000 n=10)
IndexAnyASCII/16:2             257.4n ± 0%   256.7n ± 0%   -0.27% (p=0.000 n=10)
IndexAnyASCII/16:4             275.3n ± 0%   275.3n ± 0%        ~ (p=1.000 n=10)
IndexAnyASCII/16:8             315.4n ± 0%   315.5n ± 0%   +0.03% (p=0.001 n=10)
IndexAnyASCII/16:16            405.4n ± 0%   405.4n ± 0%        ~ (p=1.000 n=10)
IndexAnyASCII/16:32            566.0n ± 0%   566.0n ± 0%        ~ (p=1.000 n=10)
IndexAnyASCII/16:64            887.0n ± 0%   887.1n ± 0%        ~ (p=0.181 n=10)
IndexAnyASCII/256:1            380.0n ± 0%   174.7n ± 0%  -54.03% (p=0.000 n=10)
IndexAnyASCII/256:2            2.826µ ± 0%   2.826µ ± 0%        ~ (p=1.000 n=10) ¹
IndexAnyASCII/256:4            2.844µ ± 0%   2.844µ ± 0%        ~ (p=1.000 n=10) ¹
IndexAnyASCII/256:8            2.884µ ± 0%   2.884µ ± 0%        ~ (p=0.087 n=10)
IndexAnyASCII/256:16           2.974µ ± 0%   2.974µ ± 0%        ~ (p=1.000 n=10)
IndexAnyASCII/256:32           3.135µ ± 0%   3.135µ ± 0%        ~ (p=1.000 n=10)
IndexAnyASCII/256:64           3.456µ ± 0%   3.456µ ± 0%        ~ (p=1.000 n=10) ¹
IndexAnyUTF8/1:1               38.13n ± 0%   38.13n ± 0%        ~ (p=1.000 n=10) ¹
IndexAnyUTF8/1:2               46.87n ± 0%   47.54n ± 0%   +1.43% (p=0.000 n=10)
IndexAnyUTF8/1:4               49.53n ± 0%   50.19n ± 0%   +1.33% (p=0.000 n=10)
IndexAnyUTF8/1:8               54.86n ± 0%   55.52n ± 0%   +1.20% (p=0.000 n=10)
IndexAnyUTF8/1:16              65.56n ± 0%   66.23n ± 0%   +1.02% (p=0.000 n=10)
IndexAnyUTF8/1:32              86.97n ± 0%   82.25n ± 0%   -5.42% (p=0.000 n=10)
IndexAnyUTF8/1:64             134.50n ± 0%   99.96n ± 0%  -25.68% (p=0.000 n=10)
IndexAnyUTF8/16:1              98.34n ± 0%   98.34n ± 0%        ~ (p=1.000 n=10)
IndexAnyUTF8/16:2              462.7n ± 0%   473.7n ± 0%   +2.38% (p=0.000 n=10)
IndexAnyUTF8/16:4              504.6n ± 0%   515.3n ± 0%   +2.11% (p=0.000 n=10)
IndexAnyUTF8/16:8              589.1n ± 0%   599.7n ± 0%   +1.80% (p=0.000 n=10)
IndexAnyUTF8/16:16             760.4n ± 0%   770.9n ± 0%   +1.38% (p=0.000 n=10)
IndexAnyUTF8/16:32             1.103µ ± 0%   1.023µ ± 0%   -7.25% (p=0.000 n=10)
IndexAnyUTF8/16:64             1.857µ ± 0%   1.294µ ± 0%  -30.32% (p=0.000 n=10)
IndexAnyUTF8/256:1             1.066µ ± 0%   1.066µ ± 0%        ~ (p=1.000 n=10) ¹
IndexAnyUTF8/256:2             6.106µ ± 0%   6.277µ ± 0%   +2.81% (p=0.000 n=10)
IndexAnyUTF8/256:4             6.787µ ± 0%   6.958µ ± 0%   +2.52% (p=0.000 n=10)
IndexAnyUTF8/256:8             8.136µ ± 0%   8.308µ ± 0%   +2.11% (p=0.000 n=10)
IndexAnyUTF8/256:16            10.88µ ± 0%   11.05µ ± 0%   +1.57% (p=0.000 n=10)
IndexAnyUTF8/256:32            16.36µ ± 0%   14.90µ ± 0%   -8.93% (p=0.000 n=10)
IndexAnyUTF8/256:64            28.51µ ± 0%   19.41µ ± 0%  -31.92% (p=0.000 n=10)
LastIndexAnyASCII/1:1          35.79n ± 0%   38.52n ± 0%   +7.63% (p=0.000 n=10)
LastIndexAnyASCII/1:2          37.12n ± 0%   39.85n ± 0%   +7.35% (p=0.000 n=10)
LastIndexAnyASCII/1:4          39.76n ± 0%   42.08n ± 0%   +5.84% (p=0.000 n=10)
LastIndexAnyASCII/1:8          44.82n ± 0%   47.22n ± 0%   +5.34% (p=0.000 n=10)
LastIndexAnyASCII/1:16         55.53n ± 0%   57.92n ± 3%   +4.30% (p=0.000 n=10)
LastIndexAnyASCII/1:32         76.94n ± 0%   70.16n ± 0%   -8.81% (p=0.000 n=10)
LastIndexAnyASCII/1:64        124.40n ± 0%   89.67n ± 0%  -27.92% (p=0.000 n=10)
LastIndexAnyASCII/16:1         245.9n ± 0%   245.9n ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/16:2         255.2n ± 0%   255.2n ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/16:4         275.1n ± 0%   275.1n ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/16:8         315.2n ± 0%   315.2n ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/16:16        400.4n ± 0%   400.4n ± 0%        ~ (p=0.087 n=10)
LastIndexAnyASCII/16:32        560.9n ± 0%   560.9n ± 0%        ~ (p=0.124 n=10)
LastIndexAnyASCII/16:64        882.1n ± 0%   882.0n ± 0%   -0.01% (p=0.003 n=10)
LastIndexAnyASCII/256:1        2.815µ ± 0%   2.815µ ± 0%        ~ (p=0.211 n=10)
LastIndexAnyASCII/256:2        2.824µ ± 0%   2.824µ ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/256:4        2.844µ ± 0%   2.844µ ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/256:8        2.884µ ± 0%   2.884µ ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/256:16       2.969µ ± 0%   2.969µ ± 0%        ~ (p=1.000 n=10)
LastIndexAnyASCII/256:32       3.130µ ± 0%   3.130µ ± 0%        ~ (p=1.000 n=10) ¹
LastIndexAnyASCII/256:64       3.451µ ± 0%   3.451µ ± 0%        ~ (p=0.474 n=10)
LastIndexAnyUTF8/1:1           35.79n ± 0%   36.13n ± 0%   +0.95% (p=0.000 n=10)
LastIndexAnyUTF8/1:2           37.11n ± 0%   37.47n ± 0%   +0.97% (p=0.000 n=10)
LastIndexAnyUTF8/1:4           39.75n ± 0%   40.14n ± 0%   +0.97% (p=0.000 n=10)
LastIndexAnyUTF8/1:8           44.82n ± 0%   45.49n ± 0%   +1.49% (p=0.000 n=10)
LastIndexAnyUTF8/1:16          55.52n ± 0%   56.20n ± 0%   +1.22% (p=0.000 n=10)
LastIndexAnyUTF8/1:32          76.93n ± 0%   74.25n ± 0%   -3.48% (p=0.000 n=10)
LastIndexAnyUTF8/1:64         124.40n ± 0%   91.15n ± 0%  -26.73% (p=0.000 n=10)
LastIndexAnyUTF8/16:1          322.5n ± 0%   322.5n ± 0%        ~ (p=0.087 n=10)
LastIndexAnyUTF8/16:2          634.2n ± 0%   616.4n ± 0%   -2.81% (p=0.000 n=10)
LastIndexAnyUTF8/16:4          674.5n ± 0%   657.9n ± 0%   -2.46% (p=0.000 n=10)
LastIndexAnyUTF8/16:8          758.3n ± 0%   741.0n ± 0%   -2.28% (p=0.000 n=10)
LastIndexAnyUTF8/16:16         929.6n ± 0%   912.3n ± 0%   -1.86% (p=0.000 n=10)
LastIndexAnyUTF8/16:32         1.272µ ± 0%   1.176µ ± 0%   -7.55% (p=0.000 n=10)
LastIndexAnyUTF8/16:64         2.018µ ± 0%   1.453µ ± 0%  -28.00% (p=0.000 n=10)
LastIndexAnyUTF8/256:1         4.015µ ± 0%   4.016µ ± 0%   +0.02% (p=0.000 n=10)
LastIndexAnyUTF8/256:2         8.896µ ± 0%   8.537µ ± 0%   -4.04% (p=0.000 n=10)
LastIndexAnyUTF8/256:4         9.553µ ± 0%   9.217µ ± 0%   -3.52% (p=0.000 n=10)
LastIndexAnyUTF8/256:8         10.90µ ± 0%   10.54µ ± 0%   -3.29% (p=0.000 n=10)
LastIndexAnyUTF8/256:16        13.64µ ± 0%   13.28µ ± 0%   -2.63% (p=0.000 n=10)
LastIndexAnyUTF8/256:32        19.12µ ± 0%   17.16µ ± 1%  -10.23% (p=0.000 n=10)
LastIndexAnyUTF8/256:64        31.11µ ± 0%   21.98µ ± 0%  -29.36% (p=0.000 n=10)
IndexPeriodic/IndexPeriodic2   625.5µ ± 0%   625.5µ ± 0%        ~ (p=0.955 n=10)
IndexPeriodic/IndexPeriodic4   625.4µ ± 0%   625.4µ ± 0%        ~ (p=0.838 n=10)
IndexPeriodic/IndexPeriodic8   625.3µ ± 0%   625.3µ ± 0%   +0.01% (p=0.009 n=10)
IndexPeriodic/IndexPeriodic16  229.8µ ± 0%   227.0µ ± 0%   -1.22% (p=0.000 n=10)
IndexPeriodic/IndexPeriodic32  168.9µ ± 3%   131.8µ ± 0%  -22.00% (p=0.000 n=10)
IndexPeriodic/IndexPeriodic64 126.36µ ± 0%   86.66µ ± 0%  -31.42% (p=0.000 n=10)
geomean                        1.361µ        1.302µ        -4.31%

As these functions are so heavily used this change impacts other
benchmarks.  I include the improvements in geomean for the all the
benchmarks in the strings and bytes packages, along with some
selected benchmarks to illustrate the impact of the change.

geomean for bytes              13.81µ          12.92µ         -6.44%
geomean for string             9.385µ          9.224µ         -1.72%

Note that when building for rva22u64 a single Zbb instruction is used
in the main loop.  This also helps to improve performance slightly.
The geomean for all the bytes benchmarks when building with
GORISCV64=rva22u64 with and without the patch is shown below.

geomean for bytes (rva22u64)   13.46µ          12.49µ         -7.21%

Examples of non-Index benchmarks affected by this commit.

ReadString uses IndexByte to search for a byte stored at the end of
32KB buffer, so we see a speed up.  SplitSingleByteSeparator searches
large buffers, but the byte being sought occurs within the first 15
bytes of the buffer, 76% of the time, hence the slowdown.  In
SplitMultiByteSeparator the first byte of the separator only occurs
in the first 15 bytes 33% of the time so we see a speed up.

ReadString               05.13µ ±  2%    74.67µ ±  0%  -28.97% (p=0.000 n=10)
SplitSingleByteSeparator 11.31m ±  2%    12.43m ±  1%   +9.83% (p=0.000 n=10)
SplitMultiByteSeparator  8.070m ±  1%    7.707m ±  1%   -4.49% (p=0.000 n=10)

Change-Id: I6210ea2f3decdc6d2e0609df72b1b66e6d6f5395
Reviewed-on: https://go-review.googlesource.com/c/go/+/561275
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>