byte,strings: improve IndexRune performance by ~45%

byte,strings: improve IndexRune performance by ~45%

Change IndexRune to search for the last byte of a multi-byte rune
instead of using the first byte. This improves search performance
by 45% on average when dealing with Unicode text.

The rationale here is that the last byte of a UTF-8 encoded multi-byte
rune is significantly more unique (evenly distributed) than the first
byte which has a 78% chance of being [240, 243, 244].

This approach is typically much faster, but can be slower when there
are a large number of false positives (see Han benchmarks) because
the more even distribution of bytes can delay/prevent falling back
to a brute-force search using bytealg.Index, which is particularly
powerful on amd64/x86_64 (particularly Skylake, but less so with
newer processors).

bytes package benchmarks:

goos: darwin
goarch: arm64
pkg: bytes
cpu: Apple M1 Max
                                 │ base.10.txt  │             new.10.txt              │
                                 │    sec/op    │   sec/op     vs base                │
IndexRune/10-10                     9.784n ± 0%   8.470n ± 0%  -13.43% (p=0.000 n=10)
IndexRune/32-10                    11.660n ± 0%   8.473n ± 0%  -27.34% (p=0.000 n=10)
IndexRune/4K-10                     83.96n ± 0%   81.08n ± 0%   -3.44% (p=0.000 n=10)
IndexRune/4M-10                     63.92µ ± 0%   64.67µ ± 0%   +1.17% (p=0.000 n=10)
IndexRune/64M-10                    1.121m ± 1%   1.125m ± 1%        ~ (p=0.218 n=10)
IndexRuneUnicode/Latin/10-10       10.125n ± 0%   7.347n ± 0%  -27.43% (p=0.000 n=10)
IndexRuneUnicode/Latin/32-10       11.435n ± 0%   7.349n ± 0%  -35.73% (p=0.000 n=10)
IndexRuneUnicode/Latin/4K-10        882.6n ± 0%   334.9n ± 1%  -62.06% (p=0.000 n=10)
IndexRuneUnicode/Latin/4M-10        977.2µ ± 0%   370.9µ ± 1%  -62.04% (p=0.000 n=10)
IndexRuneUnicode/Latin/64M-10      15.649m ± 1%   6.028m ± 1%  -61.48% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/10-10    10.070n ± 0%   8.701n ± 0%  -13.59% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/32-10    19.045n ± 0%   8.704n ± 1%  -54.30% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4K-10     2.734µ ± 0%   1.046µ ± 1%  -61.75% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4M-10     2.671m ± 0%   1.143m ± 1%  -57.22% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/64M-10    43.12m ± 1%   18.26m ± 1%  -57.64% (p=0.000 n=10)
IndexRuneUnicode/Han/10-10          10.10n ± 0%   10.82n ± 1%   +7.08% (p=0.000 n=10)
IndexRuneUnicode/Han/32-10          38.29n ± 1%   10.87n ± 1%  -71.62% (p=0.000 n=10)
IndexRuneUnicode/Han/4K-10         1409.0n ± 0%   489.1n ± 1%  -65.28% (p=0.000 n=10)
IndexRuneUnicode/Han/4M-10         1338.4µ ± 0%   821.1µ ± 2%  -38.65% (p=0.000 n=10)
IndexRuneUnicode/Han/64M-10         21.42m ± 1%   13.42m ± 2%  -37.34% (p=0.000 n=10)
geomean                             3.983µ        2.305µ       -42.14%

                                 │ base.10.txt  │               new.10.txt               │
                                 │     B/s      │      B/s       vs base                 │
IndexRune/10-10                    974.8Mi ± 0%   1126.1Mi ± 0%   +15.52% (p=0.000 n=10)
IndexRune/32-10                    2.556Gi ± 0%    3.517Gi ± 0%   +37.62% (p=0.000 n=10)
IndexRune/4K-10                    45.43Gi ± 0%    47.05Gi ± 0%    +3.56% (p=0.000 n=10)
IndexRune/4M-10                    61.12Gi ± 0%    60.41Gi ± 0%    -1.16% (p=0.000 n=10)
IndexRune/64M-10                   55.74Gi ± 1%    55.57Gi ± 1%         ~ (p=0.218 n=10)
IndexRuneUnicode/Latin/10-10       942.0Mi ± 0%   1297.9Mi ± 0%   +37.78% (p=0.000 n=10)
IndexRuneUnicode/Latin/32-10       2.606Gi ± 0%    4.055Gi ± 0%   +55.61% (p=0.000 n=10)
IndexRuneUnicode/Latin/4K-10       4.322Gi ± 0%   11.392Gi ± 1%  +163.57% (p=0.000 n=10)
IndexRuneUnicode/Latin/4M-10       3.998Gi ± 0%   10.532Gi ± 1%  +163.47% (p=0.000 n=10)
IndexRuneUnicode/Latin/64M-10      3.994Gi ± 1%   10.369Gi ± 1%  +159.61% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/10-10    947.2Mi ± 0%   1096.1Mi ± 0%   +15.72% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/32-10    1.565Gi ± 0%    3.424Gi ± 1%  +118.80% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4K-10    1.396Gi ± 0%    3.649Gi ± 1%  +161.43% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4M-10    1.462Gi ± 0%    3.418Gi ± 1%  +133.76% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/64M-10   1.450Gi ± 1%    3.422Gi ± 1%  +136.08% (p=0.000 n=10)
IndexRuneUnicode/Han/10-10         944.6Mi ± 0%    881.7Mi ± 1%    -6.66% (p=0.000 n=10)
IndexRuneUnicode/Han/32-10         797.0Mi ± 1%   2809.3Mi ± 1%  +252.47% (p=0.000 n=10)
IndexRuneUnicode/Han/4K-10         2.707Gi ± 0%    7.798Gi ± 1%  +188.04% (p=0.000 n=10)
IndexRuneUnicode/Han/4M-10         2.919Gi ± 0%    4.757Gi ± 2%   +63.01% (p=0.000 n=10)
IndexRuneUnicode/Han/64M-10        2.917Gi ± 1%    4.656Gi ± 2%   +59.60% (p=0.000 n=10)
geomean                            3.036Gi         5.246Gi        +72.82%

goos: linux
goarch: amd64
pkg: bytes
                                │   old.txt    │                new.txt                │
                                │    sec/op    │    sec/op      vs base                │
IndexRune/10-4                    10.805n ± 0%    6.999n ±  0%  -35.22% (p=0.000 n=10)
IndexRune/32-4                    12.515n ± 0%    7.539n ±  0%  -39.76% (p=0.000 n=10)
IndexRune/4K-4                     71.69n ± 0%    68.39n ±  0%   -4.60% (p=0.000 n=10)
IndexRune/4M-4                    125.19µ ± 2%    63.05µ ±  0%  -49.63% (p=0.000 n=10)
IndexRune/64M-4                    1.050m ± 1%    1.053m ±  0%        ~ (p=0.353 n=10)
IndexRuneUnicode/Latin/10-4        9.471n ± 0%    6.144n ±  1%  -35.13% (p=0.000 n=10)
IndexRuneUnicode/Latin/32-4       12.540n ± 0%    6.655n ±  0%  -46.93% (p=0.000 n=10)
IndexRuneUnicode/Latin/4K-4        522.1n ± 0%    207.2n ±  0%  -60.32% (p=0.000 n=10)
IndexRuneUnicode/Latin/4M-4        626.1µ ± 0%    297.2µ ±  0%  -52.54% (p=0.000 n=10)
IndexRuneUnicode/Latin/64M-4      13.866m ± 3%    5.069m ±  4%  -63.44% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/10-4    10.920n ± 0%    7.213n ±  0%  -33.95% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/32-4    12.515n ± 0%    7.780n ±  0%  -37.83% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4K-4    2650.0n ± 0%    621.5n ±  0%  -76.55% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4M-4    2744.7µ ± 0%    723.2µ ±  0%  -73.65% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/64M-4    44.18m ± 0%    14.22m ± 14%  -67.82% (p=0.000 n=10)
IndexRuneUnicode/Han/10-4         10.795n ± 0%    9.734n ±  1%   -9.83% (p=0.000 n=10)
IndexRuneUnicode/Han/32-4          12.79n ± 0%    10.42n ±  1%  -18.46% (p=0.000 n=10)
IndexRuneUnicode/Han/4K-4          519.7n ± 0%    288.4n ±  0%  -44.51% (p=0.000 n=10)
IndexRuneUnicode/Han/4M-4          498.2µ ± 0%    443.0µ ±  0%  -11.07% (p=0.000 n=10)
IndexRuneUnicode/Han/64M-4         9.654m ± 2%   12.223m ±  1%  +26.61% (p=0.000 n=10)
geomean                            3.168µ         1.828µ        -42.30%

                                │    old.txt    │                 new.txt                 │
                                │      B/s      │      B/s        vs base                 │
IndexRune/10-4                     882.5Mi ± 0%   1362.6Mi ±  0%   +54.41% (p=0.000 n=10)
IndexRune/32-4                     2.381Gi ± 0%    3.953Gi ±  0%   +66.00% (p=0.000 n=10)
IndexRune/4K-4                     53.21Gi ± 0%    55.77Gi ±  0%    +4.82% (p=0.000 n=10)
IndexRune/4M-4                     31.20Gi ± 2%    61.95Gi ±  0%   +98.55% (p=0.000 n=10)
IndexRune/64M-4                    59.54Gi ± 1%    59.37Gi ±  0%         ~ (p=0.353 n=10)
IndexRuneUnicode/Latin/10-4       1006.9Mi ± 0%   1552.3Mi ±  1%   +54.17% (p=0.000 n=10)
IndexRuneUnicode/Latin/32-4        2.376Gi ± 0%    4.478Gi ±  0%   +88.45% (p=0.000 n=10)
IndexRuneUnicode/Latin/4K-4        7.306Gi ± 0%   18.411Gi ±  0%  +152.01% (p=0.000 n=10)
IndexRuneUnicode/Latin/4M-4        6.239Gi ± 0%   13.145Gi ±  0%  +110.70% (p=0.000 n=10)
IndexRuneUnicode/Latin/64M-4       4.507Gi ± 3%   12.329Gi ±  4%  +173.54% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/10-4     873.0Mi ± 0%   1322.2Mi ±  0%   +51.46% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/32-4     2.382Gi ± 0%    3.831Gi ±  0%   +60.84% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4K-4     1.439Gi ± 0%    6.138Gi ±  0%  +326.43% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/4M-4     1.423Gi ± 0%    5.401Gi ±  0%  +279.52% (p=0.000 n=10)
IndexRuneUnicode/Cyrillic/64M-4    1.415Gi ± 0%    4.396Gi ± 17%  +210.79% (p=0.000 n=10)
IndexRuneUnicode/Han/10-4          883.4Mi ± 0%    979.7Mi ±  1%   +10.90% (p=0.000 n=10)
IndexRuneUnicode/Han/32-4          2.331Gi ± 0%    2.858Gi ±  1%   +22.61% (p=0.000 n=10)
IndexRuneUnicode/Han/4K-4          7.340Gi ± 0%   13.226Gi ±  0%   +80.19% (p=0.000 n=10)
IndexRuneUnicode/Han/4M-4          7.841Gi ± 0%    8.817Gi ±  0%   +12.44% (p=0.000 n=10)
IndexRuneUnicode/Han/64M-4         6.474Gi ± 2%    5.113Gi ±  1%   -21.02% (p=0.000 n=10)
geomean                            3.816Gi         6.614Gi         +73.32%

strings package benchmarks:

goos: darwin
goarch: arm64
pkg: strings
                       │ base.index_rune.10.txt │        new.index_rune.10.txt        │
                       │         sec/op         │   sec/op     vs base                │
IndexRune-10                       11.905n ± 5%   6.633n ± 6%  -44.28% (p=0.000 n=10)
IndexRuneLongString-10             13.800n ± 1%   7.330n ± 2%  -46.88% (p=0.000 n=10)
IndexRuneFastPath-10                3.477n ± 0%   3.481n ± 1%        ~ (p=0.468 n=10)
geomean                             8.297n        5.531n       -33.34%

Change-Id: I59357fda1c8ac85315b759930f620dbce1ba4721
Reviewed-on: https://go-review.googlesource.com/c/go/+/539116
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>