[libc++] Optimize lexicographical_compare (#65279)HEADmain

If the comparison operation is equivalent to < and that is a total
order, we know that we can use equality comparison on that type instead
to extract some information. Furthermore, if equality comparison on that
type is trivial, the user can't observe that we're calling it. So
instead of using the user-provided total order, we use std::mismatch,
which uses equality comparison (and is vertorized). Additionally, if the
type is trivially lexicographically comparable, we can go one step
further and use std::memcmp directly instead of calling std::mismatch.

Benchmarks:
```
-------------------------------------------------------------------------------------
Benchmark                                                         old             new
-------------------------------------------------------------------------------------
bm_lexicographical_compare<unsigned char>/1                   1.17 ns         2.34 ns
bm_lexicographical_compare<unsigned char>/2                   1.64 ns         2.57 ns
bm_lexicographical_compare<unsigned char>/3                   2.23 ns         2.58 ns
bm_lexicographical_compare<unsigned char>/4                   2.82 ns         2.57 ns
bm_lexicographical_compare<unsigned char>/5                   3.34 ns         2.11 ns
bm_lexicographical_compare<unsigned char>/6                   3.94 ns         2.21 ns
bm_lexicographical_compare<unsigned char>/7                   4.56 ns         2.11 ns
bm_lexicographical_compare<unsigned char>/8                   5.25 ns         2.11 ns
bm_lexicographical_compare<unsigned char>/16                  9.88 ns         2.11 ns
bm_lexicographical_compare<unsigned char>/64                  38.9 ns         2.36 ns
bm_lexicographical_compare<unsigned char>/512                  317 ns         6.54 ns
bm_lexicographical_compare<unsigned char>/4096                2517 ns         41.4 ns
bm_lexicographical_compare<unsigned char>/32768              20052 ns          488 ns
bm_lexicographical_compare<unsigned char>/262144            159579 ns         4409 ns
bm_lexicographical_compare<unsigned char>/1048576           640456 ns        20342 ns
bm_lexicographical_compare<signed char>/1                     1.18 ns         2.37 ns
bm_lexicographical_compare<signed char>/2                     1.65 ns         2.60 ns
bm_lexicographical_compare<signed char>/3                     2.23 ns         2.83 ns
bm_lexicographical_compare<signed char>/4                     2.81 ns         3.06 ns
bm_lexicographical_compare<signed char>/5                     3.35 ns         3.30 ns
bm_lexicographical_compare<signed char>/6                     3.90 ns         3.99 ns
bm_lexicographical_compare<signed char>/7                     4.56 ns         3.78 ns
bm_lexicographical_compare<signed char>/8                     5.20 ns         4.02 ns
bm_lexicographical_compare<signed char>/16                    9.80 ns         6.21 ns
bm_lexicographical_compare<signed char>/64                    39.0 ns         3.16 ns
bm_lexicographical_compare<signed char>/512                    318 ns         7.58 ns
bm_lexicographical_compare<signed char>/4096                  2514 ns         47.4 ns
bm_lexicographical_compare<signed char>/32768                20096 ns          504 ns
bm_lexicographical_compare<signed char>/262144              156617 ns         4146 ns
bm_lexicographical_compare<signed char>/1048576             624265 ns        19810 ns
bm_lexicographical_compare<int>/1                             1.15 ns         2.12 ns
bm_lexicographical_compare<int>/2                             1.60 ns         2.36 ns
bm_lexicographical_compare<int>/3                             2.21 ns         2.59 ns
bm_lexicographical_compare<int>/4                             2.74 ns         2.83 ns
bm_lexicographical_compare<int>/5                             3.26 ns         3.06 ns
bm_lexicographical_compare<int>/6                             3.81 ns         4.53 ns
bm_lexicographical_compare<int>/7                             4.41 ns         4.72 ns
bm_lexicographical_compare<int>/8                             5.08 ns         2.36 ns
bm_lexicographical_compare<int>/16                            9.54 ns         3.08 ns
bm_lexicographical_compare<int>/64                            37.8 ns         4.71 ns
bm_lexicographical_compare<int>/512                            309 ns         24.6 ns
bm_lexicographical_compare<int>/4096                          2422 ns          204 ns
bm_lexicographical_compare<int>/32768                        19362 ns         1947 ns
bm_lexicographical_compare<int>/262144                      155727 ns        19793 ns
bm_lexicographical_compare<int>/1048576                     623614 ns        80180 ns
bm_ranges_lexicographical_compare<unsigned char>/1            1.07 ns         2.35 ns
bm_ranges_lexicographical_compare<unsigned char>/2            1.72 ns         2.13 ns
bm_ranges_lexicographical_compare<unsigned char>/3            2.46 ns         2.12 ns
bm_ranges_lexicographical_compare<unsigned char>/4            3.17 ns         2.12 ns
bm_ranges_lexicographical_compare<unsigned char>/5            3.86 ns         2.12 ns
bm_ranges_lexicographical_compare<unsigned char>/6            4.55 ns         2.12 ns
bm_ranges_lexicographical_compare<unsigned char>/7            5.25 ns         2.12 ns
bm_ranges_lexicographical_compare<unsigned char>/8            5.95 ns         2.13 ns
bm_ranges_lexicographical_compare<unsigned char>/16           11.7 ns         2.13 ns
bm_ranges_lexicographical_compare<unsigned char>/64           45.5 ns         2.36 ns
bm_ranges_lexicographical_compare<unsigned char>/512           366 ns         6.35 ns
bm_ranges_lexicographical_compare<unsigned char>/4096         2886 ns         40.9 ns
bm_ranges_lexicographical_compare<unsigned char>/32768       23054 ns          489 ns
bm_ranges_lexicographical_compare<unsigned char>/262144     185302 ns         4339 ns
bm_ranges_lexicographical_compare<unsigned char>/1048576    741576 ns        19430 ns
bm_ranges_lexicographical_compare<signed char>/1              1.10 ns         2.12 ns
bm_ranges_lexicographical_compare<signed char>/2              1.66 ns         2.35 ns
bm_ranges_lexicographical_compare<signed char>/3              2.23 ns         2.58 ns
bm_ranges_lexicographical_compare<signed char>/4              2.82 ns         2.82 ns
bm_ranges_lexicographical_compare<signed char>/5              3.34 ns         3.06 ns
bm_ranges_lexicographical_compare<signed char>/6              3.92 ns         3.99 ns
bm_ranges_lexicographical_compare<signed char>/7              4.64 ns         4.10 ns
bm_ranges_lexicographical_compare<signed char>/8              5.21 ns         4.61 ns
bm_ranges_lexicographical_compare<signed char>/16             9.79 ns         7.42 ns
bm_ranges_lexicographical_compare<signed char>/64             38.9 ns         2.93 ns
bm_ranges_lexicographical_compare<signed char>/512             317 ns         7.31 ns
bm_ranges_lexicographical_compare<signed char>/4096           2500 ns         47.5 ns
bm_ranges_lexicographical_compare<signed char>/32768         19940 ns          496 ns
bm_ranges_lexicographical_compare<signed char>/262144       159166 ns         4393 ns
bm_ranges_lexicographical_compare<signed char>/1048576      638206 ns        19786 ns
bm_ranges_lexicographical_compare<int>/1                      1.10 ns         2.12 ns
bm_ranges_lexicographical_compare<int>/2                      1.64 ns         3.04 ns
bm_ranges_lexicographical_compare<int>/3                      2.23 ns         2.58 ns
bm_ranges_lexicographical_compare<int>/4                      2.81 ns         2.81 ns
bm_ranges_lexicographical_compare<int>/5                      3.35 ns         3.05 ns
bm_ranges_lexicographical_compare<int>/6                      3.94 ns         4.60 ns
bm_ranges_lexicographical_compare<int>/7                      4.60 ns         4.81 ns
bm_ranges_lexicographical_compare<int>/8                      5.19 ns         2.35 ns
bm_ranges_lexicographical_compare<int>/16                     9.85 ns         2.87 ns
bm_ranges_lexicographical_compare<int>/64                     38.9 ns         4.70 ns
bm_ranges_lexicographical_compare<int>/512                     318 ns         24.5 ns
bm_ranges_lexicographical_compare<int>/4096                   2494 ns          202 ns
bm_ranges_lexicographical_compare<int>/32768                 20000 ns         1939 ns
bm_ranges_lexicographical_compare<int>/262144               160433 ns        19730 ns
bm_ranges_lexicographical_compare<int>/1048576              642636 ns        80760 ns
```

0 files changed, 0 insertions, 0 deletions