goodbye simd crate, hello std::arch

[BurntSushi]

This PR ports the regex's crate use of SIMD to std::arch, which in turn drops the dependency on the simd crate and any compile time SIMD configuration requirements. As a bonus, we also add an AVX2 variant of what used to be an exclusively SSSE3 algorithm.

We do this by adding a new feature unstable, which when enabled, will cause the regex crate to automatically use SSSE3 or AVX2 optimized variants of certain literal algorithms (specifically, the Teddy multi-matcher), depending on which CPU features are available at runtime. Once std::arch is stabilized, these optimizations will be enabled automatically.

Performance improvements from no-SIMD to SSSE3 (which roughly match the status quo, when SSSE3 is enabled at compile time):

 sherlock::holmes_cochar_watson         193,079 (3081 MB/s)             160,996 (3695 MB/s)               -32,083  -16.62%   x 1.20
 sherlock::name_alt2                    166,895 (3564 MB/s)             126,387 (4707 MB/s)               -40,508  -24.27%   x 1.32
 sherlock::name_alt3                    1,090,127 (545 MB/s)            137,516 (4326 MB/s)              -952,611  -87.39%   x 7.93
 sherlock::name_alt4                    200,849 (2962 MB/s)             164,347 (3619 MB/s)               -36,502  -18.17%   x 1.22
 sherlock::name_alt4_nocase             1,155,389 (514 MB/s)            225,784 (2634 MB/s)              -929,605  -80.46%   x 5.12
 sherlock::name_alt5                    231,145 (2573 MB/s)             131,767 (4515 MB/s)               -99,378  -42.99%   x 1.75
 sherlock::name_alt5_nocase             1,158,401 (513 MB/s)            550,028 (1081 MB/s)              -608,373  -52.52%   x 2.11
 sherlock::name_holmes_nocase           943,570 (630 MB/s)              190,772 (3118 MB/s)              -752,798  -79.78%   x 4.95
 sherlock::name_sherlock_holmes_nocase  1,084,468 (548 MB/s)            170,387 (3491 MB/s)              -914,081  -84.29%   x 6.36
 sherlock::name_sherlock_nocase         1,077,668 (552 MB/s)            163,711 (3634 MB/s)              -913,957  -84.81%   x 6.58
 sherlock::the_nocase                   1,356,722 (438 MB/s)            392,886 (1514 MB/s)              -963,836  -71.04%   x 3.45

And then improvements from SSSE3 to AVX2:

 sherlock::holmes_cochar_watson         160,996 (3695 MB/s)          104,124 (5713 MB/s)              -56,872  -35.33%   x 1.55
 sherlock::holmes_coword_watson         554,179 (1073 MB/s)          495,262 (1201 MB/s)              -58,917  -10.63%   x 1.12
 sherlock::name_alt2                    126,387 (4707 MB/s)          85,083 (6992 MB/s)               -41,304  -32.68%   x 1.49
 sherlock::name_alt3                    137,516 (4326 MB/s)          94,820 (6274 MB/s)               -42,696  -31.05%   x 1.45
 sherlock::name_alt4                    164,347 (3619 MB/s)          120,466 (4938 MB/s)              -43,881  -26.70%   x 1.36
 sherlock::name_alt4_nocase             225,784 (2634 MB/s)          180,290 (3299 MB/s)              -45,494  -20.15%   x 1.25
 sherlock::name_alt5                    131,767 (4515 MB/s)          86,539 (6874 MB/s)               -45,228  -34.32%   x 1.52
 sherlock::name_holmes_nocase           190,772 (3118 MB/s)          147,946 (4021 MB/s)              -42,826  -22.45%   x 1.29
 sherlock::name_sherlock_holmes_nocase  170,387 (3491 MB/s)          124,611 (4774 MB/s)              -45,776  -26.87%   x 1.37
 sherlock::name_sherlock_nocase         163,711 (3634 MB/s)          121,786 (4885 MB/s)              -41,925  -25.61%   x 1.34

🎉

[BurntSushi]

Is it possible to use a union on Rust nightly but still compile on Rust 1.12? 🤔

[BurntSushi]

Indeed, it is possible:

macro_rules! defunion {
    () => {
        #[derive(Clone, Copy)]
        #[allow(non_camel_case_types)]
        pub union u8x32 {
            vector: __m256i,
            bytes: [u8; 32],
        }
    }
}

defunion!();