Commits on Apr 13, 2016

1. Optimize the DFA inner loop. …

   This employs a number of tricks to make the inner loop faster:
   
   1. **Elides bounds checks with `unsafe`**. This is the first use of
   `unsafe` in regex. It is justified below with benchmarks, and comments
   in the source make an argument for correctness.
   2. Store meta data about states in the upper bits of a state pointer.
   This reduces the amount of branching needed.
   3. Create a inner inner loop that handles all transitions between
   non-dead, non-match and non-start states. (i.e., The majority of cases.)
   In particular, this lets us avoid having to check specifically whether
   each state is a match state or not.
   4. Start states are only treated specially if there is a prefix detected
   that we should scan for. Otherwise, start states are no different than
   any other state.
   5. Move transitions from `State` and into one giant transition table,
   which should hopefully improve locality and make better use of the
   cache.
   
   The use of `unsafe` is unfortunate, but it significantly reduces the
   number of instructions executed in a search. When the DFA spends a lot
   of time in the inner loop, eliding the bounds checks leads to better
   performance. In most cases, the boost is worth about 5%, but in some
   extreme cases (e.g., a match is the entirety of a large haystack), the
   boost can be worth nearly 50%.
   
   Here is a comparison between code without `unsafe` and with `unsafe`:
   
   ```
   $ cargo-benchcmp rust-safe rust-unsafe --threshold 3
   name                                     rust-safe ns/iter     rust-unsafe ns/iter     diff ns/iter   diff %
   misc::anchored_literal_long_match        29 (13,448 MB/s)      26 (15,000 MB/s)                  -3  -10.34%
   misc::anchored_literal_short_match       28 (928 MB/s)         26 (1,000 MB/s)                   -2   -7.14%
   misc::easy0_1MB                          49 (21,400,061 MB/s)  42 (24,966,738 MB/s)              -7  -14.29%
   misc::easy1_1K                           79 (13,215 MB/s)      76 (13,736 MB/s)                  -3   -3.80%
   misc::easy1_32                           79 (658 MB/s)         76 (684 MB/s)                     -3   -3.80%
   misc::easy1_32K                          80 (409,850 MB/s)     76 (431,421 MB/s)                 -4   -5.00%
   misc::hard_1K                            104 (10,105 MB/s)     100 (10,510 MB/s)                 -4   -3.85%
   misc::match_class_unicode                595 (270 MB/s)        571 (281 MB/s)                   -24   -4.03%
   misc::medium_1MB                         51 (20,560,862 MB/s)  44 (23,831,909 MB/s)              -7  -13.73%
   misc::no_exponential                     378 (264 MB/s)        361 (277 MB/s)                   -17   -4.50%
   misc::not_literal                        206 (247 MB/s)        196 (260 MB/s)                   -10   -4.85%
   misc::one_pass_long_prefix               116 (224 MB/s)        111 (234 MB/s)                    -5   -4.31%
   misc::one_pass_long_prefix_not           116 (224 MB/s)        108 (240 MB/s)                    -8   -6.90%
   misc::one_pass_short                     81 (209 MB/s)         76 (223 MB/s)                     -5   -6.17%
   misc::one_pass_short_not                 79 (215 MB/s)         75 (226 MB/s)                     -4   -5.06%
   misc::reallyhard_1K                      3,796 (276 MB/s)      3,629 (289 MB/s)                -167   -4.40%
   misc::reallyhard_1MB                     3,765,536 (278 MB/s)  3,602,215 (291 MB/s)        -163,321   -4.34%
   misc::reallyhard_32                      234 (252 MB/s)        222 (265 MB/s)                   -12   -5.13%
   misc::reallyhard_32K                     117,917 (278 MB/s)    112,604 (291 MB/s)            -5,313   -4.51%
   misc::replace_all                        144                   137                               -7   -4.86%
   sherlock::before_holmes                  2,163,856 (274 MB/s)  2,077,792 (286 MB/s)         -86,064   -3.98%
   sherlock::everything_greedy              3,641,444 (163 MB/s)  2,578,502 (230 MB/s)      -1,062,942  -29.19%
   sherlock::everything_greedy_nl           2,109,164 (282 MB/s)  1,080,933 (550 MB/s)      -1,028,231  -48.75%
   sherlock::holmes_coword_watson           1,087,276 (547 MB/s)  1,037,918 (573 MB/s)         -49,358   -4.54%
   sherlock::ing_suffix                     2,419,816 (245 MB/s)  2,308,945 (257 MB/s)        -110,871   -4.58%
   sherlock::ing_suffix_limited_space       2,360,927 (251 MB/s)  2,259,791 (263 MB/s)        -101,136   -4.28%
   sherlock::letters                        27,710,372 (21 MB/s)  25,348,374 (23 MB/s)      -2,361,998   -8.52%
   sherlock::letters_lower                  26,888,541 (22 MB/s)  24,759,385 (24 MB/s)      -2,129,156   -7.92%
   sherlock::letters_upper                  3,138,611 (189 MB/s)  2,989,327 (199 MB/s)        -149,284   -4.76%
   sherlock::line_boundary_sherlock_holmes  2,132,889 (278 MB/s)  2,046,399 (290 MB/s)         -86,490   -4.06%
   sherlock::name_alt1                      35,964 (16,542 MB/s)  37,164 (16,008 MB/s)           1,200    3.34%
   sherlock::name_whitespace                88,768 (6,702 MB/s)   85,322 (6,972 MB/s)           -3,446   -3.88%
   sherlock::quotes                         800,085 (743 MB/s)    769,792 (772 MB/s)           -30,293   -3.79%
   sherlock::the_whitespace                 1,315,168 (452 MB/s)  1,238,173 (480 MB/s)         -76,995   -5.85%
   sherlock::words                          11,230,278 (52 MB/s)  9,855,296 (60 MB/s)       -1,374,982  -12.24%
   ```

   

   BurntSushi committed Apr 13, 2016
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