Commits on Apr 26, 2022

1. [compiler] use perfect hashes for switches with small strings …

   Previously, we compiled a string switch as follow:
   
   	1. compute a tag (condition) string hash
   	2. switch over precomputed hashes of cases
   	3. inside a switch case, do `equals()` for a string
   
   So, we compile a switch as a 2-level comparison: hash matching
   plus `equals()` call on a string.
   
   For shorter strings we can avoid the third step by using a
   hash that will not result in collisions and identifies a string completely.
   
   Since we're using uint64_t for switch hashes, we have 8 bits for information.
   We can store up to 8 chars inside that without any losses.
   Our hash function will be more or less `memcpy(&h, s, s_len)`.
   
   Hash of a single char becomes a value in `0 <= x <= 255` range, which
   is great for C++ switch density: it's likely to get that switch compiled
   as a jump table (useful for lexers, etc).
   
   Another common case is a shared prefix of all case values.
   
   	case 'OPERATION_DELETE'
   	case 'OPERATION_INSERT'
   	case 'OPERATION_UPDATE'
   
   All cases above have a length of 16 that is too much for our new hashing.
   But if we remove the common `OPERATION_` prefix, strings fit into 6 bytes.
   
   When we have such switch with common prefixes and varying suffixes that
   fit into 8 bytes, we compare the prefix before hashing, if it does match,
   variable cases tail is hashed (`DELETE`, `INSERT`, `UPDATE` from above).
   
   Since optimized form doesn't require extra if statement inside every case,
   the binary size gets smaller as well (but not dramatically).
   
   In average, we compile 60% of string switches using the optimized scheme.
   (~50% for short strings plus around ~10% for prefix form)
   
   Benchmark results:
   
       name                                        old time/op  new time/op  delta
       SwitchOnlyDefault                           70.0ns ± 0%  29.0ns ± 0%  -58.57%
       Lexer                                        119ns ± 1%    78ns ± 2%  -35.04%
       LexerMiss                                   87.0ns ± 0%  77.0ns ± 0%  -11.49%
       LexerComplex                                 115ns ± 1%    93ns ± 2%  -19.43%
       LexerComplexMiss                            87.0ns ± 0%  74.6ns ± 1%  -14.25%
       Switch8oneChar                               158ns ± 0%    95ns ± 1%  -39.81%
       Switch8oneCharMiss                           108ns ± 1%   102ns ± 1%   -5.48%
       Switch8oneCharNoDefault                      164ns ± 1%    87ns ± 1%  -46.67%
       Switch8oneCharNoDefaultMiss                  117ns ± 0%    97ns ± 1%  -17.35%
       Switch6perfectHash                           258ns ± 1%   204ns ± 1%  -21.19%
       Switch6perfectHashMiss                       130ns ± 0%   119ns ± 1%   -8.21%
       Switch6perfectHashNoDefault                  271ns ± 1%   193ns ± 1%  -28.72%
       Switch6perfectHashNoDefaultMiss              138ns ± 1%   115ns ± 1%  -16.58%
       Switch12perfectHash                          278ns ± 0%   194ns ± 4%  -30.19%
       Switch12perfectHashMiss                      139ns ± 1%   109ns ± 1%  -21.62%
       Switch12perfectHashNoDefault                 275ns ± 1%   190ns ± 0%  -30.74%
       Switch12perfectHashNoDefaultMiss             142ns ± 1%   108ns ± 0%  -24.05%
       Switch6perfectHashWithPrefix                 286ns ± 0%   219ns ± 3%  -23.40%
       Switch6perfectHashWithPrefixMiss             190ns ± 1%   143ns ± 2%  -24.67%
       Switch6perfectHashWithPrefixNoDefault        293ns ± 0%   230ns ± 1%  -21.58%
       Switch6perfectHashWithPrefixNoDefaultMiss    194ns ± 1%   153ns ± 2%  -20.82%
       Switch12perfectHashWithPrefix                303ns ± 1%   225ns ± 2%  -25.69%
       Switch12perfectHashWithPrefixMiss            159ns ± 0%   113ns ± 0%  -28.93%
       Switch12perfectHashWithPrefixNoDefault       308ns ± 1%   224ns ± 1%  -27.18%
       Switch12perfectHashWithPrefixNoDefaultMiss   141ns ± 1%   117ns ± 2%  -17.14%
       [Geo mean]                                   171ns        130ns       -23.93%
   
   Another change is that we now compile a default-only switch a little bit differently.
   We don't compute a hash and discard auto-generated variables to avoid C++ compiler warnings.
   
   Refs #418
   Fixes #503

   

   quasilyte committed Apr 26, 2022
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