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Feature #15631

Updated by ahorek (Pavel Rosický) over 5 years ago

right now round_capa value is rounded up to the next power of 2 
 ``` 
 round_capa(4) -> returns 8 
 round_capa(8) -> returns 16 
 round_capa(16) -> returns 32 

 round_capa(5) -> returns 8 
 round_capa(9) -> returns 16 
 round_capa(17) -> returns 32 
 etc. 
 ``` 

 it seems wasteful to allocate the extra items capacity, so this PR changes that to 
 ``` 
 round_capa(4) -> returns 4 
 round_capa(8) -> returns 8 
 round_capa(16) -> returns 16 

 round_capa(5) -> returns 8 
 round_capa(9) -> returns 16 
 round_capa(17) -> returns 32 
 etc. 
 ``` 

 the main purpose is to reduce memory usage especially during boot 

 my patch also uses BUILTIN_CLZ macro instead of hash shifts that makes it slightly faster 

 here's a benchmark 
 ```ruby 
 require 'benchmark/ips' 

 Benchmark.ips do |x| 
   x.config(time: 20, warmup: 3) 

   x.report('struct', "Struct.new(*('a'..'z').map { |x| x.to_sym })") 
 end 
 ``` 

 ``` 
 trunk 
 Warming up -------------------------------------- 
               struct     527.000    i/100ms 
 Calculating ------------------------------------- 
               struct        5.461k (± 5.5%) i/s -      109.089k in    20.040253s 

 methodmising - POW2_P (github) 
 Warming up -------------------------------------- 
               struct     544.000    i/100ms 
 Calculating ------------------------------------- 
               struct        5.570k (± 4.1%) i/s -      111.520k in    20.057245s 

 ahorek - BUILTIN_CLZ (id_table.c.patch) 
 Warming up -------------------------------------- 
               struct     571.000    i/100ms 
 Calculating ------------------------------------- 
               struct        5.812k (± 3.6%) i/s -      116.484k in    20.070607s 
 ``` 

 discussion https://github.com/ruby/ruby/pull/2083

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