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Feature #15631

Updated by ahorek (Pavel Rosický) 5 months 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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