[ruby-core:93673] [Ruby master Feature#15631] Let round_capa for ID table not allocate excess capacity for power of 2 ints >= 4

Issue #15631 has been updated by ko1 (Koichi Sasada).

Assignee set to ko1 (Koichi Sasada)

funny_falcon, do you have any opinion?
if no opinion, we'll discuss it one month later and will commit it.

in fact, I can't check algorithm, so we can try it.

ahorek:
could you give me more performance measurements?

* you should not call `map` in iteration (you should prepare IDs before)
* now you only measures 26 fields. could you measure other numbers, 1 to 50, for example.

Thanks,
Koichi

----------------------------------------
Feature #15631: Let round_capa for ID table not allocate excess capacity for power of 2 ints >= 4
https://bugs.ruby-lang.org/issues/15631#change-79284

* Author: ahorek (Pavel Rosick
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
----------------------------------------
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 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 (ア 5.5%) i/s -    109.089k in  20.040253s

methodmising - POW2_P (github)
Warming up --------------------------------------
              struct   544.000  i/100ms
Calculating -------------------------------------
              struct      5.570k (ア 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 (ア 3.6%) i/s -    116.484k in  20.070607s
```

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

---Files--------------------------------
id_table.c.patch (534 Bytes)


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