Generate populationCount const at compile time

[GULPF]

As requested in #7558

[cooldome]

Looks great, but isn’t it easier to use popcount function from bitops module

[GULPF]

Great idea, I didn't know that module existed. However, it doesn't seem to work for int8:

import bitops
echo popcount(0b11111111'i8) # 32

[Araq]

import bitops
echo popcount(cast[int](0b11111111'i8)) # force zero extend

[GULPF]

@Araq Still doesn't work. Doesn't look like cast[int](...) does zero extend:

import bitops
echo popcount(cast[int](0b11111111'i8)) # 64
doAssert cast[int32](0b11111111'i8) == 0b11111111111111111111111111111111'i32

[Araq]

It has to, that's what the spec says. :-/

[cooldome]

This seems to work:

import bitops
const populationCount = block:
  var arr: array[low(int8)..high(int8), int8]
  for it in low(int8)..high(int8):
    arr[it] = popcount(cast[uint8](it)).int8
  arr

I'll submit echo popcount(0b11111111'i8) # 32 as bug report