add faster implementation for findall(::AbstractVector{Bool})

[mzgubic]

julia> a = rand(Bool, 10000);

julia> @btime findall($a);
  40.073 μs (2 allocations: 38.95 KiB)

julia> function myfindall(a)
           return (1:length(a))[a]
       end

julia> @btime myfindall($a);
  33.978 μs (2 allocations: 38.95 KiB)

though seems to be somewhat slower with BitArrays

julia> b = BitArray(rand(Bool, 10_000));

julia> @btime findall($b);
  6.113 μs (2 allocations: 39.89 KiB)

julia> @btime myfindall($b);
  6.224 μs (2 allocations: 39.89 KiB)

Thanks to @eperim for finding this

[jakobnissen]

There is already: #42202

[mzgubic]

Oh, nice! Is your approach even faster for the benchmarks above?

[jakobnissen]

Yes - on my branch:

julia> a = rand(Bool, 10000);

julia> using BenchmarkTools

julia> @btime findall($a);
  8.097 μs (2 allocations: 38.92 KiB)

julia> function myfindall(a)
                  return (1:length(a))[a]
              end
myfindall (generic function with 1 method)

julia> @btime myfindall($a);
  29.697 μs (2 allocations: 38.92 KiB)

[mzgubic]

Excellent!