fix Dict sizehint! to allow enough space for the requested length

[JeffBezanson]

fixes #9909
noticed in #37208

Test code:

function grow_dict(szh::Bool)
    d = Dict{Int,Nothing}()
    szh && sizehint!(d, 32768)
    for i in 1:32768
        d[i] = nothing
    end
    length(d.keys)
end

before:

julia> @btime grow_dict(true)
  704.747 μs (12 allocations: 1.41 MiB)
131072

julia> @btime grow_dict(false)
  702.613 μs (27 allocations: 770.16 KiB)
65536

after:

julia> @btime grow_dict(true)
  464.426 μs (7 allocations: 576.66 KiB)
65536

julia> @btime grow_dict(false)
  702.545 μs (27 allocations: 770.16 KiB)
65536

@bkamins You'll probably want to rerun the benchmarks for allunique!

[bkamins]

All looks much better now.

Also the original use case of allunique is improved (now allunique is almost 2x faster than before my PR). Details below.

Setup:

function allunique2(C)
    seen = Dict{eltype(C), Nothing}()
    x = iterate(C)
    if haslength(C) && length(C) > 1000
        for i in OneTo(1000)
            v, s = x
            idx = ht_keyindex2!(seen, v)
            idx > 0 && return false
            _setindex!(seen, nothing, v, -idx)
            x = iterate(C, s)
        end
        sizehint!(seen, length(C))
    end
    while x !== nothing
        v, s = x
        idx = ht_keyindex2!(seen, v)
        idx > 0 && return false
        _setindex!(seen, nothing, v, -idx)
        x = iterate(C, s)
    end
    return true
end

function allunique3(C)
    seen = Dict{eltype(C), Nothing}()
    x = iterate(C)
    while x !== nothing
        v, s = x
        idx = ht_keyindex2!(seen, v)
        idx > 0 && return false
        _setindex!(seen, nothing, v, -idx)
        x = iterate(C, s)
    end
    return true
end

Before this PR:

julia> for i in 0:8
           x = [1:8000 + 20000i;]
           @show length(x)
           @btime allunique2($x)
           @btime allunique3($x)
       end
length(x) = 8000
  189.005 μs (26 allocations: 410.20 KiB)
  157.838 μs (22 allocations: 193.86 KiB)
length(x) = 28000
  650.336 μs (27 allocations: 1.46 MiB)
  651.134 μs (27 allocations: 770.16 KiB)
length(x) = 48000
  1.304 ms (27 allocations: 2.86 MiB)
  1.522 ms (32 allocations: 3.00 MiB)
length(x) = 68000
  1.188 ms (22 allocations: 1.17 MiB)
  1.863 ms (32 allocations: 3.00 MiB)
length(x) = 88000
  2.706 ms (27 allocations: 3.42 MiB)
  2.269 ms (32 allocations: 3.00 MiB)
length(x) = 108000
  3.181 ms (27 allocations: 3.42 MiB)
  2.769 ms (32 allocations: 3.00 MiB)
length(x) = 128000
  3.783 ms (27 allocations: 3.42 MiB)
  3.289 ms (32 allocations: 3.00 MiB)
length(x) = 148000
  2.863 ms (22 allocations: 2.30 MiB)
  3.854 ms (32 allocations: 3.00 MiB)
length(x) = 168000
  3.415 ms (22 allocations: 2.30 MiB)
  4.521 ms (32 allocations: 3.00 MiB)

After this PR:

julia> for i in 0:8
           x = [1:8000 + 20000i;]
           @show length(x)
           @btime allunique2($x)
           @btime allunique3($x)
       end
length(x) = 8000
  129.007 μs (22 allocations: 193.86 KiB)
  157.937 μs (22 allocations: 193.86 KiB)
length(x) = 28000
  382.389 μs (22 allocations: 625.86 KiB)
  653.205 μs (27 allocations: 770.16 KiB)
length(x) = 48000
  707.788 μs (22 allocations: 1.17 MiB)
  1.523 ms (32 allocations: 3.00 MiB)
length(x) = 68000
  1.181 ms (22 allocations: 1.17 MiB)
  1.862 ms (32 allocations: 3.00 MiB)
length(x) = 88000
  1.319 ms (22 allocations: 2.30 MiB)
  2.251 ms (32 allocations: 3.00 MiB)
length(x) = 108000
  1.762 ms (22 allocations: 2.30 MiB)
  2.690 ms (32 allocations: 3.00 MiB)
length(x) = 128000
  2.223 ms (22 allocations: 2.30 MiB)
  3.183 ms (32 allocations: 3.00 MiB)
length(x) = 148000
  2.847 ms (22 allocations: 2.30 MiB)
  3.752 ms (32 allocations: 3.00 MiB)
length(x) = 168000
  3.400 ms (22 allocations: 2.30 MiB)
  5.032 ms (32 allocations: 3.00 MiB)

Original use case:

julia> x = [1:10^8;];

julia> @time allunique(x) # version on Julia 1.5
 17.199410 seconds (82 allocations: 4.499 GiB, 1.75% gc time)
true

julia> @time allunique2(x) # version after my PR
  9.658649 seconds (22 allocations: 2.250 GiB, 0.60% gc time)
true