1.8.5 -> 1.9 regression in number of allocations from `getproperty`

[BioTurboNick]

The execution time is similar for this small example, but there are 10x more allocations on 1.9-rc3.

using BenchmarkTools
using Plots
const xx = randn(20, 3000)
@btime plot(xx; color = :blue, label = nothing);

# 1.8.5
# 1.214 s (392640 allocations: 31.88 MiB)

# 1.9-rc3
# 1.166 s (3964617 allocations: 86.43 MiB)

From JuliaPlots/Plots.jl#4742

[KristofferC]

This needs more of a reduction for this to be actionable over just a @btime plot.

[BioTurboNick]

Working on it

[BioTurboNick]

The allocations are almost entirely originating from getproperty with UnknownType, according to @profview_allocs.

Specifically, this method: https://github.com/JuliaLang/julia/blob/master/base/Base.jl#L37

[BioTurboNick]

Possibly related: #49241

[KristofferC]

Possibly related: #49241

Could check if the offending commit bisected in that PR (#47371) is the cause by looking at the results one commit before it. Could also try to bisect this but I suspect it will be a bit difficult since it might not be one commit that is to blame for the whole allocation increase.

[BioTurboNick]

I boiled it down to a simple demonstration:

const s = Set()
@time s.dict # 0.000003 seconds (1 allocation: 16 bytes)

Bisect pinpointed #47371 as you thought.

[KristofferC]

Putting that in a function makes it not appear anymore

julia> s = Set();

julia> f(s) = s.dict;

julia> @time f(s);
  0.000004 seconds

So how does this manifest itself within Plots? Is it because s is not inferred proerly where the getproperty is called or something like that?

[BioTurboNick]

Current master (e204e20) still has it

[BioTurboNick]

Via

julia/base/set.jl

Line 124 in e204e20

iterate(s::Set, i...) = iterate(KeySet(s.dict), i...)

julia/base/dict.jl

Line 700 in 8e63055

i = skip_deleted(v.dict, i)

and

julia/base/dict.jl

Line 702 in 8e63055

vals = T <: KeySet ? v.dict.keys : v.dict.vals

[BioTurboNick]

Hmm, I'd be surprised if it wasn't inferred.

One example is a for loop iterating over a global constant.

https://github.com/JuliaPlots/Plots.jl/blob/df6c09dced6f717bcb0c6cf8d6bd51701372b069/src/args.jl#L1878

https://github.com/JuliaPlots/Plots.jl/blob/df6c09dced6f717bcb0c6cf8d6bd51701372b069/src/consts.jl#L62

[BioTurboNick]

The constant is a Set{Symbol}

[BioTurboNick]

Yeah, the cases responsible are all iterating over global constant sets of symbols, or over keys() of a global constant Dict{Symbol, Any},