Stream: helpdesk (published)

Topic: ✔ Why is this not typestable?

DrChainsaw (May 09 2023 at 14:18):

Seems that when one makes a deep enough nesting of types something which is seemingly type stable just stops being so. Something something union splitting?

struct AA{T}
  x::T
end

ff(a::AA) = ff(a.x)
ff(t::Tuple) = mapreduce(ff, max, t; init=0)
ff(a::AbstractArray) = mapreduce(ff, max, a; init=0)
ff(x::Int) = x

# This is fine
let
    aa = AA(AA([1,2]))
    @code_warntype ff(aa)
end

# This too
let
    aa = AA((AA([1,2]),))
    @code_warntype ff(aa)
end
# This is not fine
let
    aa = AA((AA([AA(1), AA(2)]), AA(3)))
    @code_warntype ff(aa)
end
# This looks worse, but is still fine
let
   aa = AA((AA([AA(1), AA(2)]), AA([3, 4])))
  @code_warntype ff(aa)
end

Brian Chen (May 09 2023 at 15:27):

julia> typeof(aa2)
AA{Tuple{AA{Vector{AA{Int64}}}, AA{Int64}}}

julia> typeof(aa3)
AA{Tuple{AA{Vector{AA{Int64}}}, AA{Vector{Int64}}}}

I think the homogeneity of the 2nd level in the example that looks worse helps the compiler, but not sure how

Brian Chen (May 09 2023 at 15:28):

I did dive in with Cthulhu with remarks turned on. Saw some strange ones deeper in the stack. It may be worth exploring around with that yourself and maybe asking about this in a place where more of the compiler team hangs out.

Sukera (May 09 2023 at 15:29):

four methods for ff? sounds like union splitting to me

Sukera (May 09 2023 at 15:31):

I can't even dive into this with Cthulhu on a somewhat recent master

Sukera (May 09 2023 at 15:31):

julia> @descend ff(aa)
ERROR: unhandled LimitedAccuracy
Stacktrace:

Brian Chen (May 09 2023 at 16:35):

Even on 1.8 I see some lattice elements that don't look like they should be there (or anywhere, in a valid program). Suffice it to say that this would make a good Cthulhu bug report as well

Sukera (May 09 2023 at 16:41):

Don't think it's a cthulhu issue

Sukera (May 09 2023 at 16:41):

the stacktrace points to type inference

Sukera (May 09 2023 at 16:41):

which makes kind of sense, since @code_warntype fell back to Any too

Simone Carlo Surace (Dec 27 2023 at 21:30):

Did this lead to any issues? I recently hit something similar.

Sukera (Dec 28 2023 at 11:09):

similar as in issues with Cthulhu, or do you mean the type stability thing?

Notification Bot (Dec 28 2023 at 13:53):

cormullion has marked this topic as resolved.

Last updated: Nov 22 2024 at 04:41 UTC