Recursion disables constant propagation

[bramtayl]

So, this is a very annoying problem, and one that people don't seem to really know about. It's best demonstrated by example:

using Base: tail

function my_getindex(a_tuple, which_ones)
    rest = my_getindex(tail(a_tuple), tail(which_ones))
    if first(which_ones)
        (first(a_tuple), rest...)
    else
        rest
    end
end

function my_getindex(a_tuple::Tuple{}, which_ones::Tuple{})
    ()
end

function constant_select(a_tuple)
    my_getindex(a_tuple, (true, false, true, false))
end

@code_warntype constant_select((1, 1.0, "a", :a))
# Body::Tuple{Int64,Vararg{Any,N} where N}

My naive understanding of what's going on is that when Julia recurs into my_getindex, the compiler detects that which_ones is still a tuple of Bools (just a smaller one), figures out that we are recurring, and bails on constant propagation.

I've always gotten the impression that this is an unfixable problem, but...if it's fixable, that would be super cool.

[simeonschaub]

This looks odd to me:

julia> @code_typed constant_select((1, 1.0, "a", :a))
CodeInfo(
1 ─ %1  = Core.getfield(a_tuple, 3)::String
│   %2  = Core.getfield(a_tuple, 4)::Symbol
└──       goto #3 if not false
2 ─ %4  = Core.tuple(%2)::Tuple{Symbol}
└──       goto #4
3 ─       goto #4
4 ┄ %7  = φ (#2 => %4, #3 => ())::Union{Tuple{}, Tuple{Symbol}}
└──       goto #6 if not true
5 ─ %9  = Core.tuple(%1)::Tuple{String}
│   %10 = Core._apply_iterate(Base.iterate, Core.tuple, %9, %7)::Union{Tuple{String}, Tuple{String, Symbol}}
└──       goto #7
6 ─       goto #7
7 ┄ %13 = φ (#5 => %10, #6 => %7)::Tuple
└──       goto #8
8 ─ %15 = Base.getfield(a_tuple, 1, true)::Int64
│   %16 = Core.tuple(%15)::Tuple{Int64}
│   %17 = Core._apply_iterate(Base.iterate, Core.tuple, %16, %13)::Tuple{Int64, Vararg{Any}}
└──       goto #9
9 ─       return %17
) => Tuple{Int64, Vararg{Any}}

Shouldn't we be able to just eliminate block #2 here and therefore also avoid the type instability due to the phi node in line 7? (Same for line 13)

[Keno]

Well, fixing it is easy, you just need to delete the edgecycle check here: https://github.com/JuliaLang/julia/blob/master/base/compiler/abstractinterpretation.jl#L453. The problem is that the check guards against runaway inference (#36148). Regular inference is slightly more forgiving and uses a recursion heuristic defined here: https://github.com/JuliaLang/julia/blob/master/base/compiler/typelimits.jl#L186 to try to prove termination (which worked in this case).

That said, I'm thinking we could maybe improve the condition there to not be quite as strict. Let me play with it.

[Keno]

Shouldn't we be able to just eliminate block #2 here and therefore also avoid the type instability due to the phi node in line 7? (Same for line 13)

Inlining can often discover information that inference doesn't have access to, particularly in cases involving recursion, because we aggressively limit inference's exploration to avoid blowing up compile times. To investigate issues like this, one needs to look at unoptimized IR. In the near future, Cthulhu will display inference remarks to tell the user why it bailed. In this case it would have said: [constprop] Edge cycle encountered (https://github.com/JuliaLang/julia/blob/master/base/compiler/abstractinterpretation.jl#L460).

[Keno]

#39918

[bramtayl]

Woohoo!