The slow descent into madness
Author JuliaDebug
213 Stars
Updated Last
12 Months Ago
Started In
November 2018


The slow descent into madness

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Cthulhu can help you debug type inference issues by recursively showing the code_typed output until you find the exact point where inference gave up, messed up, or did something unexpected. Using the Cthulhu interface you can debug type inference problems faster.

Looking at type-inferred code can be a bit daunting initially, but you grow more comfortable with practice. Consider starting with a tutorial on "lowered" representation, which introduces most of the new concepts. Type-inferrred code differs mostly by having additional type annotation and (depending on whether you're looking at optimized or non-optimized code) may incorporate inlining and other fairly significant transformations of the original code as written by the programmer.

Cthulhu's main tool, descend, can be invoked like this:

descend(f, tt)     # function and argument types
@descend f(args)   # normal call

descend allows you to interactively explore the output of code_typed by descending into invoke and call statements. (invoke statements correspond to static dispatch, whereas call statements correspond to dynamic dispatch.) Press enter to select an invoke or call to descend into, select to ascend, and press q or control-c to quit.

JuliaCon 2019 Talk and Demo

Watch on YouTube
Click to watch video

The version of Cthulu in the demo is a little outdated, without the newest features, but largely it has not changed too much.

Usage: descend

function foo()
    T = rand() > 0.5 ? Int64 : Float64
    sum(rand(T, 100))

descend(foo, Tuple{})
@descend foo()

Methods: descend

  • @descend_code_typed
  • descend_code_typed
  • @descend_code_warntype
  • descend_code_warntype
  • @descend: Shortcut for @descend_code_typed
  • descend: Shortcut for descend_code_typed

Usage: ascend

Cthulhu also provides the "upwards-looking" ascend. While descend allows you to explore a call tree starting from the outermost caller, ascend allows you to explore a call chain or tree starting from the innermost callee. Its primary purpose is to support analysis of invalidation and inference triggers in conjunction with SnoopCompile, but you can use it as a standalone tool. There is a video using ascend to fix invalidations, where the part on ascend starts at minute 4:55.

For example, you can use it to examine all the inferred callers of a method instance:

julia> m = which(length, (Set{Symbol},))
length(s::Set) in Base at set.jl:55

julia> mi = m.specializations[1]
MethodInstance for length(::Set{Symbol})

julia> ascend(mi)
Choose a call for analysis (q to quit):
 >   length(::Set{Symbol})
       union!(::Set{Symbol}, ::Vector{Symbol})
         intersect!(::Set{Union{Int64, Symbol}}, ::Vector{Symbol})
           _shrink(::typeof(intersect!), ::Vector{Union{Int64, Symbol}}, ::Tuple{Vector{Symbol}})
             intersect(::Vector{Union{Int64, Symbol}}, ::Vector{Symbol})
       union!(::Set{Symbol}, ::Set{Symbol})
         union!(::Set{Symbol}, ::Set{Symbol}, ::Set{Symbol})
           union(::Set{Symbol}, ::Set{Symbol})

You use the up/down arrows to navigate this menu, enter to select a call to descend into, and your space bar to toggle branch-folding.

It also works on stacktraces:

julia> bt = try
           [sqrt(x) for x in [1, -1]]

julia> ascend(bt)
Choose a call for analysis (q to quit):
 >   throw_complex_domainerror(::Symbol, ::Float64) at ./math.jl:33
       sqrt at ./math.jl:582 => sqrt at ./math.jl:608 => iterate at ./generator.jl:47 => collect_to! at ./array.jl:710 => collect_to_with_first!(::Vector{Float64}, ::Float64, ::Base.Generator{Vector{Int64}, typeof(sqrt)}, ::Int64) at ./array.jl:688
         collect(::Base.Generator{Vector{Int64}, typeof(sqrt)}) at ./array.jl:669
           eval(::Module, ::Any) at ./boot.jl:360
             eval_user_input(::Any, ::REPL.REPLBackend) at /home/tim/src/julia-master/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:139

The calls that appear on the same line separated by => represent inlined methods; when you select such a line, you enter at the final (topmost) call on that line.

By default,

  • descend views optimized code without "warn" coloration of types
  • ascend views non-optimized code with "warn" coloration

You can toggle between these with o and w.