Access to undefined reference when using KrylovKit on a non-concrete vector

[MasonProtter]

Someone on Slack ran into this issue when they accidentally ran

exponentiate(::SparseMatrixCSC{ComplexF32, Int64}, 1, ::Vector{Complex})

instead of using a concrete type.

Here's a MWE:

#+begin_src julia
using KrylovKit, SparseArrays
let H = sprand(1000, 1000, 0.001), v = zeros(Complex, 1000)
    v[1] = 1
    exponentiate(H, 1, v)
end
#+end_src

#+RESULTS:
UndefRefError: access to undefined reference

Stacktrace:
  [1] getindex
    @ ./essentials.jl:13 [inlined]
  [2] macro expansion
    @ ~/julia/usr/share/julia/stdlib/v1.9/LinearAlgebra/src/generic.jl:183 [inlined]
  [3] macro expansion
    @ ./simdloop.jl:77 [inlined]
  [4] rmul!(X::Vector{Complex}, s::Bool)
    @ LinearAlgebra ~/julia/usr/share/julia/stdlib/v1.9/LinearAlgebra/src/generic.jl:182
  [5] expintegrator
    @ ~/.julia/packages/KrylovKit/diNbc/src/matrixfun/expintegrator.jl:106 [inlined]
  [6] expintegrator(::SparseMatrixCSC{Float64, Int64}, ::Int64, ::Vector{Complex}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ KrylovKit ~/.julia/packages/KrylovKit/diNbc/src/matrixfun/expintegrator.jl:102
  [7] expintegrator
    @ ~/.julia/packages/KrylovKit/diNbc/src/matrixfun/expintegrator.jl:98 [inlined]
  [8] #exponentiate#82
    @ ~/.julia/packages/KrylovKit/diNbc/src/matrixfun/exponentiate.jl:79 [inlined]
  [9] exponentiate(A::SparseMatrixCSC{Float64, Int64}, t::Int64, v::Vector{Complex})
    @ KrylovKit ~/.julia/packages/KrylovKit/diNbc/src/matrixfun/exponentiate.jl:79
 [10] top-level scope
    @ In[17]:4

[Jutho]

Do I infer correctly from the stack trace that this is actually a Julia Base (or actually LinearAlgebra) bug, namely with rmul!(X::Vector{Complex}, s::Bool)?

[MasonProtter]

Yeah I find that quite strange. I couldn't reproduce the bug by just calling rmul!(::Vector{Complex}, ::Bool), so I think that what’s happening is that KrylovKit is generating undefined junk somewhere and then trying to rmul! that junk

[MasonProtter]

Yeah, so if I make an undef vector of Complex then I get the error

julia> v = Vector{Complex}(undef, 2)
2-element Vector{Complex}:
 #undef
 #undef

julia> rmul!(v, false)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getindex
   @ ./essentials.jl:13 [inlined]
 [2] macro expansion
   @ ~/julia/usr/share/julia/stdlib/v1.9/LinearAlgebra/src/generic.jl:183 [inlined]
 [3] macro expansion
   @ ./simdloop.jl:77 [inlined]
 [4] rmul!(X::Vector{Complex}, s::Bool)
   @ LinearAlgebra ~/julia/usr/share/julia/stdlib/v1.9/LinearAlgebra/src/generic.jl:182
 [5] top-level scope
   @ REPL[2]:1

So maybe the fix would just be to throw a more helpful error before actually hitting rmul!?

[Jutho]

The problem is that the implementation tries to be completely agnostic about the type of vector. So checking for undef or abstract type parameters is not really possible, unless by adding a special branch for Vector instances.