Fix type instability in chebinterp

[lxvm]

I noticed a type instability in droptol due to a variable in a closure modified in a loop that could be fixed with a type annotation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dc4902b) 96.20% compared to head (d5f000c) 96.20%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #19   +/-   ##
=======================================
  Coverage   96.20%   96.20%           
=======================================
  Files           5        5           
  Lines         237      237           
=======================================
  Hits          228      228           
  Misses          9        9           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[stevengj]

Thanks!

I'm a bit unclear why ::Int is required, here. What is the inferred type if you don't have this declaration?

[lxvm]

Without the annotation the inferred type becomes Any, from checking with @code_warntype

MWE

The last three lines present the type-instability

julia> using FastChebInterp

julia> @code_warntype FastChebInterp.droptol(rand(5,5), 0.0)
MethodInstance for FastChebInterp.droptol(::Matrix{Float64}, ::Float64)
  from droptol(coefs::Array{<:Any, N}, tol::Real) where N @ FastChebInterp ~/.julia/dev/FastChebInterp/src/interp.jl:74
Static Parameters
  N = 2
Arguments
  #self#::Core.Const(FastChebInterp.droptol)
  coefs::Matrix{Float64}
  tol::Float64
Locals
  #67::FastChebInterp.var"#67#72"
  #64::FastChebInterp.var"#64#69"{2, Matrix{Float64}, Float64}
  #63::FastChebInterp.var"#63#68"{Float64}
  newsize::Tuple{Any, Any}
  abstol::Float64
Body::Any
1 ─       Core.NewvarNode(:(#67))
│         Core.NewvarNode(:(#64))
│         Core.NewvarNode(:(newsize))
│   %4  = FastChebInterp.maximum(FastChebInterp.infnorm, coefs)::Float64
│         (abstol = %4 * tol)
│   %6  = FastChebInterp.:(var"#63#68")::Core.Const(FastChebInterp.var"#63#68")
│   %7  = Core.typeof(abstol)::Core.Const(Float64)
│   %8  = Core.apply_type(%6, %7)::Core.Const(FastChebInterp.var"#63#68"{Float64})
│         (#63 = %new(%8, abstol))
│   %10 = #63::FastChebInterp.var"#63#68"{Float64}
│   %11 = FastChebInterp.all(%10, coefs)::Bool
└──       goto #4 if not %11
2 ─       return coefs
3 ─       Core.Const(:(goto %15))
4 ┄ %15 = FastChebInterp.:(var"#64#69")::Core.Const(FastChebInterp.var"#64#69")
│   %16 = $(Expr(:static_parameter, 1))::Core.Const(2)
│   %17 = Core.typeof(coefs)::Core.Const(Matrix{Float64})
│   %18 = Core.typeof(abstol)::Core.Const(Float64)
│   %19 = Core.apply_type(%15, %16, %17, %18)::Core.Const(FastChebInterp.var"#64#69"{2, Matrix{Float64}, Float64})
│         (#64 = %new(%19, coefs, abstol))
│   %21 = #64::FastChebInterp.var"#64#69"{2, Matrix{Float64}, Float64}
│   %22 = Core.apply_type(FastChebInterp.Val, $(Expr(:static_parameter, 1)))::Core.Const(Val{2})
│   %23 = (%22)()::Core.Const(Val{2}())
│         (newsize = FastChebInterp.ntuple(%21, %23))
│         (#67 = %new(FastChebInterp.:(var"#67#72")))
│   %26 = #67::Core.Const(FastChebInterp.var"#67#72"())
│   %27 = FastChebInterp.map(%26, newsize)::Tuple{Any, Any}
│   %28 = FastChebInterp.CartesianIndices(%27)::CartesianIndices{2, <:Tuple{OrdinalRange{Int64, Int64}, OrdinalRange{Int64, Int64}}}
│   %29 = Base.getindex(coefs, %28)::Any
└──       return %29

Analyzing the closure with Cthulhu shows that n in the closure gets boxed, even though it is not captured from the surrounding scope. I'm confused why this is too.

[stevengj]

This https://docs.julialang.org/en/v1/manual/performance-tips/#man-performance-captured

Maybe better to use a let block as noted there?