Add RTCallInfo for runtime calls

[timholy]

The list of call sites serves two purposes:

• it gives you options to descend into
• it summarizes the calls made by your function

Given the second purpose, I've sometimes been confused by the absence of an entry for the "worst" of all cases, runtime dispatch. This seems particularly accute for #345, but may have value even when examining the CodeInfo/IRCode.

Here's a demo: on master,

julia> function summer(list)
           s = 0      # deliberately not `zero(eltype(list))`
           for x in list
               s += x
           end
           return s
       end;

julia> @descend optimize=false summer([1.0, 2.0])
 ⋮
 • %3 = iterate(::Vector{Float64})::Union{Nothing, Tuple{Float64, Int64}}
   %10 = call → +(::Union{Float64, Int64},::Float64)::Float64
   %11 = iterate(::Vector{Float64},::Int64)::Union{Nothing, Tuple{Float64, Int64}}
   ↩

but

julia> @descend optimize=false summer(Any[1.0, 2.0])
 ⋮
 • %3 = iterate(::Vector{Any})::Union{Nothing, Tuple{Any, Int64}}
   %11 = iterate(::Vector{Any},::Int64)::Union{Nothing, Tuple{Any, Int64}}
   ↩

The absence of any "acknowledgement" of the call to + gave me pause.

In contrast, on this branch the first case is unchanged, whereas the second is

 • %3 = iterate(::Vector{Any})::Union{Nothing, Tuple{Any, Int64}}
   %10 = RT call +(::Any,::Any)::Any
   %11 = iterate(::Vector{Any},::Int64)::Union{Nothing, Tuple{Any, Int64}}
   ↩

I've set it up so that attempting to descend into the RT call gives this:

   %3 = iterate(::Vector{Any})::Union{Nothing, Tuple{Any, Int64}}
 • %10 = RT call +(::Any,::Any)::Any
   %11 = iterate(::Vector{Any},::Int64)::Union{Nothing, Tuple{Any, Int64}}
   ↩
[ Info: This is a runtime call. You cannot descend into it.
Select a call to descend into or ↩ to ascend. [q]uit. [b]ookmark.
Toggles: [o]ptimize, [w]arn, [h]ide type-stable statements, [d]ebuginfo, [r]emarks, [e]ffects, [i]nlining costs, [t]ype annotations, [s]yntax highlight for Source/LLVM/Native.
Show: [S]ource code, [A]ST, [T]yped code, [L]LVM IR, [N]ative code
Actions: [E]dit source code, [R]evise and redisplay
Advanced: dump [P]arams cache.
 • %3 = iterate(::Vector{Any})::Union{Nothing, Tuple{Any, Int64}}
   %10 = RT call +(::Any,::Any)::Any
   %11 = iterate(::Vector{Any},::Int64)::Union{Nothing, Tuple{Any, Int64}}
   ↩

I think filtering out all the Core.Builtin calls will prevent this from being too "noisy," but I'd be grateful for the thoughts of those who know much more about inference than me.

[timholy]

I'm planning to merge this tomorrow, too.

[vchuravy]

Maybe print it as runtime <+(::Any,::Any)::Any> or dynamic It might not be clear what RT stands for.