fix performance problem in bootstrapping

[aviatesk]

So EscapeAnalysis.jl has a serious performance problem, which appears
only in bootstrapping.

I made some inspections using the following print debug:

within find_escapes!

if length(argtypes) > 0
    Core.println("analyzing... ", argtypes[1], " ", length(argtypes), " ", nstmts)
end

... and it seems like the performance of find_escapes! will be really
awful when there is a large number of statements, say, nstmts > 100.

The worse case I confirmed so far happens when analyzing
construct_ssa!(::CodeInfo, ::IRCode, ::DomTree, ::Any, ::Vector{Any}),
where I got the print analyzing... Core.Const(val=Core.Compiler.construct_ssa!) 182 4262,
and it took more than 100 sec on my machine (!!!).
Note that find_escapes! works on after-inlining IR, so we often such
many statements.

The interesting property is, this performance problem seems really
specific to bootstrapping. I confirmed that find_escapes! runs in
0.34 sec when I enabled it later using Revise.

julia> @time code_typed(Core.Compiler.construct_ssa!, (Core.Compiler.CodeInfo, Core.Compiler.IRCode,
Core.Compiler.DomTree, Any, Vector{Any}));
...
Core.Const(val=Core.Compiler.setindex!) 4 5
Core.Const(val=Core.Compiler.construct_ssa!) 182 4262
0.342626 seconds (723.68 k allocations: 177.828 MiB, 15.69% gc time, 99.94% compilation time)

[jpsamaroo]

Could it just be that during bootstrapping we're interpreting instead of executing compiled code?

[aviatesk]

I'm still trying to learn the limitations that happen in boostrapping, but if we're interpreting other high-cost computations like inference should also be slow ?

[aviatesk]

Escape analysis should run much faster now, thanks @jpsamaroo and @vtjnash

[TH3CHARLie]

Cool, I'd like to merge this into my branch, what do you think?

[aviatesk]

Go ahead