Commits on Jan 8, 2022

1. optimizer: enable SROA of mutable φ-nodes …

   This commit allows elimination of mutable φ-node (and its predecessor mutables allocations).
   As an contrived example, it allows this `mutable_ϕ_elim(::String, ::Vector{String})`
   to run without any allocations at all:
   ```julia
   function mutable_ϕ_elim(x, xs)
       r = Ref(x)
       for x in xs
           r = Ref(x)
       end
       return r[]
   end
   
   let xs = String[string(gensym()) for _ in 1:100]
       mutable_ϕ_elim("init", xs)
       @test @allocated(mutable_ϕ_elim("init", xs)) == 0
   end
   ```
   
   This mutable ϕ-node elimination is still limited though.
   Most notably, the current implementation doesn't work if a mutable
   allocation forms multiple ϕ-nodes, since we check allocation eliminability
   (i.e. escapability) by counting usages counts and thus it's hard to
   reason about multiple ϕ-nodes at a time.
   For example, currently mutable allocations involved in cases like below
   will still not be eliminated:
   ```julia
   code_typed((Bool,String,String),) do cond, x, y
       if cond
           ϕ2 = ϕ1 = Ref(x)
       else
           ϕ2 = ϕ1 = Ref(y)
       end
       ϕ1[], ϕ2[]
   end
   
   \# more realistic example
   mutable struct Point{T}
       x::T
       y::T
   end
   add(a::Point, b::Point) = Point(a.x + b.x, a.y + b.y)
   function compute(a::Point{ComplexF64}, b::Point{ComplexF64})
       for i in 0:(100000000-1)
           a = add(add(a, b), b)
       end
       a.x, a.y
   end
   ```
   
   I'd say this limitation should be addressed by first introducing a better
   abstraction for reasoning escape information. More specifically, I'd like
   introduce EscapeAnalysis.jl into Julia base first, and then gradually
   adapt it to improve our SROA pass, since EA will allow us to reason about
   all escape information imposed on whatever object more easily and should
   help us get rid of the complexities of our current SROA implementation.
   
   For now, I'd like to get in this enhancement even though it has the
   limitation elaborated above, as far as this commit doesn't introduce
   latency problem (which is unlikely).

   

   aviatesk committed Jan 8, 2022
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