Type-parameterize Core.Box

[uniment]

This edit aims to take a small step toward solving some of the issues in #15276.

There are performance optimizations that do not occur when Box accesses are merely typeasserted, which instead require the box to be type-parameterized (example).

This edit type-parameterizes Core.Box, and then future improvements leveraging type annotations and inference can build on top of that.

[giordano]

Why this PR is changing permissions to 28 (out of 31) unrelated files, breaking the build process?

[uniment]

This is a practice run as I get warmed up. My setup got messed up somehow, I'm going to scratch the whole thing.

[KristofferC]

In order for something like this to happen I think it needs to be "proved" that the compiler can take advantage of it and actually generate useful parameterized Boxes in cases where it would otherwise not.

[uniment]

Oh, it appears that in Julia 1.10 the performance penalty that existed for unparameterized Box has been eliminated

Before:

julia> VERSION
v"1.9.4"

julia> @btime (()->begin # parameterized box
           x = Ref(1)
           f() = (x[] = x[] + 1; x[])
           for i=1:1000; f() end
           x[]
       end)()
  1.800 ns (0 allocations: 0 bytes)
1001

julia> @btime (()->begin # typeassert'ed box
           x::Int = 1
           f() = (x = x+1; x)
           for i=1:1000; f() end
           x
       end)()
  4.457 μs (490 allocations: 7.66 KiB)
1001

Now:

julia> VERSION
v"1.10.1"

julia> @btime (()->begin # parameterized box
           x = Ref(1)
           f() = (x[] = x[] + 1; x[])
           for i=1:1000; f() end
           x[]
       end)()
  2.000 ns (0 allocations: 0 bytes)
1001

julia> @btime (()->begin # typeassert'ed box
           x::Int = 1
           f() = (x = x+1; x)
           for i=1:1000; f() end
           x
       end)()
  1.800 ns (0 allocations: 0 bytes)
1001

[uniment]

On second look, it appears that even in Julia 1.12 there is still a performance penalty for asserted but unparameterized Box:

julia> VERSION
v"1.12.0-DEV.17"

julia> @btime let a::Int=0; for i=1:1000; a+=i end; ()->a+=1 end;
  5.000 μs (970 allocations: 15.16 KiB)

Compare with what is possible by type-parameterizing the box:

julia> @btime let a=Core.Box{Int}(0); for i=1:1000; a.contents+=i end; ()->a.contents+=1 end;
  4.300 ns (1 allocation: 16 bytes)

(notice microseconds vs nanoseconds)