Add `ThreadingUtilities.launch` for a simpler API

[chriselrod]

Simpler API, and also faster than the way I was doing it before.

On my laptop, brought this down from about 370 ns to <300ns:

using ThreadingUtilities, StaticArrays, Test

struct MulStaticArray{P} end

function (::MulStaticArray{P})(p::Ptr{UInt}) where {P}
    _, (ptry,ptrx) = ThreadingUtilities.load(p, P, 2*sizeof(UInt))
    unsafe_store!(ptry, unsafe_load(ptrx) * 2.7)
    nothing
end

@generated function mul_staticarray_ptr(::A, ::B) where {A,B}
    c = MulStaticArray{Tuple{A,B}}()
    :(@cfunction($c, Cvoid, (Ptr{UInt},)))
end

@inline function setup_mul_svector!(p, y::Base.RefValue{SVector{N,T}}, x::Base.RefValue{SVector{N,T}}) where {N,T}
    py = Base.unsafe_convert(Ptr{SVector{N,T}}, y)
    px = Base.unsafe_convert(Ptr{SVector{N,T}}, x)
    fptr = mul_staticarray_ptr(py, px)
    offset = ThreadingUtilities.store!(p, fptr, sizeof(UInt))
    ThreadingUtilities.store!(p, (py,px), offset)
end

@inline function launch_thread_mul_svector(tid, y, x)
    ThreadingUtilities.launch(tid, y, x) do p, y, x
        setup_mul_svector!(p, y, x)
    end
end

function mul_svector_threads(a::SVector{N,T}, b::SVector{N,T}, c::SVector{N,T}) where {N,T}
    ra = Ref(a)
    rb = Ref(b)
    rc = Ref(c)
    rx = Ref{SVector{N,T}}()
    ry = Ref{SVector{N,T}}()
    rz = Ref{SVector{N,T}}()
    GC.@preserve ra rb rc rx ry rz begin
        launch_thread_mul_svector(1, rx, ra)
        launch_thread_mul_svector(2, ry, rb)
        launch_thread_mul_svector(3, rz, rc)
        w = muladd.(2.7, a, b)
        ThreadingUtilities.wait(1)
        ThreadingUtilities.wait(2)
        ThreadingUtilities.wait(3)
    end
    rx[],ry[],rz[],w
end

a = @SVector rand(16);
b = @SVector rand(16);
c = @SVector rand(16);

w1,x1,y1,z1 = mul_svector_threads(a, b, c);

@test w1 == a*2.7
@test x1 == b*2.7
@test y1 == c*2.7

@test z1 ≈ muladd(2.7, a, b)

@benchmark mul_svector_threads($(Ref(a))[], $(Ref(b))[], $(Ref(c))[])

[chriselrod]

ERROR: Too many threads requested for JULIA_EXCLUSIVE option.
Error: Process completed with exit code 1.

@DilumAluthge any idea?

[chriselrod]

I also need to find out how to enable codecov.