Support `Threads.Atomic{Ptr}`

[oxinabox]

There is a TODO for this in the code, but no open issue AFAICT.

julia/base/atomics.jl

Line 27 in 0fde275

# TODO: Support Ptr

That todo used to be for Bool as well, but that was solved in #26542

Without atomic operations on pointers, it is impossible to make lock-free multithreaded data structures.
which is a huge bummer for:
https://github.com/JuliaCollections/ThreadSafeDataStructures.jl

[chethega]

Which operations do we want, anything else besides atomic_cas! and atomic_xchg!?

[oxinabox]

We should support as much of the Ptr methods as can be done atomically.

julia> methodswith(Ptr)
[1] +(x::Integer, y::Ptr) in Base at pointer.jl:157
[2] +(x::Ptr, y::Integer) in Base at pointer.jl:155
[3] -(x::Ptr, y::Ptr) in Base at pointer.jl:153
[4] -(x::Ptr, y::Integer) in Base at pointer.jl:156
[5] <(x::Ptr, y::Ptr) in Base at pointer.jl:152
[6] ==(x::Ptr, y::Ptr) in Base at pointer.jl:151
[7] ==(x::Union{Cstring, Cwstring}, y::Ptr) in Base at c.jl:190
[8] ==(x::Ptr, y::Union{Cstring, Cwstring}) in Base at c.jl:191
[9] convert(::Type{T}, x::Ptr) where T<:Integer in Base at pointer.jl:23
[10] convert(::Type{Ptr{T}}, p::Ptr{T}) where T in Base at pointer.jl:29
[11] convert(::Type{Ptr{T}}, p::Ptr) where T in Base at pointer.jl:30
[12] dump(io::IOContext, x::Ptr, n::Int64, indent) in Base at show.jl:1641
[13] isequal(x::Ptr, y::Ptr) in Base at pointer.jl:148
[14] isless(x::Ptr{T}, y::Ptr{T}) where T in Base at pointer.jl:149
[15] show(io::IO, p::Ptr) in Base at show.jl:572
[16] unsafe_copyto!(dest::Ptr{T}, src::Ptr{T}, n) where T in Base at array.jl:225
[17] unsafe_load(p::Ptr) in Base at pointer.jl:105
[18] unsafe_load(p::Ptr, i::Integer) in Base at pointer.jl:105
[19] unsafe_pointer_to_objref(x::Ptr) in Base at pointer.jl:128
[20] unsafe_read(s::IO, p::Ptr, n::Integer) in Base at io.jl:585
[21] unsafe_store!(p::Ptr{T}, x) where T in Base at pointer.jl:118
[22] unsafe_store!(p::Ptr{T}, x, i::Integer) where T in Base at pointer.jl:118
[23] unsafe_wrap(::Union{Type{Array{T,N} where N}, Type{Array{T,N}}, Type{Array}}, p::Ptr{T}, dims::Tuple{Vararg{Int64,N}}) where {T, N} in Base at pointer.jl:84
[24] unsafe_wrap(::Union{Type{Array{T,N} where N}, Type{Array{T,1}}, Type{Array}}, p::Ptr{T}, d::Integer) where T in Base at pointer.jl:89
[25] unsafe_wrap(Atype::Type, p::Ptr, dims::Tuple{Vararg{#s55,N}} where #s55<:Integer) where N in Base at pointer.jl:92
[26] unsafe_write(s::IO, p::Ptr, n::Integer) in Base at io.jl:509
[27] write(to::IO, p::Ptr) in Base at io.jl:517
[28] dlclose(p::Ptr) in Libdl at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Libdl\src\Libdl.jl:119
[29] dlsym(hnd::Ptr, s::Union{AbstractString, Symbol}) in Libdl at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Libdl\src\Libdl.jl:54
[30] dlsym_e(hnd::Ptr, s::Union{AbstractString, Symbol}) in Libdl at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Libdl\src\Libdl.jl:64
[31] serialize(s::Serialization.AbstractSerializer, p::Ptr) in Serialization at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Serialization\src\Serialization.jl:181