function feature_string()
    llvmlib_path = VERSION ≥ v"1.6.0-DEV.1429" ? Base.libllvm_path() : only(filter(lib->occursin(r"LLVM\b", basename(lib)), Libdl.dllist()))
    libllvm = Libdl.dlopen(llvmlib_path)
    gethostcpufeatures = Libdl.dlsym(libllvm, :LLVMGetHostCPUFeatures)
    features_cstring = ccall(gethostcpufeatures, Cstring, ())
    features = filter(ext -> (ext ≠ "" && (m = match(r"\d", ext); isnothing(m) ? true : m.offset != 2)), split(unsafe_string(features_cstring), ','))
    features, features_cstring
end

const FEATURE_SET = Set{String}()
_has_feature(str::String) = str ∈ FEATURE_SET

archstr() = Sys.ARCH === :i686 ? "x86_64_" : string(Sys.ARCH) * '_'

# feature_name(ext) = archstr() * replace(ext[2:end], r"\." => "_")
feature_name(ext) = archstr() * ext[2:end]
process_feature(ext) = (feature_name(ext), first(ext) == '+')

has_feature(_) = False()
@noinline function set_featue(feature::String, has::Bool)
    featqn = QuoteNode(Symbol(feature))
    if has
        @eval has_feature(::Val{$featqn}) = True()
    else
        @eval has_feature(::Val{$featqn}) = False()
    end
end

function set_features!()
    features, features_cstring = feature_string()
    for ext ∈ features
      feature, has = process_feature(ext)
      has && push!(FEATURE_SET, feature)
      set_featue(feature, has)
    end
    Libc.free(features_cstring)
end
set_features!()



function reset_features!()
    features, features_cstring = feature_string()
    for ext ∈ features
        feature, has = process_feature(ext)
        if _has_feature(feature) ≠ has
            @debug "Defining $(has ? "presence" : "absense") of feature $feature."
            set_featue(feature, has)
        end
    end
    Libc.free(features_cstring)
end

register_size(::Type{T}) where {T} = register_size()
register_size(::Type{T}) where {T<:Union{Signed,Unsigned}} = simd_integer_register_size()

function define_cpu_name()
    cpu = QuoteNode(Symbol(Sys.CPU_NAME::String))
    @eval cpu_name() = Val{$cpu}()
end
define_cpu_name()
