improve performance for ascii(::String)

[KristofferC]

julia> using Random

julia> s = randstring(10000);

# Before
julia> @btime ascii(s);
  7.886 μs (0 allocations: 0 bytes)

# After
julia> @btime ascii(s);
  4.093 μs (0 allocations: 0 bytes)

[JeffBezanson]

Is the improvement due to the inbounds or moving the throw or both? Might be something we should be compiling better here.

[KristofferC]

Both, e.g.

 function ascii2(s::String)
   for i = 1:sizeof(s)
       b = @inbounds codeunit(s,i)
       b < 0x80 || throw(ArgumentError("invalid ASCII at index $i in $(repr(s))"))
   end
   return s
end

function ascii3(s::String)
   for i = 1:sizeof(s)
       b = @inbounds codeunit(s,i)
       b < 0x80 || __throw_invalid_ascii(s, i)
   end
   return s
end

@noinline __throw_invalid_ascii(s, i) = throw(ArgumentError("invalid ASCII at index $i in $(repr(s))"))

julia> @btime ascii2("a")
  7.395 ns (0 allocations: 0 bytes)
"a"

julia> @btime ascii3("a")
  4.451 ns (0 allocations: 0 bytes)
"a"

Guessing it is setting up gc frames and stuff, ref #23281.