improve performance for materializing a string from a vector of Char

[KristofferC]

Before

julia> a = rand(Char, 10^4);

julia> @btime String(a);
  363.761 μs (6 allocations: 49.20 KiB)

After

julia> a = rand(Char, 10^4);

julia> @btime String(a);
  51.856 μs (1 allocation: 38.56 KiB)

The unrolling of the loop to write the char made a significant difference here (but is not where the major performance benefit comes from).

cc @bkamins

[KristofferC]

@nanosoldier runbenchmarks(ALL, vs = ":master")

[KristofferC]

@ararslan, any idea how nanosoldier is doing?

[ararslan]

Hm, there were no errors in the log or anything, it just seemed to be stuck. I've restarted the server, so let's just try again. @nanosoldier runbenchmarks(ALL, vs=":master")

[bkamins]

Maybe the signature could be a::AbstractVector{Char}?

[bkamins]

maybe name it __unsafe_string! to highlight the fact that it does not perform bounds checking?

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[vtjnash]

Looks like a good optimization, but I disagree that this should be backported.

[KristofferC]

fine

[ararslan]

but I disagree that this should be backported.

Can you elaborate on why?

[StefanKarpinski]

Looks like a good optimization, but I disagree that this should be backported.

Why?