add derivation parser benchmark

[Mic92]

the current identified bottlenecks are parseString in derivations.cc and dirOf (because of std::filessystem creation).

[Mic92]

So this line is expensive:

nix/src/libutil/file-system.cc

Line 158 in 47f5e5f

return std::filesystem::path{path}.parent_path().string();

And this function is hot:

nix/src/libstore/derivations.cc

Line 192 in 47f5e5f

for (; c != end && *c != '"'; c++) {

This is what I did in rust: https://github.com/Mic92/nix-diff-rs/blob/bd5d3d2ba4470b3b4a9acddcd0d5ba9968235416/src/parser.rs#L181

What is the equivalent of memchr2 in C++?

[Mic92]

Looks like this option is not the right approach w.r.t. componentized builds? It works in a devshell only.

[xokdvium]

Maybe add the option only to the subprojects that have benchmarks and make that option yield?

[Mic92]

I could move benchmarks to its own subprojects maybe? Or we just always build them but not run them.

[xokdvium]

I could move benchmarks to its own subprojects maybe

Not sure if it's warranted for now. Can we stick with just 2 options: in libstore-tests and the top-level file, and the one in libstore-tests with yield: true.

Or we just always build them but not run them.

Not a huge fan, because we'd have an unconditional dep on gbenchmark in nixpkgs.

[Ericson2314]

Or we just always build them but not run them.

Not a huge fan, because we'd have an unconditional dep on gbenchmark in nixpkgs.

Does the componentized build help here? We would always build the benchmarks, but we don't have to run time, and the thing users actually install won't depend on them.

[xokdvium]

We would always build the benchmarks, but we don't have to run time, and the thing users actually install won't depend on them.

Makes sense, yeah. I suppose introducing configurability isn't a top priority here.

[Ericson2314]

I think we can merge this now, and maybe then consider removing the flags.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/getting-started-with-optimizing-the-nix-project/67342/1

[xokdvium]

Very good start. Very excited about improved tooling. We can improve on it in follow-ups.