meson: use optimization level 3 and LTO by default

[ConnorBaker]

Motivation

Let Meson handle optimization arguments for us.

Additionally, enables building with LTO, which was lost in the transition to Meson.

Testing

I'm not sure what the state-of-the-art is for benchmarking Nix. Any pointers?

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[Ericson2314]

Won't this slow down debug builds?

[ConnorBaker]

Won't this slow down debug builds?

In which sense? Speed of compilation, speed of execution, or something else?

I guess it could slow down compilation depending on how costly the level three optimizations are. I don’t know that LTO is a bad idea, generally.

I could refactor this to conditionally set optimization level to three when it isn’t a debug build, similar to what I removed in the refactor.

I’d also really appreciate any pointers you can give on reliably benchmarking Nix!

[MagicRB]

This should be 100% disabled for debug builds, optimization can make chabnes to the code which make debugging really hard, inlining, the infamous "where did my code go? right it was optimized away/unrolled"

[ConnorBaker]

Should debug builds be built without any optimization? Because that’s not what happens currently :/

[MagicRB]

I'm not part of the Nix dev team, but in my experience having all optimizations off greatly helps staring at code in gdb/lldb. Since the machine code is closer to what's actually written the source code

[Ericson2314]

Yes for debug builds we like less optimization both because it means faster builds, and because it means the debug symbols are easier to understand / closer to the code.

[ConnorBaker]

@Ericson2314 so should debug builds happen without any optimization, or with optimization level two (which is what happens prior to this PR)?

[Ericson2314]

I think without any optimization, right @edolstra?

We could have the full nix builds instead specify more optimization with mesonFlags I think?

[Mic92]

@Ericson2314 so should debug builds happen without any optimization, or with optimization level two (which is what happens prior to this PR)?

We should use -Og rather than -O0. It's optimization with debugger in mind.