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?

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[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