Understand build optimization for runtime speed

[frederikhors]

I read both:

• https://doc.rust-lang.org/cargo/reference/profiles.html#opt-level
• https://github.com/johnthagen/min-sized-rust#optimize-for-size

What I don't understand is which speed gain I'm changing if I use:

[profile.release]
opt-level = "z"

instead of:

[profile.release]
opt-level = 3

• Is it right that today opt-level = 3 is the best setting (for opt-level section) for runtime speed?

• If I instead use opt-level = "z" I'm decreasing runtime performance, right?

I'm not interested in building/compiling speed.

[alice]

Yes, opt-level 3 is the most efficient setting. You may also want to turn on lto and set codegen-units to 1 for optimal runtime performance.

And yes, opt-level z is the least performant optimization level.