Explain better for speed at runtime

[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.

[MartinKavik]

My experience with these options as a Seed maintainer - some algorithms with s/z can be almost 2x slower, but the app can be also 2x smaller (vs 3).
So it really depends on the specific use-case - you should write at least some benchmarks and try multiple options.

[johnthagen]

@frederikhors This repo is concerned with helping Rust developers generate the smallest possible binary size at the expense of all else. Many of the recommendations in this repo would not make sense unless you were trying to optimize soley for size.

For what the opt-level's specifically do, I do agree that the cargo docs could be improved but I suggest that an issue be opened in that repo to request. That question is mostly for rustc / LLVM.

[frederikhors]

@johnthagen rust-lang/cargo#8031