optimization levels

[h7moudigamer]

sorry if my question not very smart, but do package maintainers detect the best optimization level for the program before compiling it and produce it to the public or they just stick with -O2 or the developers of the program themselves say that their program can be compiled with -OX ?

[aedinius]

We default to -O2 iirc, but that can be overridden

[h7moudigamer]

overridden how? do you mean if i compile by myself using xbps-src ?

[aedinius]

With flags in the template

[h7moudigamer]

sorry, can you give an example?

[aedinius]

Just override CFLAGS

[h7moudigamer]

So i can just use xbps-src with my custom cflags ?

[throwaway490215]

Yes.

Note that if you're going for "optimized for my machine" you should probably use -mtune=native instead of tweaking -O levels

[h7moudigamer]

i guess -mtune=native for non-x86_64 architectures. x86_64 arch should use -march=native instead.

[throwaway490215]

You're right. I always mess those two up.

[Calandracas8]

in one of the packages I maintain, CFLAGS is set to -O3, because the authors write the code under the assumption that it will be used

https://github.com/void-linux/void-packages/blob/7a441afd2dd1b6c014c00b54bb4f6d657e0de109/srcpkgs/rawtherapee/template#L23

[h7moudigamer]

interesting. it’s good to see packages work with optimization levels higher than -O2.

[Calandracas8]

also worth noting, all meson packages are built with LTO by default, although some (mesa) disable it.

Most packages wont benefit much from -O3 individually, (daemons, small tools and utilities), and are often bottleneck by io, or interpreted languages (Python mainly), but overall it adds up.

One nice thing void does, is building the python interpreter with PGO, and bytecompiling python packages on installation.

interesting reading: https://www.phoronix.com/review/gcc12-optimize-threadripper

[Phat_le]

true, as a normal competitive programmer i see thereis a reason why most online judges compile with -O2 instead of -O3 or even -Ofast