foot: Add PGO build option

[travankor]

PGO optimization speed foot up by a significant amount. Add a build option that enables PGO during build.

[ericonr]

Definitely not on by default, since PGO is affected by the CPU that's being used.

[dnkl]

since PGO is affected by the CPU that's being used.

It's actually not. PGO counts functions and branches executed, and helps re-organize the code; it clusters "hot" functions, to improve cache locality. It can also re-arrange code within a function to make the "normal" path the fast path (i.e. placing the not-often executed branches at e.g. the end of the function). But PGO in itself does not emit CPU specific instructions, nor does it depend on CPU-specific knowledge for e.g. instruction scheduling.

Over at nix, @sternenseemann has been able to do reproducable PGO builds. That is, builds on different systems produce identical output. I can't for my life find the issue, or comment, where I saw this, but pretty sure sterni can fill in the details, if needed.

Oh, and as a bonus, PGO builds tend to be smaller (in binary size) than regular release builds. This is at least true for LTO builds (which I strongly recommend building foot with, with or without PGO).

[sternenseemann]

I can confirm that building foot with PGO can be reproducible, i. e. I can reproduce the exact hash of the binary package produced by our builders on two different local machines (at different times). This also leads me to believe that a reproducible gcc with PGO is possible, but most likely very hard to get to work.

The only thing necessary to achieve a reproducible foot build is generating the random inputs which are used for profiling while building using a fixed seed.

[github-actions[bot]]

Issues become stale 90 days after last activity and are closed 14 days after that. If this issue is still relevant bump it or assign it.

[ericonr]

Not stale. @ifreund do you have anything in effect for this? I think I had a branch attempting it a while back

[ifreund]

@ericonr nope, I haven't tried to implement this yet.

[github-actions[bot]]

Issues become stale 90 days after last activity and are closed 14 days after that. If this issue is still relevant bump it or assign it.