[RFC] Improve build time

[rolfbjarne]

This is a list of ideas that can make our build faster.

• [x] Mono SDK as binaries (this is already in progress)
• [x] Put binaries of the F# libraries in macios-binaries. Fixed Building F# takes ~2:20 on the public jenkins bots (1:27 for me locally).
• [ ] Reduce the number of source files in runtime/, each source file is compiled ~30 times (we have 595 .o files from 20 .m/.c files). Building runtime/ takes ~1:14 on the bots (0:23 for me locally).

• [ ] A big chunk of time in src/ is spent in the generator, which we’ve never optimized. It might be worth it to spend some time profiling if there are any easy things to improve/fix. Building all of src/ takes ~5:53 on the bots (1:27 for me locally) Locally running the generator for all platforms takes 36s (~40% of the total time):

  $ cd src
  $ git clean -xfd
  $ make -j build/ios/native/generated_sources build/mac/full/generated-sources build/mac/mobile/generated-sources build/tvos/tvos/generated_sources build/watch/watch/generated_sources
  [...]
  $ rm build/ios/native/generated_sources build/mac/full/generated-sources build/mac/mobile/generated-sources build/tvos/tvos/generated_sources build/watch/watch/generated_sources
  $ time make -j build/ios/native/generated_sources build/mac/full/generated-sources build/mac/mobile/generated-sources build/tvos/tvos/generated_sources build/watch/watch/generated_sources
  [...]
  real    0m36.732s

Bot timings are from https://jenkins.mono-project.com/view/Xamarin.MaciOS/job/xamarin-macios-pr-builder/8386/consoleFull.

[spouliot]

Beside binaries (like mono and F#) but before optimizing any code I think we need a pie chart (or the data to make one) of where the time is spent, even if made without -jX, in comparison with the total build time.

That will also give us a baseline to see the % gain of using mono binaries in the build.