Quick test from a branch which has the deploy_windows.ps1 changes from this PR, but does not install jom in the prepare script: https://github.com/hoffie/jamulus/runs/5387646863?check_suite_focus=true#step:9:126

(This was before @ann0see's suggestion to add an explicit echo for this case, so don't search for it ;))

• Build logic still works even without jom installed

Should we test this locally too?

I can't. I think I would be OK without explicit testing:

• I've tried a run without jom installed (posted a comment in the same minute as yours)

• I'd argue that only developers manually build on Windows anyway (contrary to Linux). So if there's really an issue, it'll hit someone qualified. :)

• We're still in main 3.9.0 development phase

• There are hints that this build logic does not kick in when building from Qt creator or the MSVC GUI, so it'll only affect people who build manually (similar to autobuild). Source: https://developex.com/blog/how-to-make-your-c-qt-project-build-10x-faster-with-4-optimizations/

  If you build with MSVC – you will internally use ‘jom’ or ‘nmake’ for which you don’t need to specify any extra parameters.
  [...]
  In cases where you work with Qt Creator – you’ll likely use ‘jom’ internally, you can see it in “Projects” -> “Build Settings” -> “Build Steps” : “Make: jom.exe …”, if there’s ‘nmake.exe’ – you can click “Details” and override it with ‘jom’, which is a free addition to Qt.
  [...]
  The Microsoft compiler handles parallelism by itself, while ‘nmake’ – doesn’t, so custom build steps won’t run in parallel unless you use ‘make’ or ‘jom’.

I'm going to add the other suggestion (/MP) as well, maybe it provides even more benefits.

I'm going to add the other suggestion (/MP) as well, maybe it provides even more benefits.

Well, I won't -- it'll only help if multiple files are compiled at once in the Makefile. This doesn't seem to be the case, at least I don't see any cl calls with multiple .cpp targets.

In Linux I thought multiple files were compiled in parallel (at least locally) and I don’t know if you can judge via CI log.

There are two places where "parallelization" can happen:

• By running compiler invocations in parallel (that's what make -j and jom /N do. That's not necessarily visible in build logs, yep.
• By compiling multiple source files in a single compiler invocation. This isn't real parallelization, but it'll likely reduce overall compiler warm up times, .h parsing, etc. This should be visible in the build logs as the invocation would have to change for that. That's where /MP would be relevant. But if we don't expect benefits, I'm a bit hesistant to add it.