if using MSVC, then add /MP flag to speed up builds

[imrichardcole]

From a basic test using GitHub actions, I compared the Windows build with all configurations workflows.

From here I can see an average of around 2h15m - https://github.com/lballabio/QuantLib/actions/workflows/msvc-all-configs.yml

And in my test it was just over 1h.

[boring-cyborg]

Thanks for opening this pull request! It might take a while before we look at it, so don't worry if there seems to be no feedback. We'll get to it.

[CLAassistant]

All committers have signed the CLA.

[lballabio]

Hi, thanks but I'm a bit confused -- the msvc-all-configs.yml workflow you linked doesn't use cmake...

[imrichardcole]

Hi - oh that's strange indeed. Maybe I just got lucky with my runs, but saw it was much faster.

Looking at the yml file:

msbuild ./QuantLib.sln /verbosity:normal /property:Configuration="${{ matrix.configuration }}" /property:Platform=${{ matrix.platform }}

How is this solution file generated in these pipelines?

[lballabio]

It's not generated, it's maintained.

[imrichardcole]

Noted. So it's interesting, I've run my PR three times now - resulting in 57mins, 1h2mins and 59mins. I had incorrectly put this down to my change, but I guess it must be unrelated.

Do you have a workflow that does use cmake on windows/msvc?

[lballabio]

I think the 2-hours time you're seeing is because the workflow is run weekly at night together with other workflows, and the jobs spend time being queued which is also counted in the overall workflow time. If you click on "show all jobs" in your run at https://github.com/imrichardcole/QuantLib/actions/runs/11765594786 and our latest run at https://github.com/lballabio/QuantLib/actions/runs/11760510340, the times for the single jobs are comparable (you win some, you lose some, but they're always in the same ballpark.)

For a workflow that uses cmake, see https://github.com/lballabio/QuantLib/actions/workflows/cmake.yml; however, on the one hand, it uses sccache which makes it harder to compare times, and on the other hand, it uses ninja as makefile generator which already enables parallel builds.

[coveralls]

coverage: 72.193% (-0.6%) from 72.803%
when pulling 42b550f on imrichardcole:task/use-msvc-mp-flag-for-parallel-compilation
into 585bb09 on lballabio:master.