CMake: Add support for UNITY_BUILD

[jschueller]

Allows to speed-up compilation, or at least avoid erros in vtk where netcdf is used as a third-party

[WardF]

Can you explain what this option is doing, and/or what the errors being encountered & fixed by this are?

[jschueller]

yes, this option allows to merge several source files into a single compilation unit to speed up builds
but when doing that some declarations from different files may conflict, so a workaround is to exclude these files from unity builds
the big picture is avoiding these errors in vtk unity builds, as vtk that uses netcdf as a third-party
see also
https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html

[WardF]

Thanks for providing the inference, I was unfamiliar with UNITY_BUILD. Out of curiosity, how is netCDF being built in a unity build; if I'm reading the reference correctly, this is something that would be enabled via setting target properties in the various CMakeLists.txt files¹, rather than something that would be specified on the command line².

Am I misreading this? Looking at the warning, it says that developers should not, but as well all know, that doesn't mean developers aren't XD. But unless that's what's happening, I wonder if these changes would make more sense wherever the UNITY_BUILD_MODE is being defined in the underlying infrastructure. RIght now, these changes exist in a vacuum, and, absent any context, aren't immediately clear what they're doing or why they're in these files.

A fix might be to modify our project to use UNITY_BUILD natively (thoughts, @K20shores)? That might be a bit down the road, however.

Footnotes

1. https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD_MODE.html#prop_tgt:UNITY_BUILD_MODE ↩

2. https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html ↩

[jschueller]

it can be enabled at configure time with cmake -DCMAKE_UNITY_BUILD=ON which enables UNITY_BUILD for all targets (thats why it was enabled in vtk),

I dont advise to enable it by default because it can be slower for incremental build needed when developing,
its faster for one-time builds (interesting on CI etc)

[WardF]

Ah, I see, the CMAKE_UNITY_BUILD is infact intended for command-line use. Thanks!

I'll take a closer look at this on Monday, we should be able to get it in but I'll want to provide some comments or accompanying information so that these changes don't exist in a contextless vacuum. There's very little chance I'll remember any of these 18 months from now XD.

[WardF]

I added comments to future-proof against my own future confusion, and updated the release notes. Once the tests pass, I'll get this merged in. Thanks @jschueller!