[CMake] Unity builds (was: Re: [ANNOUNCE] CMake 3.16.0-rc1 is ready for testing)

[Alan W. Irwin]

On 2019-10-10 18:21-0400 Paul Smith wrote:

> On Thu, 2019-10-10 at 14:57 -0400, Robert Maynard via CMake wrote:
>> * The "UNITY_BUILD" target property was added to tell generators to
>>   batch include source files for faster compilation times.
>
> Are there any instructions on how to make this work?  I tried this:
>
>  cmake -G 'Unix Makefiles' -DCMAKE_UNITY_BUILD=ON .
>
> Then ran "make".  The output showed I had just as many output .o files
> as input .cpp files and that make ran one compile command per .cpp
> file.
>
> Is there something else I need to do to enable unity builds in my cmake
> files, than just give the above option?  The docs imply that the above
> is all that's needed.

Hi Paul:

I have never tried unity builds, but your question did inspire me to
look at [the unity build
documentation](https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html)
which if you follow the links to other related documentation does imply what
you have done above is correct and should by default group 8
source-code files into a lump to be compiled together.  Since you are
not getting that behaviour, I am wondering if one or more of the
documented reasons for skipping unity builds is making a difference in your
case. Those reasons are the following:

"The source files marked by GENERATED will be skipped from unity build. This applies also for the source files marked with SKIP_UNITY_BUILD_INCLUSION.

The source files that have COMPILE_OPTIONS, COMPILE_DEFINITIONS, COMPILE_FLAGS, or INCLUDE_DIRECTORIES will also be skipped."

Good luck, and I hope you will keep the list informed of any further
progress you make with your unity build experiments.

Alan
__________________________
Alan W. Irwin

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.org); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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Linux-powered Science
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