[cmake-developers] Patch to only consider build dependencies between files in the source directory

[Attila Krasznahorkay]

Hi Brad,

All valid points. Will try to make a new patch today.

Cheers,
           Attila

> On 15 Mar 2016, at 19:13, Brad King <brad.king at kitware.com> wrote:
> 
> On 03/15/2016 10:48 AM, Attila Krasznahorkay wrote:
>> https://cmake.org/pipermail/cmake-developers/2015-December/027197.html
>> 
>> With the following patch, when setting the CMAKE_ONLY_IN_SOURCE_DEPENDENCIES
>> variable to 1/ON/TRUE, this file size goes down to just a few kilobytes.
>> Speeding up our build noticeably.
> 
> Thanks.  As Ben pointed out in the original thread the build tree should
> be included as well.  Perhaps a name like
> 
>  CMAKE_DEPENDS_IN_PROJECT_ONLY
> 
> could be used to indicate that it includes only dependencies in the project
> directories (source or build tree).
> 
>> As far as I can see, the "cmake -E cmake_depends" call is only used by
>> the Makefile generator at the moment. So I only taught that generator
>> about this new variable.
> 
> The Makefile generator is the only one for which CMake takes responsibility
> for implicit dependency scanning.
> 
>> I just didn't see what I should do in the Ninja generator for instance
>> to apply this setting. Even though I would be very interested in implementing
>> the feature in that generator as well.
> 
> The Ninja generator uses the compiler's information about dependencies
> as generated by options like `gcc -MD`.  Since that occurs along with
> the compilation there is little cost to including all dependencies
> except for stat during rebuilds.  See discussion here:
> 
>  https://cmake.org/Bug/view.php?id=14914
> 
> about the opposite use case (including system dependencies instead of
> excluding them).
> 
>> +  infoFileStream
>> +    << "set(CMAKE_ONLY_IN_SOURCE_DEPENDENCIES "
>> +    << ( this->Makefile->IsOn( "CMAKE_ONLY_IN_SOURCE_DEPENDENCIES" ) ?
>> +         "1" : "0" )
>> +    << ")\n";
> 
> Rather than communicating this to the dependency scanning step and
> doing the filtering there, please look at applying the filter during
> generation of CMAKE_<LANG>_TARGET_INCLUDE_PATH.
> 
> Thanks,
> -Brad
>