Dear All, In our software infrastructure I now got to the point that in building "high level" packages/projects the CMake build spends a very noticeable amount of time in calculating the dependencies of source files. To give you a feel of the issue, in one example that I was profiling, I build a small "package" that uses >180 other "packages", which at this point are all installed on a network file system (AFS). Those low level packages themselves depend on N external packages as well (like Boost, Oracle, etc.).
So the include list for the build becomes very long, with most of the needed
headers being outside of the source tree. In this case evaluating all the
dependencies for a shared library made up of just 3 source files can take >10
seconds.
I tried to find a way to tell CMake to ignore the dependencies that are outside
of the source tree. As at this point for my use case those files can be
considered fixed. I don't need to consider that they may change without me
knowing. But as far as I could see, there is no mechanism in the code at the
moment to achieve this.
I did consider teaching cmDepends to only take non-system include paths into
account for the dependency calculation. So that whatever is specified as a
system include path, would be ignored. But since I found that GCC becomes
significantly slower in building my sources if I specify >100 -isystem /afs/...
paths to it compared to specifying all those paths with -I, I gave up on this
idea.
I also considered adding a new regular expression to cmDependsC to be able to
specify using a regular expression which include paths to take into account and
which to ignore. Which could still be a good idea to implement in cmDepends
(not cmDependsC) later on, but for my own use case something simpler was enough.
In the end I applied the following patch to CMake 3.4.1 locally to speed it up
for my use case very significantly. Of course this is not a patch that could be
applied to CMake for a general audience. But I do think that if this
code/behaviour could be switched on using something like a directory property /
global variable, a lot of users could make good use of it. As it can be a
reasonable assumption in many development environments that only the changes
inside of the source tree should be tracked by the build system.
If you think that this is a good idea, I would be willing to provide some help
in implementing it "properly" as well.
Cheers,
Attila
cmDepends-sourceOnly.diff
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