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For A Parallel World. Theory lesson n.3: on recursive make

There is one particular topic that was in my TODO list of things to write about in the “For A Parallel World” series, and that topic is recursive make, the most common form of build system in use in this world.

The problems with recursive make has been exposed since at least 1997 by the almost famous Recursive Make Considered Harmful paper. I suggest the reading of the paper to everybody who’s interested in the problems of parallelisation of build systems.

As it turns out, automake supports non-recursive forms quite well, and indeed I use it on at least one project of mine . Lennart also uses it in PulseAudio, as all the modules are built in the same file even though their sources (even the generated ones) are split among different directories.

Unfortunately the paper has to be taken with a grain of salt. The reason why I’m saying that is that if you read it there are at least a couple of places where the author seems to misknown his make rules and defines a rule with two output files, and one where he ignores temporary files naming problems .

There are, of course, other solutions to this problem, for instance I remember the make command for FreeBSD to be able to run into directories in parallel just fine, but I sincerely think here we have to stop one particular problem first. I don’t have too many problems with recursive make for different binaries or final libraries, or for libraries that are shared among targets. Yes they tend to put more serialisation into the process but it’s not tremendously bad, especially not for the non-parallel case where the problem does not seem to appear for most users at all.

What I think is a problem, and I seriously detest it, is when the only reason to use recursive make is to keep the layout of the built object files the same as the source files, and then create sub-directories for logical parts of the same target. With automake this tends to require the creation of convenience noinst libraries that are built and linked against, but never installed. While this works, it tends to increase tremendously the complexity of the whole build, and the time required to build them, since sometimes they get compiled not only into archive file (.a static libraries) but also in final ELF shared objects, depending on their use. Since we know that linking is slow we should try to avoid doing it for no good reason, don’t you think?

In general, the noinst_LTLIBRARIES presence means that either you’re grouping sources that will be used just one in a recursive make system, or you’re creating internal convenience libraries which might even be more evil than that, since it can create two huge binaries like the case of Inkscape.

Once again, if you want your build system reviewed, feel free to drop me an email, depending on how much time I’ve got I’ll do my best to point out eventual fixes, or actually fix it.

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