ATF's Makefile.am, which is a single Makefile for the whole tree, was already at the 1300 lines mark and growing. At this size, it is unmanageable, and a quick look at its contents reveals tons of repeated delicate code.
Why so much repeated code, you ask, if the whole point of Automake is to simplify Makefiles? Automake does in fact simplify Makefile code when you define targets known by Automake, such as binaries and/or libraries. However, as soon as you start doing fancy things with documentation, building tons of small programs or messing with shell scripts, things get out of control because you are left on your own to define their targets and their necessary build logic.
Up until now, I had just kept up with the boilerplate code... but now that I'm starting to add pretty complex rules to generate HTML and plain text documentation out of XML files, the complexity must go. And here comes my solution:
I've just committed an experiment to process Makefile.am with M4. I've been trying to look for prior art behind this idea and couldn't find any, so I'm not sure how well this will work. But, so far, this has cut down 350 lines of Makefile.am code.
How does this work? First of all, I've written a script to generate the Makefile.am from the Makefile.am.m4 and put it in admin/generate-makefile.sh. All this script does is call M4, but I want to keep this logic in a single place because it has to be used from two call sites as described below.
Then, I've added an autogen.sh script to the top-level directory that generates Makefile.am (using the previous script) and calls autoreconf -is. I'm against autogen.sh scripts that pretend to be smart instead of just calling autoreconf, but in this case I see no other way around it.
At last, I've modified Makefile.am to add an extra rule to generate itself based on the M4 version. This, of course, also uses generate-makefile.sh.
We'll see how this scales, but I'm so far happy with the results.