如果 Makefile 更改，则自动生成源文件

Question

When using an autoconf/automake build system if the compiler flags or other variables in a Makefile.am (or even higher level like configure.ac) change, the C++ source files associated with that Makefile will not be automatically rebuilt. This becomes especially important as we use automake as part of a continuous build system that only recompiles as needed.

My thought was to include Makefile as a dependency for the.o files which would theoretically solve the above issue. So a couple of questions:

First, is it possible to add a rule like that? I would prefer to not have to add that custom rule to every single Makefile.am, so something that could be placed into a top-level file (like configure.ac) would be great.

Second, the downside to this approach is that in some cases the change to the Makefile did not actually affect the compilation so I will end up rebuilding when it is not really needed. I guess I'm willing to live with this (or at least try it to see how painful it is) to have a better guarantee that my builds will be correct, but is there a better way to solve this problem? I believe clearmake solves this by saving the actual compiler command (along with other dependencies) then comparing the current command with the previous to determine if a file needs to be regenerated.

Answer 1

If you use ccache ( ./configure CXX='ccache g++' , or just add ccache's g++ to the path), spurious rebuilds should be very cheap and still safe. Also make sure never to use the AM_MAINTAINER_MODE autoconf macro, which makes dependency tracking optional (conditional on the --enable-maintainer-mode flag).