automake-history: First Take on Dependencies
2.1 First Take on Dependency Tracking
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Description
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Our first attempt at automatic dependency tracking was based on the
method recommended by GNU 'make'. (⇒Generating Prerequisites
Automatically (make)Automatic Prerequisites.)
This version worked by precomputing dependencies ahead of time. For
each source file, it had a special '.P' file that held the dependencies.
There was a rule to generate a '.P' file by invoking the compiler
appropriately. All such '.P' files were included by the 'Makefile',
thus implicitly becoming dependencies of 'Makefile'.
Bugs
----
This approach had several critical bugs.
* The code to generate the '.P' file relied on 'gcc'. (A limitation,
not technically a bug.)
* The dependency tracking mechanism itself relied on GNU 'make'. (A
limitation, not technically a bug.)
* Because each '.P' file was a dependency of 'Makefile', this meant
that dependency tracking was done eagerly by 'make'. For instance,
'make clean' would cause all the dependency files to be updated,
and then immediately removed. This eagerness also caused problems
with some configurations; if a certain source file could not be
compiled on a given architecture for some reason, dependency
tracking would fail, aborting the entire build.
* As dependency tracking was done as a pre-pass, compile times were
doubled-the compiler had to be run twice per source file.
* 'make dist' re-ran 'automake' to generate a 'Makefile' that did not
have automatic dependency tracking (and that was thus portable to
any version of 'make'). In order to do this portably, Automake had
to scan the dependency files and remove any reference that was to a
source file not in the distribution. This process was error-prone.
Also, if 'make dist' was run in an environment where some object
file had a dependency on a source file that was only conditionally
created, Automake would generate a 'Makefile' that referred to a
file that might not appear in the end user's build. A special,
hacky mechanism was required to work around this.
Historical Note
---------------
The code generated by Automake is often inspired by the 'Makefile' style
of a particular author. In the case of the first implementation of
dependency tracking, I believe the impetus and inspiration was Jim
Meyering. (I could be mistaken. If you know otherwise feel free to
correct me.)
Info Catalog
automake-history: Dependency Tracking Evolution
automake-history: Dependencies As Side Effects