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由 Jonathan Nieder 提交于
Use the gcc -MMD -MP -MF options to generate dependency rules as a byproduct when building .o files if the COMPUTE_HEADER_DEPENDENCIES variable is defined. That variable is left undefined by default for now. As each object file is built, write a makefile fragment containing its dependencies in the deps/ subdirectory of its containing directory. The deps/ directories should be generated if they are missing at the start of each build. So let each object file depend on $(missing_dep_dirs), which lists only the directories of this kind that are missing to avoid needlessly regenerating files when the directories' timestamps change. gcc learned the -MMD -MP -MF options in version 3.0, so most gcc users should have them by now. The dependencies this option computes are more specific than the rough estimates hard-coded in the Makefile, greatly speeding up rebuilds when only a little-used header file has changed. Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
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