This is a GNU Make dependency database plugin inspired by a similar functionality in Ninja (cf. .ninja_deps). It can reduce the startup time of the projects consisting of an extremely large number of source files by merging all .d files into a single database.
An example usage:
ifneq (,$(filter load,$(.FEATURES))) # load supported
DEPDIR := .deps # output directory
CPPFLAGS += -MMD -MP # compiler flags to generate dependencies
load dbdeps.dll # load plugin
$(dbdeps $(DEPDIR)) # read in all dependencies
all : $(DEPDIR)/data.mdb # mark database as a target for update
# the rules to update database from .d files
# note: double colon is needed for splitting a single rule into several ones
$(DEPDIR)/data.mdb :: $(objects1) ;@updbdeps --remove $(?:.o=.d) -d $(@D)
$(DEPDIR)/data.mdb :: $(objects2) ;@updbdeps --remove $(?:.o=.d) -d $(@D)
endif