[PATCH] Record dependencies during build instead of before

[Austin Clements]

Previously, the makefile created dependency files in a separate, first
pass.  In particular, include-ing the dependency files would cause
make to attempt to rebuild those files using the dependency-generation
rules in the makefile.  Unfortunately, this approach required obtuse
rules and silently delayed the start of the build process (by quite a
bit on a clean tree without any dependency files).  Worse, this
required the dependency files to themselves depend on all of the
headers the source file depended on, which meant that, if a header
file was removed, the depedency file could not be updated because of a
missing dependency (!), which would cause make to silently fail.

This patch eliminates the dependency generation rules and instead
generates dependency files as a side-effect of the regular build rule.
On the first build, we don't need to know the dependencies beforehand;
the object file doesn't exist, so it will be built anyway.  On
subsequent builds, if a header file is updated, the dependency rules
generated by the previous build will force a rebuild.  If a source
file is updated, the dependency rules may be stale, but it doesn't
matter because the updated source file will force a rebuild.

In the final case above, the stale dependency rules may refer to a
header file that no longer exists but is also no longer needed.  In
order to prevent this from breaking the build, we also pass gcc the
-MP option, which generates phony targets for every depended-on header
file, so make won't complain if it can't find them during a later
build.
---
The description is way more complicated than the patch.

 Makefile.local |   18 ++++--------------
 1 files changed, 4 insertions(+), 14 deletions(-)

diff --git a/Makefile.local b/Makefile.local
index 1131dea..525eda0 100644
--- a/Makefile.local
+++ b/Makefile.local
@@ -256,23 +256,12 @@ endif
 quiet ?= $($(shell echo $1 | sed -e s'/ .*//'))
 
 %.o: %.cc $(global_deps)
+	@mkdir -p .deps/$(@D)
+	$(call quiet,CXX $(CXXFLAGS)) -c $(FINAL_CXXFLAGS) $< -o $@ -MD -MP -MF .deps/$*.d
-	$(call quiet,CXX $(CXXFLAGS)) -c $(FINAL_CXXFLAGS) $< -o $@
 
 %.o: %.c $(global_deps)
+	@mkdir -p .deps/$(@D)
+	$(call quiet,CC $(CFLAGS)) -c $(FINAL_CFLAGS) $< -o $@ -MD -MP -MF .deps/$*.d
-	$(call quiet,CC $(CFLAGS)) -c $(FINAL_CFLAGS) $< -o $@
-
-.deps/%.d: %.c $(global_deps)
-	@set -e; rm -f $@; mkdir -p $$(dirname $@) ; \
-	$(CC) -M $(CPPFLAGS) $(FINAL_CFLAGS) $< > $@.$$$$ 2>/dev/null ; \
-	sed 's,'$$(basename $*)'\.o[ :]*,$*.o $@ : ,g' < $@.$$$$ > $@; \
-	rm -f $@.$$$$
-
-.deps/%.d: %.cc $(global_deps)
-	@set -e; rm -f $@; mkdir -p $$(dirname $@) ; \
-	$(CXX) -M $(CPPFLAGS) $(FINAL_CXXFLAGS) $< > $@.$$$$ 2>/dev/null ; \
-	sed 's,'$$(basename $*)'\.o[ :]*,$*.o $@ : ,g' < $@.$$$$ > $@; \
-	rm -f $@.$$$$
 
 .PHONY : clean
 clean:
-- 
1.7.9.1