From a2f50d4a0a253e9cb6eaaf0e0e1c67dd02b717ac Mon Sep 17 00:00:00 2001 From: Brian Norris Date: Thu, 27 Aug 2015 15:48:22 -0700 Subject: autogenerated dependency files are not being utilized properly TL;DR Acked-by: Mike Frysinger ===== Auto-generated dependency rules are not being written correctly, so changes to dependent files (e.g., headers) do not actually trigger rebuilds. The problem =========== It appears that when a dependency generation flag is passed directly to the preprocessor (with '-Wp,...'), it loses information about the output path. So, it just makes up the output name as $(basename).o, with no path information. This yields .*.c.dep files that look like this: flash_lock.o: flash_lock.c /usr/include/stdc-predef.h flash_unlock.c \ (...) and nanddump.o: nanddump.c /usr/include/stdc-predef.h /usr/include/ctype.h \ (...) include/libmtd.h This is the case for both in-tree *and* out-of-tree builds. Naturally, this is a problem for out-of-tree builds. But it is also a problem for in-tree builds, because we use rules like this for builds: $(BUILDDIR)/%.o: %.c and make doesn't recognize $(BUILDDIR)/%.o as the same as %.o even when $(BUILDDIR) == $(PWD). Example failures ================ ## Rebuilding after touching common header doesn't recompile anything $ make (...) $ touch include/libmtd.h $ make CHK include/version.h ## Same for out-of-tree builds $ BUILDDIR=test make (...) $ touch include/libmtd.h $ BUILDDIR=test make CHK include/version.h I noticed this when seeing that flash_lock would not get rebuilt when modifying flash_unlock.c (where 99% of the source code lies): $ make (...) $ touch flash_unlock.c $ make CHK include/version.h CC flash_unlock.o LD flash_unlock The fix ======= Just pass -MD straight to the compiler, and make sure to specify the output file for the dependency info with -MF. Signed-off-by: Brian Norris Cc: Mike Frysinger Cc: David Woodhouse Signed-off-by: Brian Norris --- common.mk | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/common.mk b/common.mk index ba87377..7c09ab0 100644 --- a/common.mk +++ b/common.mk @@ -80,7 +80,7 @@ ifneq ($(BUILDDIR),$(CURDIR)) $(Q)mkdir -p $(dir $@) endif $(call BECHO,CC) - $(Q)$(CC) $(CPPFLAGS) $(CFLAGS) -c -o $@ $< -g -Wp,-MD,$(BUILDDIR)/.$(