autogenerated dependency files are not being utilized properly
TL;DR
Acked-by: Mike Frysinger <vapier@gentoo.org>
=====
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 <computersforpeace@gmail.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
1 file changed
