[PATCH resend] Makefile: explicitly set target name for autogenerated dependencies

[Jonathan Nieder]

Date: Fri, 18 Nov 2011 17:23:24 -0600

gcc -MF depfile -MMD -MP -c -o path/to/file.o produces a makefile
snippet named depfile describing what files are needed to build the
target given by -o.  When ccache versions before v3.0pre0~187 (Fix
handling of the -MD and -MDD options, 2009-11-01) run, they execute

gcc -MF depfile -MMD -MP -E

instead to get the final content for hashing.  Notice that the -c -o
combination is replaced by -E.  The result is a target name without
a leading path.

Thus when building git with such versions of ccache with
COMPUTE_HEADER_DEPENDENCIES enabled, the generated makefile snippets
define dependencies for the wrong target:

$ make builtin/add.o
GIT_VERSION = 1.7.8.rc3
* new build flags or prefix
CC builtin/add.o
$ head -1 builtin/.depend/add.o.d
add.o: builtin/add.c cache.h git-compat-util.h compat/bswap.h strbuf.h \

After a change in a header file, object files in a subdirectory are
not automatically rebuilt by make:

$ touch cache.h
$ make builtin/add.o
$

Luckily we can prevent trouble by explicitly supplying the name of the
target to ccache and gcc, using the -MQ option.  Do so.

Reported-and-tested-by: Nguyễn Thái Ngọc Duy pclo...@gmail.com
Reported-by: : 허종만 jongman@samsung.com
Signed-off-by: Jonathan Nieder jrnie...@gmail.com
---
 Makefile | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 731b6a8..5a2e02d 100644
--- a/Makefile
+++ b/Makefile
@@ -973,7 +973,8 @@ endif
 
 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),auto)
 dep_check = $(shell $(CC) $(ALL_CFLAGS) \
-   -c -MF /dev/null -MMD -MP -x c /dev/null -o /dev/null 21; \
+   -c -MF /dev/null -MQ /dev/null -MMD -MP \
+   -x c /dev/null -o /dev/null 21; \
echo $$?)
 ifeq ($(dep_check),0)
 override COMPUTE_HEADER_DEPENDENCIES = yes
@@ -1843,7 +1844,7 @@ $(dep_dirs):
 
 missing_dep_dirs := $(filter-out $(wildcard $(dep_dirs)),$(dep_dirs))
 dep_file = $(dir $@).depend/$(notdir $@).d
-dep_args = -MF $(dep_file) -MMD -MP
+dep_args = -MF $(dep_file) -MQ $@ -MMD -MP
 ifdef CHECK_HEADER_DEPENDENCIES
 $(error cannot compute header dependencies outside a normal build. \
 Please unset CHECK_HEADER_DEPENDENCIES and try again)
-- 
1.8.1

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Re: [PATCH resend] Makefile: explicitly set target name for autogenerated dependencies

[Jeff King]

On Fri, Feb 01, 2013 at 02:45:19PM -0800, Jonathan Nieder wrote:

 After a change in a header file, object files in a subdirectory are
 not automatically rebuilt by make:
 
   $ touch cache.h
   $ make builtin/add.o
   $
 
 Luckily we can prevent trouble by explicitly supplying the name of the
 target to ccache and gcc, using the -MQ option.  Do so.

Thanks, I missed the original thread last year, but this does look like
the same problem. The fixed version of ccache is a few years old, but
the OP did mention RHEL5, so that makes sense.

  missing_dep_dirs := $(filter-out $(wildcard $(dep_dirs)),$(dep_dirs))
  dep_file = $(dir $@).depend/$(notdir $@).d
 -dep_args = -MF $(dep_file) -MMD -MP
 +dep_args = -MF $(dep_file) -MQ $@ -MMD -MP

This looks like a nice simple solution. The only downside would be if
-MQ is not supported by some gcc versions (in which case we would do
better to improve the can we use computed dependencies check, and
punish people on old ccache, not people on old gcc who otherwise could
work without -MQ).

As far as I can tell, though, -MQ came along with the other dependency
generation options in 2001:

  http://gcc.gnu.org/news/dependencies.html

which means it's not a problem to rely on. So the patch looks good to
me.

-Peff
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