Re: [PATCH resend] Makefile: Use computed header dependencies if the compiler supports it
- From
Fredrik Kuivinen <frekui@gmail.com>
- Date
- Aug 18, 2011, 18:34 UTC
- Message-ID
- <20110818183439.GA21560@fredrik-Q430-Q530>
- In-Reply-To
- <20110814200255.GC16819@elie.gateway.2wire.net>
On Sun, Aug 14, 2011 at 03:02:55PM -0500, Jonathan Nieder wrote:
Show 15 quoted lines
> Fredrik Kuivinen wrote: > > On Sun, Aug 14, 2011 at 21:00, Jonathan Nieder <jrnieder@gmail.com> wrote: > > >> I wonder if we can make do with a faster check, like > >> > >> $(CC) -c -MF /dev/null -MMD -MP git.c --help >/dev/null 2>&1 > >> > >> What do you think? > [...] > > Without patch (with COMPUTE_HEADER_DEPENDENCIES=Yes): > > The case to compare against is when COMPUTE_HEADER_DEPENDENCIES is not > set, I'd think, since that is the status quo. And I was talking about > commands like "make clean" that do not care about that feature, not > "make all".
There is no measurable difference between setting and unsetting COMPUTE_HEADER_DEPENDENCIES for me (not surprising as nothing is actually built). "make clean" (in a clean tree) takes more time than "make" (in a fully built tree), so the relative overhead in the make clean case is even smaller. The absolute overhead is, of course, the same.
> [...] > > With patch, but changed to use git.c instead of ++empty.c: > > Did you try with "--help"?
Oh, I missed "--help". But for me gcc always exits with status code 0 when I give it "--help", regardless of what other flags I provide. Therefore, I don't see how "--help" can be used to test for support of -MMD.
Here is an updated patch. It avoids the ++empty.c file by giving "-x c" to the compiler. It also avoids the auto-detection when COMPUTE_HEADER_DEPENDENCIES is set, so if you want to avoid the overhead you can set that in you config.mak.
-- 8< --
Subject: [PATCH] Makefile: Use computed header dependencies if the compiler supports it
Previously you had to manually define COMPUTE_HEADER_DEPENDENCIES to enable this feature. It seemed a bit sad that such a useful feature had to be enabled manually.
To avoid the small overhead we don't do the auto-detection if COMPUTE_HEADER_DEPENDENCIES is already set.
Signed-off-by: Fredrik Kuivinen <frekui@gmail.com> --- Makefile | 13 +++++++++---- 1 files changed, 9 insertions(+), 4 deletions(-)
diff --git a/Makefile b/Makefile index 89cc624..c131439 100644 --- a/Makefile +++ b/Makefile @@ -250,10 +250,6 @@ all:: # DEFAULT_EDITOR='$GIT_FALLBACK_EDITOR', # DEFAULT_EDITOR='"C:\Program Files\Vim\gvim.exe" --nofork' # -# Define COMPUTE_HEADER_DEPENDENCIES if your compiler supports the -MMD option -# and you want to avoid rebuilding objects when an unrelated header file -# changes. -# # Define CHECK_HEADER_DEPENDENCIES to check for problems in the hard-coded # dependency rules. # @@ -1236,6 +1232,15 @@ endif ifdef CHECK_HEADER_DEPENDENCIES COMPUTE_HEADER_DEPENDENCIES = USE_COMPUTED_HEADER_DEPENDENCIES = +else +ifndef COMPUTE_HEADER_DEPENDENCIES +dep_check = $(shell sh -c \ + '$(CC) -c -MF /dev/null -MMD -MP -x c /dev/null -o /dev/null 2>&1; \ + echo $$?') +ifeq ($(dep_check),0) +COMPUTE_HEADER_DEPENDENCIES=YesPlease +endif +endif endif ifdef COMPUTE_HEADER_DEPENDENCIES
-- 1.7.5.3.368.g8b1b7.dirty