Re: [PATCH resend] Makefile: Use computed header dependencies if the compiler supports it

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Aug 14, 2011, 19:00 UTC
Message-ID: <20110814190050.GA16819@elie.gateway.2wire.net>
In-Reply-To: <1313347512-7815-1-git-send-email-frekui@gmail.com>

Hi,

Fredrik Kuivinen wrote:

> 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.

Yes!  Thanks for this.

I have a few thoughts about the implementation:

> --- a/Makefile
> +++ b/Makefile

[...]

Show 13 quoted lines

> @@ -1236,6 +1232,15 @@ endif
>  ifdef CHECK_HEADER_DEPENDENCIES
>  COMPUTE_HEADER_DEPENDENCIES =
>  USE_COMPUTED_HEADER_DEPENDENCIES =
> +else
> +dep_check = $(shell sh -c \
> +	': > ++empty.c; \
> +	$(CC) -c -MF /dev/null -MMD -MP ++empty.c -o /dev/null 2>&1; \
> +	echo $$?; \
> +	$(RM) ++empty.c')
> +ifeq ($(dep_check),0)
> +COMPUTE_HEADER_DEPENDENCIES=YesPlease
> +endif

This causes "make foo" to run gcc and create a temporary file unconditionally, regardless of what foo is. In an ideal world:

 - the autodetection would only happen when building targets that
   care about it

 - the detection would happen once (creating some file to store the
   result) and not be repeated with each invocation of "make"

 - (maybe) there would be a way to override the detection with
   either a "yes" or "no" result, for those who really care to
   save a little time.

I was about to say that the GIT_VERSION variable has some of these properties, but now that I check, from the point of view of the Makefile it doesn't. ./GIT-VERSION-GEN is just very fast. :)

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?