> On Fri, 28 Dec 2001, Legacy Fishtank wrote:
> >
> > I think one thing to note is that dependencies is that if you are smart
> > about it, dependencies -really- do not even change when your .config
> > changes.
>
> Absolutely. I detest "gcc -MD", exactly because it doesn't get this part
> right. "mkdep.c" gets this _right_.
Well, -MD gets this right. The dependencies it generates will cause a
recompile when necessary. Unfortunately, though, it's too good, because
the dependency on include/linux/autoconf.h will cause lots of unnecessary
recompiles.
But yes, it seems possible to replace the -MD dependency file, which
depends on a specific config, with a generic dependency file, which knows
about our #ifdef CONFIG_XXX and translates them to the corresponding
ifeq(CONFIG_,) Makefile syntax. It'd make an interesting project, but it
effectively means re-implementing a C preprocessor.
I don't think you can blame gcc -MD for not knowing about the kernel's
CONFIG_ system, though ;-)
From
--- #ifdef CONFIG_XXX #include <linux/xxx.h> #endif#ifdef CONFIG_YYY const int nr = 10; #else const int nr = 100; #endif
---you'd have to generate
--- ifeq(CONFIG_XXX,y) DEPS += include/linux/xxx.h endifDEPS += include/config/yyy
---i.e. the include/config trick has to stay any way.
I don't think the above is necessary, though, the following does work pretty good (I did it this way, inspired by mec, and I think kbuild-2.5 does it similarly):
Generate dependencies for a .o file when compiling it.
[ Doing make dep in advance is unnessary. Actually, it's pretty stupid to generate dependencies for *all* possible object files which you are never going to compile (think arch/*). If you don't have the object yet, you don't need to know the dependencies, dependencies only make sense for recompiles. It's also cheaper to generate dependencies during the compile, as you need to read the file anyway. Also, dependencies on generated files cannot be found correctly until these files have been generated. ]
The generated dependencies will always include linux/autoconf.h, which is correct, but will cause too many recompiles. So, replace linux/autoconf.h with linux/config/xxx, where xxx are all the config options which appear in all of the files used to build the object file (which is what -MD gave you).
The result is still dependencies which are 100% correct. It's that simple. The object file gcc generates depends on the command line and all the files it reads during the compile. Why make it more complex?
--Kai
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