[PATCH v3 09/11] cocci: optimistically use COMPUTE_HEADER_DEPENDENCIES

Improve the incremental rebuilding support of "coccicheck" by piggy-backing on the computed dependency information of the corresponding *.o file, rather than rebuilding all <RULE>/<FILE> pairs if either their corresponding file changes, or if any header changes.

This in effect uses the same method that the "sparse" target was made to use in c234e8a0ecf (Makefile: make the "sparse" target non-.PHONY, 2021-09-23), except that the dependency on the *.o file isn't a hard one, we check with $(wildcard) if the *.o file exists, and if so we'll depend on it.

This means that the common case of:

	make
	make coccicheck

Will benefit from incremental rebuilding, now changing e.g. a header will only re-run "spatch" on those those *.c files that make use of it:

By depending on the *.o we piggy-back on COMPUTE_HEADER_DEPENDENCIES. See c234e8a0ecf (Makefile: make the "sparse" target non-.PHONY, 2021-09-23) for prior art of doing that for the *.sp files. E.g.:

    make contrib/coccinelle/free.cocci.patch
    make -W column.h contrib/coccinelle/free.cocci.patch

Will take around 15 seconds for the second command on my 8 core box if I didn't run "make" beforehand to create the *.o files. But around 2 seconds if I did and we have those "*.o" files.

Notes about the approach of piggy-backing on *.o for dependencies:

 * It *is* a trade-off since we'll pay the extra cost of running the C
   compiler, but we're probably doing that anyway. The compiler is much
   faster than "spatch", so even though we need to re-compile the *.o to
   create the dependency info for the *.c for "spatch" it's
   faster (especially if using "ccache").

 * There *are* use-cases where some would like to have *.o files
   around, but to have the "make coccicheck" ignore them. See:
   https://lore.kernel.org/git/20220826104312.GJ1735@szeder.dev/

   For those users a:

	make
	make coccicheck SPATCH_USE_O_DEPENDENCIES=

   Will avoid considering the *.o files.

 * If that *.o file doesn't exist we'll depend on an intermediate file
   of ours which in turn depends on $(FOUND_H_SOURCES).

   This covers both an initial build, or where "coccicheck" is run
   without running "all" beforehand, and because we run "coccicheck"
   on e.g. files in compat/* that we don't know how to build unless
   the requisite flag was provided to the Makefile.

   Most of the runtime of "incremental" runs is now spent on various
   compat/* files, i.e. we conditionally add files to COMPAT_OBJS, and
   therefore conflate whether we *can* compile an object and generate
   dependency information for it with whether we'd like to link it
   into our binary.

   Before this change the distinction didn't matter, but now one way
   to make this even faster on incremental builds would be to peel
   those concerns apart so that we can see that e.g. compat/mmap.c
   doesn't depend on column.h.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)