[PATCH v4] Makefile: add a non-.PHONY "sparse-incr" target

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add a "sparse-incr" target to compliment the existing "sparse"
target. The new "sparse-incr" target doesn't depend on "FORCE", and
will create empty *.sp files as markers for the corresponding *.c file
having been checked.

Those *.sp files in turn depend on the *.o files, so we can be certain
that the dependencies are correct by either depending on all header
files, or under "COMPUTE_HEADER_DEPENDENCIES=yes" the headers the
relevant file needs.

The "sparse-incr" target is slower on a fresh git.git checkout, as it
depends on the creation of the *.o files. But once the *.o and *.sp
files are built it's able to incrementally update them. It's thus
viable to run:

    make all sparse-incr

As part of a regular edit/compile/test cycle, or as a fast "git rebase
--exec" command.

On my box (with -j8) the initial run of the "sparse" target takes ~5s,
but ~16s for "sparse-incr". When using CC="ccache cc" the difference
between the two is negligible.

Running e.g.:

    make grep.sp

Will behave the same way and will always re-run "cgcc", we'll only use
the new dependency chain of "sparse-incr" is part of the MAKECMDGOALS.

I think it would make sense to just remove the "sparse" target
entirely, and to say that anyone who cares about the speed of an
initial "sparse" run should use "CC='ccache cc'". But per [1] and [2]
there are existing users of "make sparse" and "make <file>.sp" that
prefer the current semantics.

I.e. per [2] want "make <file>.sp" to *always* run "sparse", even
though a corresponding "make <file>.o" would only re-run the "real"
compilation if needed. I don't think that makes any sense, especially
in combination with DEVELOPER=1 which'll ensure that -Werror would
have made any errors in a "make <file>.sp" sticky.

But since we have existing users relying on it, and I don't really
care at al about "make <file>.sp", I just want an incremental target I
can use, let's create this new "make sparse-incr" instead of "fixing"
the existing "make sparse".

See 0bcd9ae85d7 (sparse: Fix errors due to missing target-specific
variables, 2011-04-21) for the modern implementation of the "sparse"
target being changed here.

Appending to $@ without a move is OK here because we're using the
.DELETE_ON_ERROR Makefile feature. See 7b76d6bf221 (Makefile: add and
use the ".DELETE_ON_ERROR" flag, 2021-06-29). GNU make ensures that on
error this file will be removed.

1. https://lore.kernel.org/git/xmqqtuib199x.fsf@gitster.g/
2. https://lore.kernel.org/git/457ec039-1e26-9da9-55f6-9ea79b962bfe@ramsayjones.plus.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---

Not so long since v3, it was hopelessly broken, sorry. I was doing
some last-minute tweaking of the ifeq/ifneq conditions, so any target
in the Makefile that wasn't "sparse" or "sparse-incr" broke.

Now we do the right thing by just splitting up the check, if you
supply "sparse" and "sparse-incr" (in any order, and even among other
targets) we'll error, they're incompatible.

Then we separately check if the "sparse-incr" target has been
specified, if it has we'll use the new dependency mechanism, if not
we'll use the old behavior.

Range-diff against v3:
1:  b6ba99ca4cc ! 1:  f31fa3e8282 Makefile: add a non-.PHONY "sparse-incr" target
    @@ Makefile: check-sha1:: t/helper/test-tool$X
      		$(SPARSE_FLAGS) $(SP_EXTRA_FLAGS) $<
     +endef
     +
    -+ifneq ($(filter sparse-incr,$(MAKECMDGOALS)),sparse-incr)
    -+ifneq ($(filter sparse,$(MAKECMDGOALS)),)
    ++ifeq ($(sort $(filter sparse sparse-incr,$(MAKECMDGOALS))),sparse sparse-incr)
     +$(error The sparse and sparse-incr targets cannot be combined!)
     +endif
    ++
    ++ifneq ($(filter sparse-incr,$(MAKECMDGOALS)),sparse-incr)
     +$(SP_OBJ): %.sp: %.c GIT-CFLAGS FORCE
     +	$(cmd_run_sparse)
     +else

 .gitignore |  1 +
 Makefile   | 19 +++++++++++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/.gitignore b/.gitignore
index 311841f9bed..b02250a50c4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -224,6 +224,7 @@
 *.lib
 *.res
 *.sln
+*.sp
 *.suo
 *.ncb
 *.vcproj
diff --git a/Makefile b/Makefile
index a9f9b689f0c..fd623523394 100644
--- a/Makefile
+++ b/Makefile
@@ -2896,12 +2896,26 @@ check-sha1:: t/helper/test-tool$X
 
 SP_OBJ = $(patsubst %.o,%.sp,$(C_OBJ))
 
-$(SP_OBJ): %.sp: %.c GIT-CFLAGS FORCE
+define cmd_run_sparse
 	$(QUIET_SP)cgcc -no-compile $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) \
 		$(SPARSE_FLAGS) $(SP_EXTRA_FLAGS) $<
+endef
+
+ifeq ($(sort $(filter sparse sparse-incr,$(MAKECMDGOALS))),sparse sparse-incr)
+$(error The sparse and sparse-incr targets cannot be combined!)
+endif
+
+ifneq ($(filter sparse-incr,$(MAKECMDGOALS)),sparse-incr)
+$(SP_OBJ): %.sp: %.c GIT-CFLAGS FORCE
+	$(cmd_run_sparse)
+else
+$(SP_OBJ): %.sp: %.c %.o GIT-CFLAGS
+	$(cmd_run_sparse) >$@
+endif
 
-.PHONY: sparse $(SP_OBJ)
+.PHONY: sparse sparse-incr
 sparse: $(SP_OBJ)
+sparse-incr: $(SP_OBJ)
 
 EXCEPT_HDRS := command-list.h config-list.h unicode-width.h compat/% xdiff/%
 ifndef GCRYPT_SHA256
@@ -3227,6 +3241,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
 	$(RM) $(FUZZ_PROGRAMS)
+	$(RM) $(SP_OBJ)
 	$(RM) $(HCC)
 	$(RM) -r bin-wrappers $(dep_dirs) $(compdb_dir) compile_commands.json
 	$(RM) -r po/build/
-- 
2.33.0.1326.g5e4342b7bef