[PATCH v2] Makefile: make the "sparse" target non-.PHONY

Change the "sparse" target and its *.sp dependencies to be non-.PHONY. Before this change "make sparse" would take ~5s to re-run all the *.c files through "cgcc", after it it'll create an empty *.sp file sitting alongside the *.c file, only if the *.c file or its dependencies are newer than the *.sp is the *.sp re-made.

We ensure that the recursive dependencies are correct by depending on the *.o file, which in turn will have correct dependencies by either depending on all header files, or under "COMPUTE_HEADER_DEPENDENCIES=yes" the headers it needs.

This means that a plain "make sparse" is much slower, as we'll now need to make the *.o files just to create the *.sp files, but incrementally creating the *.sp files is *much* faster and less verbose, it thus becomes viable to run "sparse" along with "all" as e.g. "git rebase --exec 'make all sparse'".

On my box with -j8 "make sparse" was fast before, or around 5 seconds, now it only takes that long the first time, and the common case is <100ms, or however long it takes GNU make to stat the *.sp file and see that all the corresponding *.c file and its dependencies are older.

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.

It is critical that we use -Wsparse-error here, otherwise the error would only show up once, but we'd successfully create the empty *.sp file, and running a second time wouldn't show the error. I'm therefore not putting it into SPARSE_FLAGS or SP_EXTRA_FLAGS, it's not optional, the Makefile logic won't behave properly without it.

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.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
Range-diff against v1:
1:  e03fde1b642 ! 1:  059829f2195 Makefile: make the "sparse" target non-.PHONY
    @@ Commit message
         Makefile: make the "sparse" target non-.PHONY
     
         Change the "sparse" target and its *.sp dependencies to be
    -    non-.PHONY. It's now viable to run it as part of a normal compilation
    -    target, as we'll only re-generate these checks if the source *.c file
    -    has changed.
    +    non-.PHONY. Before this change "make sparse" would take ~5s to re-run
    +    all the *.c files through "cgcc", after it it'll create an empty *.sp
    +    file sitting alongside the *.c file, only if the *.c file or its
    +    dependencies are newer than the *.sp is the *.sp re-made.
     
    -    On my box with -j8 it was fast before, or around 5 seconds, now it
    -    only takes that long the first time, and the common case is <100ms, or
    -    however long it takes GNU make to stat the *.sp file and see that all
    -    the corresponding *.c files are older.
    +    We ensure that the recursive dependencies are correct by depending on
    +    the *.o file, which in turn will have correct dependencies by either
    +    depending on all header files, or under
    +    "COMPUTE_HEADER_DEPENDENCIES=yes" the headers it needs.
    +
    +    This means that a plain "make sparse" is much slower, as we'll now
    +    need to make the *.o files just to create the *.sp files, but
    +    incrementally creating the *.sp files is *much* faster and less
    +    verbose, it thus becomes viable to run "sparse" along with "all" as
    +    e.g. "git rebase --exec 'make all sparse'".
    +
    +    On my box with -j8 "make sparse" was fast before, or around 5 seconds,
    +    now it only takes that long the first time, and the common case is
    +    <100ms, or however long it takes GNU make to stat the *.sp file and
    +    see that all the corresponding *.c file and its dependencies are
    +    older.
     
         See 0bcd9ae85d7 (sparse: Fix errors due to missing target-specific
         variables, 2011-04-21) for the modern implementation of the sparse
    @@ Makefile: check-sha1:: t/helper/test-tool$X
      SP_OBJ = $(patsubst %.o,%.sp,$(C_OBJ))
      
     -$(SP_OBJ): %.sp: %.c GIT-CFLAGS FORCE
    -+$(SP_OBJ): %.sp: %.c GIT-CFLAGS
    ++$(SP_OBJ): %.sp: %.c %.o GIT-CFLAGS
      	$(QUIET_SP)cgcc -no-compile $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) \
     -		$(SPARSE_FLAGS) $(SP_EXTRA_FLAGS) $<
     +		-Wsparse-error \
    @@ Makefile: check-sha1:: t/helper/test-tool$X
      sparse: $(SP_OBJ)
      
      EXCEPT_HDRS := command-list.h config-list.h unicode-width.h compat/% xdiff/%
    +@@ Makefile: 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:  9c4cedacdce < -:  ----------- Makefile: do one append in %.hcc rule
3:  e2ad1700f9e < -:  ----------- Makefile: make the "hdr-check" target non-.PHONY

 .gitignore | 1 +
 Makefile   | 9 ++++++---
 2 files changed, 7 insertions(+), 3 deletions(-)