[PATCH 1/3] 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.

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.

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 <redacted>
---
 .gitignore | 1 +
 Makefile   | 8 +++++---
 2 files changed, 6 insertions(+), 3 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..5b09f67aab0 100644
--- a/Makefile
+++ b/Makefile

@@ -2896,11 +2896,13 @@ 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
 	$(QUIET_SP)cgcc -no-compile $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) \
-		$(SPARSE_FLAGS) $(SP_EXTRA_FLAGS) $<
+		-Wsparse-error \
+		$(SPARSE_FLAGS) $(SP_EXTRA_FLAGS) $< && \
+	>$@
 
-.PHONY: sparse $(SP_OBJ)
+.PHONY: sparse
 sparse: $(SP_OBJ)
 
 EXCEPT_HDRS := command-list.h config-list.h unicode-width.h compat/% xdiff/%

-- 
2.33.0.1098.gf02a64c1a2d