[PATCH] Makefile: add support for generating JSON compilation database

["Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com>]

From: Philippe Blain <levraiphilippeblain@gmail.com>

Tools based on LibClang [1] can make use of a 'JSON Compilation
Database' [2] that keeps track of the exact options used to compile a set
of source files.

The Clang compiler can generate JSON fragments when compiling [3],
using the `-MJ` flag. These JSON fragments (one per compiled source
file) can then be concatenated to create the compilation database,
commonly called 'compile_commands.json'.

Add support to the Makefile for generating these JSON fragments as well
as the compilation database itself, if the environment variable
'GENERATE_COMPILATION_DATABASE' is set.

If this variable is set, check that $(CC) indeed supports the `-MJ`
flag, following what is done for automatic dependencies.

All JSON fragments are placed in the 'compile_commands/' directory, and
the compilation database 'compile_commands.json' is generated as a
dependency of the 'all' target using a `sed` invocation.

[1] https://clang.llvm.org/docs/Tooling.html
[2] https://clang.llvm.org/docs/JSONCompilationDatabase.html
[3] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
    Add support for generating JSON compilation database
    
    I don't have a lot of knowledge of Make double-colon rules, or insight
    into why they are used for the 'all' target, but I think the approach I
    chose makes sense. In particular, I do not list any prerequisite for the
    'compile_commands.json' file, but from what I tested it is still rebuilt
    anytime the 'all' target is rebuilt, which is what we want.
    
    Note: CMakeLists.txt in contrib/buildsystems does not need to be updated
    to also support this feature because CMake supports it out-of-the-box
    [1].
    
    [1] 
    https://cmake.org/cmake/help/latest/variable/CMAKE_EXPORT_COMPILE_COMMANDS.html

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-714%2Fphil-blain%2Fcompiledb-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-714/phil-blain/compiledb-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/714

 .gitignore |  2 ++
 Makefile   | 52 +++++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/.gitignore b/.gitignore
index ee509a2ad2..f4c51300e0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -197,6 +197,7 @@
 /git.spec
 *.exe
 *.[aos]
+*.o.json
 *.py[co]
 .depend/
 *.gcda
@@ -218,6 +219,7 @@
 /tags
 /TAGS
 /cscope*
+/compile_commands.json
 *.hcc
 *.obj
 *.lib
diff --git a/Makefile b/Makefile
index 65f8cfb236..954bd2aa47 100644
--- a/Makefile
+++ b/Makefile
@@ -462,6 +462,12 @@ all::
 # the global variable _wpgmptr containing the absolute path of the current
 # executable (this is the case on Windows).
 #
+# Define GENERATE_COMPILATION_DATABASE to generate JSON compilation database
+# entries during compilation if your compiler supports it, using the `-MJ` flag.
+# The JSON entries will be placed in the `compile_commands/` directory,
+# and the JSON compilation database can be created afterwards with
+# `make compile_commands.json`.
+#
 # Define DEVELOPER to enable more compiler warnings. Compiler version
 # and family are auto detected, but could be overridden by defining
 # COMPILER_FEATURES (see config.mak.dev). You can still set
@@ -1258,6 +1264,20 @@ $(error please set COMPUTE_HEADER_DEPENDENCIES to yes, no, or auto \
 endif
 endif
 
+ifdef GENERATE_COMPILATION_DATABASE
+compdb_check = $(shell $(CC) $(ALL_CFLAGS) \
+	-c -MJ /dev/null \
+	-x c /dev/null -o /dev/null 2>&1; \
+	echo $$?)
+ifeq ($(compdb_check),0)
+override GENERATE_COMPILATION_DATABASE = yes
+else
+override GENERATE_COMPILATION_DATABASE = no
+$(warning GENERATE_COMPILATION_DATABASE is set, but your compiler does not \
+support generating compilation database entries)
+endif
+endif
+
 ifdef SANE_TOOL_PATH
 SANE_TOOL_PATH_SQ = $(subst ','\'',$(SANE_TOOL_PATH))
 BROKEN_PATH_FIX = 's|^\# @@BROKEN_PATH_FIX@@$$|git_broken_path_fix "$(SANE_TOOL_PATH_SQ)"|'
@@ -2381,16 +2401,30 @@ missing_dep_dirs =
 dep_args =
 endif
 
+compdb_dir = compile_commands/
+
+ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
+missing_compdb_dir = $(compdb_dir)
+$(missing_compdb_dir):
+	@mkdir -p $@
+
+compdb_file = $(compdb_dir)$(subst .-,,$(subst /,-,$(dir $@)))$(notdir $@).json
+compdb_args = -MJ $(compdb_file)
+else
+missing_compdb_dir =
+compdb_args =
+endif
+
 ASM_SRC := $(wildcard $(OBJECTS:o=S))
 ASM_OBJ := $(ASM_SRC:S=o)
 C_OBJ := $(filter-out $(ASM_OBJ),$(OBJECTS))
 
 .SUFFIXES:
 
-$(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs)
-	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
-$(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs)
-	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
+$(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
+$(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
 
 %.s: %.c GIT-CFLAGS FORCE
 	$(QUIET_CC)$(CC) -o $@ -S $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
@@ -2413,6 +2447,14 @@ else
 $(OBJECTS): $(LIB_H) $(GENERATED_H)
 endif
 
+ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
+all:: compile_commands.json
+compile_commands.json:
+	@$(RM) $@
+	$(QUIET_GEN)sed -e '1s/^/[/' -e '$$s/,$$/]/' $(compdb_dir)*.o.json > $@+
+	@if test -s $@+; then mv $@+ $@; else $(RM) $@+; fi
+endif
+
 exec-cmd.sp exec-cmd.s exec-cmd.o: GIT-PREFIX
 exec-cmd.sp exec-cmd.s exec-cmd.o: EXTRA_CPPFLAGS = \
 	'-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' \
@@ -3117,7 +3159,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) $(TEST_PROGRAMS)
 	$(RM) $(FUZZ_PROGRAMS)
 	$(RM) $(HCC)
-	$(RM) -r bin-wrappers $(dep_dirs)
+	$(RM) -r bin-wrappers $(dep_dirs) $(compdb_dir) compile_commands.json
 	$(RM) -r po/build/
 	$(RM) *.pyc *.pyo */*.pyc */*.pyo $(GENERATED_H) $(ETAGS_TARGET) tags cscope*
 	$(RM) -r $(GIT_TARNAME) .doc-tmp-dir

base-commit: d9cd4331470f4d9d78677f12dc79063dab832f53
-- 
gitgitgadget