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

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

[Philippe Blain via GitGitGadget]

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

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

[brian m. carlson]

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On 2020-08-30 at 19:28:27, Philippe Blain via GitGitGadget wrote:
> 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.

For additional context why this is valuable, clangd, which is a C
language server protocol implementation, can use these files to
determine the flags needed to compile a file so it can provide proper
editor integration.  As a result, editors supporting the language server
protocol (such as VS Code, or Vim with a suitable plugin) can provide
better searching, integration, and refactoring tools.

So I'm very much in favor of a change like this.

> +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

How are those commas at the end of the line added?  Are they natively
part of the files?  If so, this seems reasonable.
-- 
brian m. carlson: Houston, Texas, US

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 263 bytes --]

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

[Philippe Blain]

> #
> +# 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`.
> +#

I'm realizing that the way I'm describing how it works here is wrong: there is no
separate 'make compile_commands.json' step needed (it was needed in my first draft).

I'll fix that for v2.

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

[Philippe Blain]

Hi Brian, 

> Le 30 août 2020 à 18:10, brian m. carlson <sandals@crustytoothpaste.net> a écrit :
> 
> On 2020-08-30 at 19:28:27, Philippe Blain via GitGitGadget wrote:
>> 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.
> 
> For additional context why this is valuable, clangd, which is a C
> language server protocol implementation, can use these files to
> determine the flags needed to compile a file so it can provide proper
> editor integration.  As a result, editors supporting the language server
> protocol (such as VS Code, or Vim with a suitable plugin) can provide
> better searching, integration, and refactoring tools.
> 
> So I'm very much in favor of a change like this.

Thanks!

> 
>> +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
> 
> How are those commas at the end of the line added?  Are they natively
> part of the files?  If so, this seems reasonable.

Yes: the '*.o.json' files generated by the compiler contain one JSON object per file, 
with a trailing comma.
This 'sed' invocation turns these files into a proper JSON array by:
- adding a '[' at the beginning and a ']' at the end of the list of objects
- removing the comma after the last entry (before the closing ']')

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

[Junio C Hamano]

"brian m. carlson" <sandals@crustytoothpaste.net> writes:

> On 2020-08-30 at 19:28:27, Philippe Blain via GitGitGadget wrote:
>> 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.
>
> For additional context why this is valuable, clangd, which is a C
> language server protocol implementation, can use these files to
> determine the flags needed to compile a file so it can provide proper
> editor integration.  As a result, editors supporting the language server
> protocol (such as VS Code, or Vim with a suitable plugin) can provide
> better searching, integration, and refactoring tools.

I found that the proposed commit log was very weak to sell the
change; some of what you gave above should definitely help strenthen
it.

Thanks.

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

[Jeff King]

On Sun, Aug 30, 2020 at 09:24:03PM -0700, Junio C Hamano wrote:

> "brian m. carlson" <sandals@crustytoothpaste.net> writes:
> 
> > On 2020-08-30 at 19:28:27, Philippe Blain via GitGitGadget wrote:
> >> 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.
> >
> > For additional context why this is valuable, clangd, which is a C
> > language server protocol implementation, can use these files to
> > determine the flags needed to compile a file so it can provide proper
> > editor integration.  As a result, editors supporting the language server
> > protocol (such as VS Code, or Vim with a suitable plugin) can provide
> > better searching, integration, and refactoring tools.
> 
> I found that the proposed commit log was very weak to sell the
> change; some of what you gave above should definitely help strenthen
> it.

Likewise. Looking at the output, I'm confused how it would help with
things like searching and refactoring. It might be nice to spell it out
for those of us exposed to it for the first time (I tried following the
links but remained unenlightened).

I'd also be curious to hear what advantages it gives to add a new
Makefile knob rather than just letting interested parties add -MJ to
their CFLAGS. Is it just a convenience to create the concatenated form?
It seems weird that projects would need to do so themselves with sed
hackery (i.e., I'd expect whatever consumes this json to be able to
handle multiple files).

-Peff

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

[Philippe Blain]

> Le 1 sept. 2020 à 03:38, Jeff King <peff@peff.net> a écrit :
> 
> On Sun, Aug 30, 2020 at 09:24:03PM -0700, Junio C Hamano wrote:
> 
>> "brian m. carlson" <sandals@crustytoothpaste.net> writes:
>> 
>>> On 2020-08-30 at 19:28:27, Philippe Blain via GitGitGadget wrote:
>>>> 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.
>>> 
>>> For additional context why this is valuable, clangd, which is a C
>>> language server protocol implementation, can use these files to
>>> determine the flags needed to compile a file so it can provide proper
>>> editor integration.  As a result, editors supporting the language server
>>> protocol (such as VS Code, or Vim with a suitable plugin) can provide
>>> better searching, integration, and refactoring tools.
>> 
>> I found that the proposed commit log was very weak to sell the
>> change; some of what you gave above should definitely help strenthen
>> it.
> 
> Likewise. Looking at the output, I'm confused how it would help with
> things like searching and refactoring. It might be nice to spell it out
> for those of us exposed to it for the first time (I tried following the
> links but remained unenlightened).

OK, I'll improve the commit message. I'm not at all an expert in this subject,
I just had to generate a compilation database myself to use the Sourcetrail source
explorer [1] with Git so I figured I'd share what I had done. Further exploration of the 
topic are in [2] and [3]. Note that I did try some of the tools listed in [2] before resorting
to modifying the Makefile, but these tools either did not work at all or produced wrong 
output (ex. strings in the JSON were not properly quoted, etc.)

> I'd also be curious to hear what advantages it gives to add a new
> Makefile knob rather than just letting interested parties add -MJ to
> their CFLAGS. Is it just a convenience to create the concatenated form?

Unfortunately this would not work because the '-MJ' flag needs a file name
to know where to put the JSON fragment.

Thanks,
Philippe.


[1] www.sourcetrail.com
[2] https://sarcasm.github.io/notes/dev/compilation-database.html
[3] https://eli.thegreenplace.net/2014/05/21/compilation-databases-for-clang-based-tools

[PATCH v2] Makefile: add support for generating JSON compilation database

[Philippe Blain via GitGitGadget]

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.

For example, clangd [3], which is a C language server protocol
implementation, can use a JSON compilation database to determine the
flags needed to compile a file so it can provide proper editor
integration.  As a result, editors supporting the language server
protocol (such as VS Code, Emacs, or Vim, with suitable plugins) can
provide better searching, integration, and refactoring tools.

The Clang compiler can generate JSON fragments when compiling [4],
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://clangd.llvm.org/
[4] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
    Add support for generating JSON compilation database
    
    Changes since v1:
    
     * Added a paragraph to the commit message to better explain why this
       feature would help Git developers (Thanks Brian!)
     * Corrected the description of the new GENERATE_COMPILATION_DATABASE 
       Makefile knob to make it reflect its actual behavior (cf. 
       https://lore.kernel.org/git/FC95CFF7-F9DA-4CDA-9923-99C3432DCAD5@gmail.com/
       )
    
    v1: 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-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-714/phil-blain/compiledb-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/714

Range-diff vs v1:

 1:  411fefaafb ! 1:  da9544e4aa Makefile: add support for generating JSON compilation database
     @@ Commit message
          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],
     +    For example, clangd [3], which is a C language server protocol
     +    implementation, can use a JSON compilation database to determine the
     +    flags needed to compile a file so it can provide proper editor
     +    integration.  As a result, editors supporting the language server
     +    protocol (such as VS Code, Emacs, or Vim, with suitable plugins) can
     +    provide better searching, integration, and refactoring tools.
     +
     +    The Clang compiler can generate JSON fragments when compiling [4],
          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'.
     @@ Commit message
      
          [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
     +    [3] https://clangd.llvm.org/
     +    [4] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg
      
     +    Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
          Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
      
       ## .gitignore ##
     @@ Makefile: all::
      +# 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`.
     ++# and the JSON compilation database 'compile_commands.json' will be created 
     ++# at the root of the repository. 
      +#
       # Define DEVELOPER to enable more compiler warnings. Compiler version
       # and family are auto detected, but could be overridden by defining


 .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..51cd0f302b 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 'compile_commands.json' will be created 
+# at the root of the repository. 
+#
 # 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

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

[brian m. carlson]

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On 2020-09-01 at 07:38:27, Jeff King wrote:
> Likewise. Looking at the output, I'm confused how it would help with
> things like searching and refactoring. It might be nice to spell it out
> for those of us exposed to it for the first time (I tried following the
> links but remained unenlightened).

Traditionally, editors had to learn about every language if they wanted
to add special functionality like refactoring (e.g., renaming "struct
foo" to "struct bar"), finding all the instances of a type, finding
where a type or function was declared, or similar IDE features.  When
Microsoft developed Visual Studio Code, they decided that they did not
want to implement this functionality for every language under the sun,
and instead developed the Language Server Protocol[0].

With LSP, each editor needs functionality to speak its portion (either
natively, as with VS Code, or with a plugin, such as Vim's ALE) and each
language implements a language server to implement its part of the
functionality.  The protocol is capability based, so implementations can
support those features which make sense for their editor or language and
omit those which don't.  This way, all editors can benefit and language
communities can implement one program to provide features, and the
problem becomes an O(M + N) problem instead of an O(M * N) problem.

In some languages, like Rust, it's pretty obvious how to compile your
project: you use cargo, the built-in build tool.  There is also a
standard layout to find and enumerate files within a project.  However,
C is not so standardized, so clangd, which is a clang-based C and C++
LSP implementation, needs help to find out which flags are needed to
compile, and therefore find the header files to make sense of parsing
the C code and implementing its side of the protocol.  That's what this
patch implements.

I use Vim and ALE extensively, and it pretty much just works for most
languages, including Go and Rust, once you install the LSP server.  Git
is one of the few projects I work on which is still C and therefore
needs help here.

Hopefully this is at least more enlightening about the functionality
that clangd provides, why it's interesting, how it works, and why it's
valuable.

> I'd also be curious to hear what advantages it gives to add a new
> Makefile knob rather than just letting interested parties add -MJ to
> their CFLAGS. Is it just a convenience to create the concatenated form?
> It seems weird that projects would need to do so themselves with sed
> hackery (i.e., I'd expect whatever consumes this json to be able to
> handle multiple files).

I believe clangd does need the concatenated form, and at least the ALE
plugin for Vim uses that specific file name to detect whether clangd
should be used.  The problem is that clangd doesn't know where your
source code is actually located and it's very expensive to traverse an
entire repository which might contain literally millions of files if
you're only really interested in a handful.

[0] https://microsoft.github.io/language-server-protocol/
-- 
brian m. carlson: Houston, Texas, US

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

[Jeff King]

On Wed, Sep 02, 2020 at 01:33:51AM +0000, brian m. carlson wrote:

> Traditionally, editors had to learn about every language if they wanted
> to add special functionality like refactoring (e.g., renaming "struct
> foo" to "struct bar"), finding all the instances of a type, finding
> where a type or function was declared, or similar IDE features.  When
> Microsoft developed Visual Studio Code, they decided that they did not
> want to implement this functionality for every language under the sun,
> and instead developed the Language Server Protocol[0].
> [...]

Thanks for the explanation. I understand what LSP does, but the missing
link for me was how "here are the command-line flags to the compiler"
turned into something useful like "here's a list of identifiers". And
clangd fills in that gap (presumably re-running the front-end bits of
clang on the fly to pull out that kind of information).

-Peff

Re: [PATCH v2] Makefile: add support for generating JSON compilation database

[Junio C Hamano]

"Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com> writes:

> diff --git a/.gitignore b/.gitignore
> index ee509a2ad2..f4c51300e0 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -197,6 +197,7 @@
>  /git.spec
>  *.exe
>  *.[aos]
> +*.o.json

Does this naming/suffix follow the common practice followed among
those projects that use the -MJ option?  Even though I may have
heard of it, I am not familiar with the use of the feature at all,
and to such a person, naming a file after what format it is written
in (i.e. 'json') feels a lot less useful than what it contains
(i.e. compilation database entries).  

It's like naming our source files with .txt suffix ;-)

> +# 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 'compile_commands.json' will be created 
> +# at the root of the repository. 

Likewise for the name of the directory and the resulting single
database file.  I just want to make sure that we are following the
common convention, so people familiar with the use of the feature
would feel at home, so a simple answer "yes" would suffice.

> +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

This feels strange.  If the end user said to GENERATE and we find we
are capable, we still override to 'yes'?  What if the end user set
'no' to the GENERATE_COMPILATION_DATABASE macro?  Shouldn't we be
honoring that wish?

> +else
> +override GENERATE_COMPILATION_DATABASE = no
> +$(warning GENERATE_COMPILATION_DATABASE is set, but your compiler does not \
> +support generating compilation database entries)

This side is perfectly understandable and I think it is a valid use
of override.  But I do not understand the other side.

> @@ -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

This detail does not matter as long as the end result ensures unique
output for all source files, but I am having trouble guessing what
the outermost subst, which removes ".-" sequence, is trying to make
prettier.  Care to explain?

> +compdb_args = -MJ $(compdb_file)
> +else
> +missing_compdb_dir =
> +compdb_args =

These are unfortunate.  I briefly wondered if we can make GIT-CFLAGS
depend on these two variables so that ASM_OBJ and C_OBJ do not have
to depend on them, but the actual rules need to be updated anyway,
so it cannot be helped.

I do wonder if we can unify dep_args and compdb_args into a single
extra_args (and similarly dep_dirs and compdb_dir to extra_dirs) so
that future similar options can all piggyback on it, but this can do
for now.

> @@ -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 > $@+

OK.  The entire thing is concatenated and enclosed by a single pair
of '[' and ']'.

If we are planning to allow developers customize compdb_dir,
requiring trailing slash in the value of $(compdb_dir) is not very
friendly.


Thanks.

Re: [PATCH v2] Makefile: add support for generating JSON compilation database

[Philippe Blain]

Hi Junio, 

> Le 2 sept. 2020 à 13:21, Junio C Hamano <gitster@pobox.com> a écrit :
> 
> "Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> diff --git a/.gitignore b/.gitignore
>> index ee509a2ad2..f4c51300e0 100644
>> --- a/.gitignore
>> +++ b/.gitignore
>> @@ -197,6 +197,7 @@
>> /git.spec
>> *.exe
>> *.[aos]
>> +*.o.json
> 
> Does this naming/suffix follow the common practice followed among
> those projects that use the -MJ option?  Even though I may have
> heard of it, I am not familiar with the use of the feature at all,
> and to such a person, naming a file after what format it is written
> in (i.e. 'json') feels a lot less useful than what it contains
> (i.e. compilation database entries).  
> 
> It's like naming our source files with .txt suffix ;-)

This addition to the .gitignore is for the individual JSON files (one per source file), 
that are placed in the $(compdb_dir). 
I think naming "rebase.o.json" the JSON file that describes how to compile "rebase.c"
into "rebase.o" makes sense. I don't know what is the convention for other projects.

>> +# 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 'compile_commands.json' will be created 
>> +# at the root of the repository. 
> 
> Likewise for the name of the directory and the resulting single
> database file.  I just want to make sure that we are following the
> common convention, so people familiar with the use of the feature
> would feel at home, so a simple answer "yes" would suffice.

The name `compile_commands.json` for the database itself is standard. 
The name of the directory where the '*.o.json' files are placed is a name
I chose, and I don't feel strongly about it. I thought it made sense to name
it like that, then its purpose is clear.  We could make it a hidden directory 
if we don't want to add a new folder to the root of the repo when using this feature.

>> +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
> 
> This feels strange.  If the end user said to GENERATE and we find we
> are capable, we still override to 'yes'?  What if the end user set
> 'no' to the GENERATE_COMPILATION_DATABASE macro?  Shouldn't we be
> honoring that wish?

We should. I'll tweak (and simplify) that for v3.

>> @@ -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
> 
> This detail does not matter as long as the end result ensures unique
> output for all source files, but I am having trouble guessing what
> the outermost subst, which removes ".-" sequence, is trying to make
> prettier.  Care to explain?

Yes, it is because the `$(dir $@)` Makefile function will return `./` for source files 
at the base of the repo, so the JSON files get named eg. `.-rebase.o.json` and then they are 
hidden. So it's just to make them non-hidden, so as not to confuse someone that would
count the number of source files and compare with the number of (non-hidden)
 '*.o.json' files in $(comdb_dir) and get a different number.

>> @@ -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 > $@+
> 
> OK.  The entire thing is concatenated and enclosed by a single pair
> of '[' and ']'.

Yes, and the comma after the last entry removed for JSON compliance.

> If we are planning to allow developers customize compdb_dir,
> requiring trailing slash in the value of $(compdb_dir) is not very
> friendly.

OK I'll change that.

> Thanks.

Thank you for your comments!

Philippe.

Re: [PATCH v2] Makefile: add support for generating JSON compilation database

[Junio C Hamano]

Philippe Blain <levraiphilippeblain@gmail.com> writes:

> This addition to the .gitignore is for the individual JSON files (one per source file), 
> that are placed in the $(compdb_dir). 
> I think naming "rebase.o.json" the JSON file that describes how to compile "rebase.c"
> into "rebase.o" makes sense. I don't know what is the convention for other projects.

I agree rebase.o.$somesuffix does make sense, but I do not know
'json' is a great value for $somesuffix.  I wouldn't be surprised if
'cdb' or some other silly abbreviation for "compilation database" is
how other people use this feature.

Those watching from the sidelines.  Does anybody know if there is an
established convention used by other projects?  If we hear nothing
by early next week, let's declare 'json' is good enough and move on.

> The name `compile_commands.json` for the database itself is standard. 
> The name of the directory where the '*.o.json' files are placed is a name
> I chose, and I don't feel strongly about it. I thought it made sense to name
> it like that, then its purpose is clear.  We could make it a hidden directory 
> if we don't want to add a new folder to the root of the repo when using this feature.

I think both of these are sensible.  Again if we hear nothing about
common practice, let's move on with these constants as-is.

>>> +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
>> 
>> This feels strange.  If the end user said to GENERATE and we find we
>> are capable, we still override to 'yes'?  What if the end user set
>> 'no' to the GENERATE_COMPILATION_DATABASE macro?  Shouldn't we be
>> honoring that wish?
>
> We should. I'll tweak (and simplify) that for v3.

I think

 - GENERATE_COMPILATION_DATABASE is set to 'no': don't even probe

 - GENERATE_COMPILATION_DATABASE is set to 'yes': probe and turn it
   to 'no' if unavailable.

 - GENERATE_COMPILATION_DATABASE is set to anything else: either
   error out, or turn it into 'no' (I have no preference between
   them).

would cover all the cases.

>>> +compdb_file = $(compdb_dir)$(subst .-,,$(subst /,-,$(dir $@)))$(notdir $@).json
>> 
>> This detail does not matter as long as the end result ensures unique
>> output for all source files, but I am having trouble guessing what
>> the outermost subst, which removes ".-" sequence, is trying to make
>> prettier.  Care to explain?
>
> Yes, it is because the `$(dir $@)` Makefile function will return `./` for source files 
> at the base of the repo, so the JSON files get named eg. `.-rebase.o.json` and then they are 
> hidden. So it's just to make them non-hidden, so as not to confuse someone that would
> count the number of source files and compare with the number of (non-hidden)
>  '*.o.json' files in $(comdb_dir) and get a different number.

Hmph.  Would $(subst /,-,$@) instead of "only substitute leading
directory part, and concatenate the basename part unmolested" work
better then?  After all, by definition the basename part would not
have a slash in it, so substituting all '/' to '-' in the whole
pathname should do the same thing and we won't have to worry about
the spurious './', no?

Re: [PATCH v2] Makefile: add support for generating JSON compilation database

[Philippe Blain]

> Le 3 sept. 2020 à 17:31, Junio C Hamano <gitster@pobox.com> a écrit :
> 
> Philippe Blain <levraiphilippeblain@gmail.com> writes:
> 
>> This addition to the .gitignore is for the individual JSON files (one per source file), 
>> that are placed in the $(compdb_dir). 
>> I think naming "rebase.o.json" the JSON file that describes how to compile "rebase.c"
>> into "rebase.o" makes sense. I don't know what is the convention for other projects.
> 
> I agree rebase.o.$somesuffix does make sense, but I do not know
> 'json' is a great value for $somesuffix.  I wouldn't be surprised if
> 'cdb' or some other silly abbreviation for "compilation database" is
> how other people use this feature.
> 
> Those watching from the sidelines.  Does anybody know if there is an
> established convention used by other projects?  If we hear nothing
> by early next week, let's declare 'json' is good enough and move on.
> 
>> The name `compile_commands.json` for the database itself is standard. 
>> The name of the directory where the '*.o.json' files are placed is a name
>> I chose, and I don't feel strongly about it. I thought it made sense to name
>> it like that, then its purpose is clear.  We could make it a hidden directory 
>> if we don't want to add a new folder to the root of the repo when using this feature.
> 
> I think both of these are sensible.  Again if we hear nothing about
> common practice, let's move on with these constants as-is.

OK. 

> 
>>>> +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
>>> 
>>> This feels strange.  If the end user said to GENERATE and we find we
>>> are capable, we still override to 'yes'?  What if the end user set
>>> 'no' to the GENERATE_COMPILATION_DATABASE macro?  Shouldn't we be
>>> honoring that wish?
>> 
>> We should. I'll tweak (and simplify) that for v3.
> 
> I think
> 
> - GENERATE_COMPILATION_DATABASE is set to 'no': don't even probe
> 
> - GENERATE_COMPILATION_DATABASE is set to 'yes': probe and turn it
>   to 'no' if unavailable.
> 
> - GENERATE_COMPILATION_DATABASE is set to anything else: either
>   error out, or turn it into 'no' (I have no preference between
>   them).
> 
> would cover all the cases.

I agree. I'll do that.

> 
>>>> +compdb_file = $(compdb_dir)$(subst .-,,$(subst /,-,$(dir $@)))$(notdir $@).json
>>> 
>>> This detail does not matter as long as the end result ensures unique
>>> output for all source files, but I am having trouble guessing what
>>> the outermost subst, which removes ".-" sequence, is trying to make
>>> prettier.  Care to explain?
>> 
>> Yes, it is because the `$(dir $@)` Makefile function will return `./` for source files 
>> at the base of the repo, so the JSON files get named eg. `.-rebase.o.json` and then they are 
>> hidden. So it's just to make them non-hidden, so as not to confuse someone that would
>> count the number of source files and compare with the number of (non-hidden)
>> '*.o.json' files in $(comdb_dir) and get a different number.
> 
> Hmph.  Would $(subst /,-,$@) instead of "only substitute leading
> directory part, and concatenate the basename part unmolested" work
> better then?  After all, by definition the basename part would not
> have a slash in it, so substituting all '/' to '-' in the whole
> pathname should do the same thing and we won't have to worry about
> the spurious './', no?

This indeed works, and reads better. Thanks!

[PATCH v3] Makefile: add support for generating JSON compilation database

[Philippe Blain via GitGitGadget]

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.

For example, clangd [3], which is a C language server protocol
implementation, can use a JSON compilation database to determine the
flags needed to compile a file so it can provide proper editor
integration.  As a result, editors supporting the language server
protocol (such as VS Code, Emacs, or Vim, with suitable plugins) can
provide better searching, integration, and refactoring tools.

The Clang compiler can generate JSON fragments when compiling [4],
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://clangd.llvm.org/
[4] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
    Add support for generating JSON compilation database
    
    Changes since v2: Incorporated Junio's suggestions:
    
     * improved the logic around values of the new 
       GENERATE_COMPILATION_DATABASE macro
     * simplified the Makefile functions used to generate the path of the
       JSON fragments
     * removed the trailing slash after the default value of compdb_dir
    
    Changes since v1:
    
     * Added a paragraph to the commit message to better explain why this
       feature would help Git developers (Thanks Brian!)
     * Corrected the description of the new GENERATE_COMPILATION_DATABASE 
       Makefile knob to make it reflect its actual behavior (cf. 
       https://lore.kernel.org/git/FC95CFF7-F9DA-4CDA-9923-99C3432DCAD5@gmail.com/
       )
    
    v1: 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-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-714/phil-blain/compiledb-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/714

Range-diff vs v2:

 1:  da9544e4aa ! 1:  1955b6b390 Makefile: add support for generating JSON compilation database
     @@ Makefile: 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 'compile_commands.json' will be created 
     -+# at the root of the repository. 
     ++# Define GENERATE_COMPILATION_DATABASE to "yes" 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 'compile_commands.json' will be
     ++# created at the root of the repository.
      +#
       # Define DEVELOPER to enable more compiler warnings. Compiler version
       # and family are auto detected, but could be overridden by defining
     @@ Makefile: $(error please set COMPUTE_HEADER_DEPENDENCIES to yes, no, or auto \
       endif
       endif
       
     -+ifdef GENERATE_COMPILATION_DATABASE
     ++ifndef GENERATE_COMPILATION_DATABASE
     ++GENERATE_COMPILATION_DATABASE = no
     ++endif
     ++
     ++ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
      +compdb_check = $(shell $(CC) $(ALL_CFLAGS) \
      +	-c -MJ /dev/null \
      +	-x c /dev/null -o /dev/null 2>&1; \
      +	echo $$?)
     -+ifeq ($(compdb_check),0)
     -+override GENERATE_COMPILATION_DATABASE = yes
     -+else
     ++ifneq ($(compdb_check),0)
      +override GENERATE_COMPILATION_DATABASE = no
     -+$(warning GENERATE_COMPILATION_DATABASE is set, but your compiler does not \
     ++$(warning GENERATE_COMPILATION_DATABASE is set to "yes", but your compiler does not \
      +support generating compilation database entries)
      +endif
     ++else
     ++ifneq ($(GENERATE_COMPILATION_DATABASE),no)
     ++$(error please set GENERATE_COMPILATION_DATABASE to "yes" or "no" \
     ++(not "$(GENERATE_COMPILATION_DATABASE)"))
     ++endif
      +endif
      +
       ifdef SANE_TOOL_PATH
     @@ Makefile: missing_dep_dirs =
       dep_args =
       endif
       
     -+compdb_dir = compile_commands/
     ++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_file = $(compdb_dir)/$(subst /,-,$@.json)
      +compdb_args = -MJ $(compdb_file)
      +else
      +missing_compdb_dir =
     @@ Makefile: else
      +all:: compile_commands.json
      +compile_commands.json:
      +	@$(RM) $@
     -+	$(QUIET_GEN)sed -e '1s/^/[/' -e '$$s/,$$/]/' $(compdb_dir)*.o.json > $@+
     ++	$(QUIET_GEN)sed -e '1s/^/[/' -e '$$s/,$$/]/' $(compdb_dir)/*.o.json > $@+
      +	@if test -s $@+; then mv $@+ $@; else $(RM) $@+; fi
      +endif
      +


 .gitignore |  2 ++
 Makefile   | 59 +++++++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 56 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..39ca667689 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 "yes" 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 'compile_commands.json' will be
+# created at the root of the repository.
+#
 # 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 +1264,27 @@ $(error please set COMPUTE_HEADER_DEPENDENCIES to yes, no, or auto \
 endif
 endif
 
+ifndef GENERATE_COMPILATION_DATABASE
+GENERATE_COMPILATION_DATABASE = no
+endif
+
+ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
+compdb_check = $(shell $(CC) $(ALL_CFLAGS) \
+	-c -MJ /dev/null \
+	-x c /dev/null -o /dev/null 2>&1; \
+	echo $$?)
+ifneq ($(compdb_check),0)
+override GENERATE_COMPILATION_DATABASE = no
+$(warning GENERATE_COMPILATION_DATABASE is set to "yes", but your compiler does not \
+support generating compilation database entries)
+endif
+else
+ifneq ($(GENERATE_COMPILATION_DATABASE),no)
+$(error please set GENERATE_COMPILATION_DATABASE to "yes" or "no" \
+(not "$(GENERATE_COMPILATION_DATABASE)"))
+endif
+endif
+
 ifdef SANE_TOOL_PATH
 SANE_TOOL_PATH_SQ = $(subst ','\'',$(SANE_TOOL_PATH))
 BROKEN_PATH_FIX = 's|^\# @@BROKEN_PATH_FIX@@$$|git_broken_path_fix "$(SANE_TOOL_PATH_SQ)"|'
@@ -2381,16 +2408,30 @@ 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 /,-,$@.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 +2454,14 @@ 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 +3166,7 @@ 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