Re: [PATCH v5 3/5] p1689r5: initial support

[Jason Merrill <jason@redhat.com>]

On 1/25/23 13:06, Ben Boeckel wrote:
> This patch implements support for [P1689R5][] to communicate to a build
> system the C++20 module dependencies to build systems so that they may
> build `.gcm` files in the proper order.

Thanks again.

> Support is communicated through the following three new flags:
> 
> - `-fdeps-format=` specifies the format for the output. Currently named
>    `p1689r5`.
> 
> - `-fdeps-file=` specifies the path to the file to write the format to.
> 
> - `-fdep-output=` specifies the `.o` that will be written for the TU
>    that is scanned. This is required so that the build system can
>    correlate the dependency output with the actual compilation that will
>    occur.

I notice that the actual flags are all -fdep-*, though some of them are 
-fdeps-* here, and the internal variables all seem to be fdeps_*.  I 
lean toward harmonizing on "deps", I think.

I don't love the three separate options, but I suppose it's fine.  I'd 
prefer "target" instead of "output".

It should be possible to omit both -file and -target and get reasonable 
defaults, like the ones for -MD/-MQ in gcc.cc:cpp_unique_options.

> CMake supports this format as of 17 Jun 2022 (to be part of 3.25.0)
> using an experimental feature selection (to allow for future usage
> evolution without committing to how it works today). While it remains
> experimental, docs may be found in CMake's documentation for
> experimental features.
> 
> Future work may include using this format for Fortran module
> dependencies as well, however this is still pending work.
> 
> [P1689R5]: https://isocpp.org/files/papers/P1689R5.html
> [cmake-experimental]: https://gitlab.kitware.com/cmake/cmake/-/blob/master/Help/dev/experimental.rst
> 
> TODO:
> 
> - header-unit information fields
> 
> Header units (including the standard library headers) are 100%
> unsupported right now because the `-E` mechanism wants to import their
> BMIs. A new mode (i.e., something more workable than existing `-E`
> behavior) that mocks up header units as if they were imported purely
> from their path and content would be required.

I notice that the cpp dependency generation tries (in open_file_failed) 
to continue after encountering a missing file, is that not sufficient 
for header units?  Or adjustable to be sufficient?

> - non-utf8 paths
> 
> The current standard says that paths that are not unambiguously
> represented using UTF-8 are not supported (because these cases are rare
> and the extra complication is not worth it at this time). Future
> versions of the format might have ways of encoding non-UTF-8 paths. For
> now, this patch just doesn't support non-UTF-8 paths (ignoring the
> "unambiguously represetable in UTF-8" case).

typo "representable"

> - figure out why junk gets placed at the end of the file
> 
> Sometimes it seems like the file gets a lot of `NUL` bytes appended to
> it. It happens rarely and seems to be the result of some
> `ftruncate`-style call which results in extra padding in the contents.
> Noting it here as an observation at least.
> 
> libcpp/
> 
> 	* include/cpplib.h: Add cpp_deps_format enum.
> 	(cpp_options): Add format field
> 	(cpp_finish): Add dependency stream parameter.
> 	* include/mkdeps.h (deps_add_module_target): Add new preprocessor
> 	parameter used for C++ module tracking.
> 	* init.cc (cpp_finish): Add new preprocessor parameter used for C++
> 	module tracking.
> 	* mkdeps.cc (mkdeps): Implement P1689R5 output.
> 
> gcc/
> 
> 	* doc/invoke.texi: Document -fdeps-format=, -fdep-file=, and
> 	-fdep-output= flags.
> 
> gcc/c-family/
> 
> 	* c-opts.cc (c_common_handle_option): Add fdeps_file variable and
> 	-fdeps-format=, -fdep-file=, and -fdep-output= parsing.
> 	* c.opt: Add -fdeps-format=, -fdep-file=, and -fdep-output= flags.
> 
> gcc/cp/
> 
> 	* module.cc (preprocessed_module): Pass whether the module is
> 	exported to dependency tracking.
> 
> gcc/testsuite/
> 
> 	* g++.dg/modules/depflags-f-MD.C: New test.
> 	* g++.dg/modules/depflags-f.C: New test.
> 	* g++.dg/modules/depflags-fi.C: New test.
> 	* g++.dg/modules/depflags-fj-MD.C: New test.
> 	* g++.dg/modules/depflags-fj.C: New test.
> 	* g++.dg/modules/depflags-fjo-MD.C: New test.
> 	* g++.dg/modules/depflags-fjo.C: New test.
> 	* g++.dg/modules/depflags-fo-MD.C: New test.
> 	* g++.dg/modules/depflags-fo.C: New test.
> 	* g++.dg/modules/depflags-j-MD.C: New test.
> 	* g++.dg/modules/depflags-j.C: New test.
> 	* g++.dg/modules/depflags-jo-MD.C: New test.
> 	* g++.dg/modules/depflags-jo.C: New test.
> 	* g++.dg/modules/depflags-o-MD.C: New test.
> 	* g++.dg/modules/depflags-o.C: New test.
> 	* g++.dg/modules/p1689-1.C: New test.
> 	* g++.dg/modules/p1689-1.exp.json: New test expectation.
> 	* g++.dg/modules/p1689-2.C: New test.
> 	* g++.dg/modules/p1689-2.exp.json: New test expectation.
> 	* g++.dg/modules/p1689-3.C: New test.
> 	* g++.dg/modules/p1689-3.exp.json: New test expectation.
> 	* g++.dg/modules/p1689-4.C: New test.
> 	* g++.dg/modules/p1689-4.exp.json: New test expectation.
> 	* g++.dg/modules/p1689-5.C: New test.
> 	* g++.dg/modules/p1689-5.exp.json: New test expectation.
> 	* g++.dg/modules/modules.exp: Load new P1689 library routines.
> 	* g++.dg/modules/test-p1689.py: New tool for validating P1689 output.
> 	* lib/modules.exp: Support for validating P1689 outputs.
> 
> Signed-off-by: Ben Boeckel <ben.boeckel@kitware.com>
> ---
>   gcc/c-family/c-opts.cc                        |  40 +++-
>   gcc/c-family/c.opt                            |  12 +
>   gcc/cp/module.cc                              |   3 +-
>   gcc/doc/invoke.texi                           |  15 ++
>   gcc/testsuite/g++.dg/modules/depflags-f-MD.C  |   2 +
>   gcc/testsuite/g++.dg/modules/depflags-f.C     |   1 +
>   gcc/testsuite/g++.dg/modules/depflags-fi.C    |   3 +
>   gcc/testsuite/g++.dg/modules/depflags-fj-MD.C |   3 +
>   gcc/testsuite/g++.dg/modules/depflags-fj.C    |   4 +
>   .../g++.dg/modules/depflags-fjo-MD.C          |   4 +
>   gcc/testsuite/g++.dg/modules/depflags-fjo.C   |   5 +
>   gcc/testsuite/g++.dg/modules/depflags-fo-MD.C |   3 +
>   gcc/testsuite/g++.dg/modules/depflags-fo.C    |   4 +
>   gcc/testsuite/g++.dg/modules/depflags-j-MD.C  |   2 +
>   gcc/testsuite/g++.dg/modules/depflags-j.C     |   3 +
>   gcc/testsuite/g++.dg/modules/depflags-jo-MD.C |   3 +
>   gcc/testsuite/g++.dg/modules/depflags-jo.C    |   4 +
>   gcc/testsuite/g++.dg/modules/depflags-o-MD.C  |   2 +
>   gcc/testsuite/g++.dg/modules/depflags-o.C     |   3 +
>   gcc/testsuite/g++.dg/modules/modules.exp      |   1 +
>   gcc/testsuite/g++.dg/modules/p1689-1.C        |  18 ++
>   gcc/testsuite/g++.dg/modules/p1689-1.exp.json |  27 +++
>   gcc/testsuite/g++.dg/modules/p1689-2.C        |  16 ++
>   gcc/testsuite/g++.dg/modules/p1689-2.exp.json |  16 ++
>   gcc/testsuite/g++.dg/modules/p1689-3.C        |  14 ++
>   gcc/testsuite/g++.dg/modules/p1689-3.exp.json |  16 ++
>   gcc/testsuite/g++.dg/modules/p1689-4.C        |  14 ++
>   gcc/testsuite/g++.dg/modules/p1689-4.exp.json |  14 ++
>   gcc/testsuite/g++.dg/modules/p1689-5.C        |  14 ++
>   gcc/testsuite/g++.dg/modules/p1689-5.exp.json |  14 ++
>   gcc/testsuite/g++.dg/modules/test-p1689.py    | 222 ++++++++++++++++++
>   gcc/testsuite/lib/modules.exp                 |  71 ++++++
>   libcpp/include/cpplib.h                       |  12 +-
>   libcpp/include/mkdeps.h                       |  17 +-
>   libcpp/init.cc                                |  13 +-
>   libcpp/mkdeps.cc                              | 149 +++++++++++-
>   36 files changed, 745 insertions(+), 19 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-f-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-f.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fi.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fj-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fj.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fjo-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fjo.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fo-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-fo.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-j-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-j.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-jo-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-jo.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-o-MD.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/depflags-o.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-1.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-1.exp.json
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-2.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-2.exp.json
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-3.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-3.exp.json
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-4.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-4.exp.json
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-5.C
>   create mode 100644 gcc/testsuite/g++.dg/modules/p1689-5.exp.json
>   create mode 100644 gcc/testsuite/g++.dg/modules/test-p1689.py
>   create mode 100644 gcc/testsuite/lib/modules.exp
> 
> diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
> index c68a2a27469..1c14ce3fe8e 100644
> --- a/gcc/c-family/c-opts.cc
> +++ b/gcc/c-family/c-opts.cc
> @@ -77,6 +77,9 @@ static bool verbose;
>   /* Dependency output file.  */
>   static const char *deps_file;
>   
> +/* Enhanced dependency output file.  */

Maybe "structured", as in the docs?  It isn't really a direct 
enhancement of the makefile dependencies.

> +static const char *fdeps_file;
> +
>   /* The prefix given by -iprefix, if any.  */
>   static const char *iprefix;
>   
> @@ -360,6 +363,23 @@ c_common_handle_option (size_t scode, const char *arg, HOST_WIDE_INT value,
>         deps_file = arg;
>         break;
>   
> +    case OPT_fdep_format_:
> +      if (!strcmp (arg, "p1689r5"))
> +	cpp_opts->deps.format = DEPS_FMT_P1689R5;
> +      else
> +	error ("%<-fdep-format=%> unknown format %<%s%>", arg);
> +      break;
> +
> +    case OPT_fdep_file_:
> +      deps_seen = true;
> +      fdeps_file = arg;
> +      break;
> +
> +    case OPT_fdep_output_:
> +      deps_seen = true;
> +      defer_opt (code, arg);
> +      break;
> +
>       case OPT_MF:
>         deps_seen = true;
>         deps_file = arg;
> @@ -1272,6 +1292,7 @@ void
>   c_common_finish (void)
>   {
>     FILE *deps_stream = NULL;
> +  FILE *fdeps_stream = NULL;
>   
>     /* Note that we write the dependencies even if there are errors. This is
>        useful for handling outdated generated headers that now trigger errors
> @@ -1300,9 +1321,24 @@ c_common_finish (void)
>        locations with input_location, which would be incorrect now.  */
>     override_libcpp_locations = false;
>   
> +  if (cpp_opts->deps.format != DEPS_FMT_NONE)
> +    {
> +      if (!fdeps_file)
> +	fdeps_stream = out_stream;
> +      else if (fdeps_file[0] == '-' && fdeps_file[1] == '\0')
> +	fdeps_stream = stdout;

You probably want to check that deps_stream and fdeps_stream don't end 
up as the same stream.

> +      else
> +	{
> +	  fdeps_stream = fopen (fdeps_file, "w");
> +	  if (!fdeps_stream)
> +	    fatal_error (input_location, "opening dependency file %s: %m",
> +			 fdeps_file);
> +	}
> +    }
> +
>     /* For performance, avoid tearing down cpplib's internal structures
>        with cpp_destroy ().  */
> -  cpp_finish (parse_in, deps_stream);
> +  cpp_finish (parse_in, deps_stream, fdeps_stream);
>   
>     if (deps_stream && deps_stream != out_stream && deps_stream != stdout
>         && (ferror (deps_stream) || fclose (deps_stream)))
> @@ -1374,6 +1410,8 @@ handle_deferred_opts (void)
>   
>   	if (opt->code == OPT_MT || opt->code == OPT_MQ)
>   	  deps_add_target (deps, opt->arg, opt->code == OPT_MQ);
> +	else if (opt->code == OPT_fdep_output_)
> +	  deps_add_output (deps, opt->arg, true);

How about fdeps_add_target?

>         }
>   }
>   
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index ef371ca8c26..630781fdf8a 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -256,6 +256,18 @@ MT
>   C ObjC C++ ObjC++ Joined Separate MissingArgError(missing makefile target after %qs)
>   -MT <target>	Add a target that does not require quoting.
>   
> +fdep-format=
> +C ObjC C++ ObjC++ NoDriverArg Joined MissingArgError(missing format after %qs)
> +Format for output dependency information.  Supported (\"p1689r5\").

I think we want "structured" here, as well.

> +fdep-file=
> +C ObjC C++ ObjC++ NoDriverArg Joined MissingArgError(missing output path after %qs)
> +File for output dependency information.
> +
> +fdep-output=
> +C ObjC C++ ObjC++ NoDriverArg Joined MissingArgError(missing path after %qs)
> +-fdep-output=obj.o Output file for the compile step.
> +
>   P
>   C ObjC C++ ObjC++
>   Do not generate #line directives.
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index ac2fe66b080..ebd30f63d81 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -19832,7 +19832,8 @@ preprocessed_module (cpp_reader *reader)
>   		  && (module->is_interface () || module->is_partition ()))
>   		deps_add_module_target (deps, module->get_flatname (),
>   					maybe_add_cmi_prefix (module->filename),
> -					module->is_header());
> +					module->is_header (),
> +					module->is_exported ());
>   	      else
>   		deps_add_module_dep (deps, module->get_flatname ());
>   	    }
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 06d77983e30..b61c3ebd3ec 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -2791,6 +2791,21 @@ is @option{-fpermitted-flt-eval-methods=c11}.  The default when in a GNU
>   dialect (@option{-std=gnu11} or similar) is
>   @option{-fpermitted-flt-eval-methods=ts-18661-3}.
>   
> +@item -fdep-file=@var{file}
> +@opindex fdep-file
> +Where to write structured dependency information.
> +
> +@item -fdep-format=@var{format}
> +@opindex fdep-format
> +The format to use for structured dependency information. @samp{p1689r5} is the
> +only supported format right now.  Note that when this argument is specified, the
> +output of @samp{-MF} is stripped of some information (namely C++ modules) so
> +that it does not use extended makefile syntax not understood by most tools.
> +
> +@item -fdep-output=@var{file}
> +@opindex fdep-output
> +Analogous to @option{-MT} but for structured dependency information.

Please add more detail about how these are intended to be used.

>   @item -fplan9-extensions
>   @opindex fplan9-extensions
>   Accept some non-standard constructs used in Plan 9 code.
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-f-MD.C b/gcc/testsuite/g++.dg/modules/depflags-f-MD.C
> new file mode 100644
> index 00000000000..90e1c9983bd
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-f-MD.C
> @@ -0,0 +1,2 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-format=p1689r5 }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-f.C b/gcc/testsuite/g++.dg/modules/depflags-f.C
> new file mode 100644
> index 00000000000..6192300879d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-f.C
> @@ -0,0 +1 @@
> +// { dg-additional-options -fdep-format=p1689r5 }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fi.C b/gcc/testsuite/g++.dg/modules/depflags-fi.C
> new file mode 100644
> index 00000000000..4f649a11bdd
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fi.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options -fdep-format=invalid }
> +
> +// { dg-prune-output "error: '-fdep-format=' unknown format 'invalid'"  }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fj-MD.C b/gcc/testsuite/g++.dg/modules/depflags-fj-MD.C
> new file mode 100644
> index 00000000000..a361d81f37f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fj-MD.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +// { dg-additional-options -fdep-format=p1689r5 }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fj.C b/gcc/testsuite/g++.dg/modules/depflags-fj.C
> new file mode 100644
> index 00000000000..4a140ec1f13
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fj.C
> @@ -0,0 +1,4 @@
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +
> +// { dg-prune-output "error: to generate dependencies you must specify either '-M' or '-MM'" }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fjo-MD.C b/gcc/testsuite/g++.dg/modules/depflags-fjo-MD.C
> new file mode 100644
> index 00000000000..18d765211b4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fjo-MD.C
> @@ -0,0 +1,4 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> +// { dg-additional-options -fdep-format=p1689r5 }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fjo.C b/gcc/testsuite/g++.dg/modules/depflags-fjo.C
> new file mode 100644
> index 00000000000..6d239f63017
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fjo.C
> @@ -0,0 +1,5 @@
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +
> +// { dg-prune-output "error: to generate dependencies you must specify either '-M' or '-MM'" }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fo-MD.C b/gcc/testsuite/g++.dg/modules/depflags-fo-MD.C
> new file mode 100644
> index 00000000000..a3a775b606a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fo-MD.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-fo.C b/gcc/testsuite/g++.dg/modules/depflags-fo.C
> new file mode 100644
> index 00000000000..29839978e59
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-fo.C
> @@ -0,0 +1,4 @@
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> +
> +// { dg-prune-output "error: to generate dependencies you must specify either '-M' or '-MM'" }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-j-MD.C b/gcc/testsuite/g++.dg/modules/depflags-j-MD.C
> new file mode 100644
> index 00000000000..d95c8e6c2e6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-j-MD.C
> @@ -0,0 +1,2 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-file=depflags-3.json }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-j.C b/gcc/testsuite/g++.dg/modules/depflags-j.C
> new file mode 100644
> index 00000000000..5f100b0f6e5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-j.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +
> +// { dg-prune-output "error: to generate dependencies you must specify either '-M' or '-MM'" }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-jo-MD.C b/gcc/testsuite/g++.dg/modules/depflags-jo-MD.C
> new file mode 100644
> index 00000000000..44330794abc
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-jo-MD.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-jo.C b/gcc/testsuite/g++.dg/modules/depflags-jo.C
> new file mode 100644
> index 00000000000..8eec6bba1d1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-jo.C
> @@ -0,0 +1,4 @@
> +// { dg-additional-options -fdep-file=depflags-3.json }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> +
> +// { dg-prune-output "error: to generate dependencies you must specify either '-M' or '-MM'" }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-o-MD.C b/gcc/testsuite/g++.dg/modules/depflags-o-MD.C
> new file mode 100644
> index 00000000000..429f1f85684
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-o-MD.C
> @@ -0,0 +1,2 @@
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fdep-output=depflags-1.C }
> diff --git a/gcc/testsuite/g++.dg/modules/depflags-o.C b/gcc/testsuite/g++.dg/modules/depflags-o.C
> new file mode 100644
> index 00000000000..9a7326cc812
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/depflags-o.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options -fdep-output=depflags-1.C }
> +
> +// { dg-prune-output "error: to generate dependencies you must specify either '-M' or '-MM'" }
> diff --git a/gcc/testsuite/g++.dg/modules/modules.exp b/gcc/testsuite/g++.dg/modules/modules.exp
> index 61994b05945..c06e1cbca8a 100644
> --- a/gcc/testsuite/g++.dg/modules/modules.exp
> +++ b/gcc/testsuite/g++.dg/modules/modules.exp
> @@ -28,6 +28,7 @@
>   # { dg-module-do [link|run] [xfail] [options] } # link [and run]
>   
>   load_lib g++-dg.exp
> +load_lib modules.exp
>   
>   # If a testcase doesn't have special options, use these.
>   global DEFAULT_CXXFLAGS
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-1.C b/gcc/testsuite/g++.dg/modules/p1689-1.C
> new file mode 100644
> index 00000000000..245e30d09ce
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-1.C
> @@ -0,0 +1,18 @@
> +// { dg-additional-options -E }
> +// { dg-additional-options -MT }
> +// { dg-additional-options p1689-1.json }
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fmodules-ts }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=p1689-1.o }
> +// { dg-additional-options -fdep-file=p1689-1.json }

Note that with dg-additional-options you can put multiple flags in "" 
rather than put each on its own line.

So, I take it this is the common use case you have in mind, generating 
Make dependencies for the p1689 file?  When are you thinking the Make 
dependencies for the .o are generated?  At build time?

I'm a bit surprised you're using .json rather than an extension that 
indicates what the information is.

> +// Export a module that uses modules, re-exports modules, and partitions.
> +
> +export module foo;
> +export import foo:part1;
> +import foo:part2;
> +
> +export import bar;
> +
> +// { dg-final { run-check-p1689-valid p1689-1.json p1689-1.exp.json } }
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-1.exp.json b/gcc/testsuite/g++.dg/modules/p1689-1.exp.json
> new file mode 100644
> index 00000000000..c5648ac7ae5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-1.exp.json
> @@ -0,0 +1,27 @@
> +{
> +    "rules": [
> +        {
> +            "primary-output": "p1689-1.o",
> +            "provides": [
> +                {
> +                    "logical-name": "foo",
> +                    "is-interface": true
> +                }
> +            ],
> +            "requires": [
> +                "__P1689_unordered__",
> +                {
> +                    "logical-name": "bar"
> +                },
> +                {
> +                    "logical-name": "foo:part1"
> +                },
> +                {
> +                    "logical-name": "foo:part2"
> +                }
> +            ]
> +        }
> +    ],
> +    "version": 0,
> +    "revision": 0
> +}
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-2.C b/gcc/testsuite/g++.dg/modules/p1689-2.C
> new file mode 100644
> index 00000000000..add07f59a0e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-2.C
> @@ -0,0 +1,16 @@
> +// { dg-additional-options -E }
> +// { dg-additional-options -MT }
> +// { dg-additional-options p1689-2.json }
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fmodules-ts }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=p1689-2.o }
> +// { dg-additional-options -fdep-file=p1689-2.json }
> +
> +// Export a module partition that uses modules.
> +
> +export module foo:part1;
> +
> +#include <iostream>
> +
> +// { dg-final { run-check-p1689-valid p1689-2.json p1689-2.exp.json } }
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-2.exp.json b/gcc/testsuite/g++.dg/modules/p1689-2.exp.json
> new file mode 100644
> index 00000000000..6901172f277
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-2.exp.json
> @@ -0,0 +1,16 @@
> +{
> +    "rules": [
> +        {
> +            "primary-output": "p1689-2.o",
> +            "provides": [
> +                {
> +                    "logical-name": "foo:part1",
> +                    "is-interface": true
> +                }
> +            ],
> +            "requires": []
> +        }
> +    ],
> +    "version": 0,
> +    "revision": 0
> +}
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-3.C b/gcc/testsuite/g++.dg/modules/p1689-3.C
> new file mode 100644
> index 00000000000..3482c2f4903
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-3.C
> @@ -0,0 +1,14 @@
> +// { dg-additional-options -E }
> +// { dg-additional-options -MT }
> +// { dg-additional-options p1689-3.json }
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fmodules-ts }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=p1689-3.o }
> +// { dg-additional-options -fdep-file=p1689-3.json }
> +
> +// Provide a module partition.
> +
> +module foo:part2;
> +
> +// { dg-final { run-check-p1689-valid p1689-3.json p1689-3.exp.json } }
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-3.exp.json b/gcc/testsuite/g++.dg/modules/p1689-3.exp.json
> new file mode 100644
> index 00000000000..5a40beacd22
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-3.exp.json
> @@ -0,0 +1,16 @@
> +{
> +    "rules": [
> +        {
> +            "primary-output": "p1689-3.o",
> +            "provides": [
> +                {
> +                    "logical-name": "foo:part2",
> +                    "is-interface": false
> +                }
> +            ],
> +            "requires": []
> +        }
> +    ],
> +    "version": 0,
> +    "revision": 0
> +}
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-4.C b/gcc/testsuite/g++.dg/modules/p1689-4.C
> new file mode 100644
> index 00000000000..88bac77a8f8
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-4.C
> @@ -0,0 +1,14 @@
> +// { dg-additional-options -E }
> +// { dg-additional-options -MT }
> +// { dg-additional-options p1689-4.json }
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fmodules-ts }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=p1689-4.o }
> +// { dg-additional-options -fdep-file=p1689-4.json }
> +
> +// Module implementation unit.
> +
> +module foo;
> +
> +// { dg-final { run-check-p1689-valid p1689-4.json p1689-4.exp.json } }
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-4.exp.json b/gcc/testsuite/g++.dg/modules/p1689-4.exp.json
> new file mode 100644
> index 00000000000..b119f5654b1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-4.exp.json
> @@ -0,0 +1,14 @@
> +{
> +    "rules": [
> +        {
> +            "primary-output": "p1689-4.o",
> +            "requires": []
> +                {
> +                    "logical-name": "foo"
> +                }
> +            ]
> +        }
> +    ],
> +    "version": 0,
> +    "revision": 0
> +}
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-5.C b/gcc/testsuite/g++.dg/modules/p1689-5.C
> new file mode 100644
> index 00000000000..e985277368b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-5.C
> @@ -0,0 +1,14 @@
> +// { dg-additional-options -E }
> +// { dg-additional-options -MT }
> +// { dg-additional-options p1689-5.json }
> +// { dg-additional-options -MD }
> +// { dg-additional-options -fmodules-ts }
> +// { dg-additional-options -fdep-format=p1689r5 }
> +// { dg-additional-options -fdep-output=p1689-5.o }
> +// { dg-additional-options -fdep-file=p1689-5.json }
> +
> +// Use modules, don't provide anything.
> +
> +import bar;
> +
> +// { dg-final { run-check-p1689-valid p1689-5.json p1689-5.exp.json } }
> diff --git a/gcc/testsuite/g++.dg/modules/p1689-5.exp.json b/gcc/testsuite/g++.dg/modules/p1689-5.exp.json
> new file mode 100644
> index 00000000000..18704ac8820
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/p1689-5.exp.json
> @@ -0,0 +1,14 @@
> +{
> +    "rules": [
> +        {
> +            "primary-output": "p1689-5.o",
> +            "requires": [
> +                {
> +                    "logical-name": "bar"
> +                }
> +            ]
> +        }
> +    ],
> +    "version": 0,
> +    "revision": 0
> +}
> diff --git a/gcc/testsuite/g++.dg/modules/test-p1689.py b/gcc/testsuite/g++.dg/modules/test-p1689.py
> new file mode 100644
> index 00000000000..2f07cc361aa
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/test-p1689.py
> @@ -0,0 +1,222 @@
> +import json
> +
> +
> +# Parameters.
> +ALL_ERRORS = False
> +REPLACEMENTS = {}
> +
> +
> +def _print_path(path):
> +    '''Format a JSON path for output.'''
> +    return '/'.join(path)
> +
> +
> +def _report_error(msg):
> +    '''Report an error.'''
> +    full_msg = 'ERROR: ' + msg
> +    if ALL_ERRORS:
> +        print(full_msg)
> +    else:
> +        raise RuntimeError(full_msg)
> +
> +
> +def _error_type_mismatch(path, actual, expect):
> +    '''Report that there is a type mismatch.'''
> +    _report_error('type mismatch at %s: actual: "%s" expect: "%s"' % (_print_path(path), actual, expect))
> +
> +
> +def _error_unknown_type(path, typ):
> +    '''Report that there is an unknown type in the JSON object.'''
> +    _report_error('unknown type at %s: "%s"' % (_print_path(path), typ))
> +
> +
> +def _error_length_mismatch(path, actual, expect):
> +    '''Report a length mismatch in an object or array.'''
> +    _report_error('length mismatch at %s: actual: "%s" expect: "%s"' % (_print_path(path), actual, expect))
> +
> +
> +def _error_unexpect_value(path, actual, expect):
> +    '''Report a value mismatch.'''
> +    _report_error('value mismatch at %s: actual: "%s" expect: "%s"' % (_print_path(path), actual, expect))
> +
> +
> +def _error_extra_key(path, key):
> +    '''Report on a key that is unexpected.'''
> +    _report_error('extra key at %s: "%s"' % (_print_path(path), key))
> +
> +
> +def _error_missing_key(path, key):
> +    '''Report on a key that is missing.'''
> +    _report_error('extra key at %s: %s' % (_print_path(path), key))
> +
> +
> +def _compare_object(path, actual, expect):
> +    '''Compare a JSON object.'''
> +    is_ok = True
> +
> +    if not len(actual) == len(expect):
> +        _error_length_mismatch(path, len(actual), len(expect))
> +        is_ok = False
> +
> +    for key in actual:
> +        if key not in expect:
> +            _error_extra_key(path, key)
> +            is_ok = False
> +        else:
> +            sub_error = compare_json(path + [key], actual[key], expect[key])
> +            if sub_error:
> +                is_ok = False
> +
> +    for key in expect:
> +        if key not in actual:
> +            _error_missing_key(path, key)
> +            is_ok = False
> +
> +    return is_ok
> +
> +
> +def _compare_array(path, actual, expect):
> +    '''Compare a JSON array.'''
> +    is_ok = True
> +
> +    if not len(actual) == len(expect):
> +        _error_length_mismatch(path, len(actual), len(expect))
> +        is_ok = False
> +
> +    for (idx, (a, e)) in enumerate(zip(actual, expect)):
> +        sub_error = compare_json(path + [str(idx)], a, e)
> +        if sub_error:
> +            is_ok = False
> +
> +    return is_ok
> +
> +
> +def _make_replacements(value):
> +    for (old, new) in REPLACEMENTS.values():
> +        value = value.replace(old, new)
> +    return value
> +
> +
> +def _compare_string(path, actual, expect):
> +    '''Compare a JSON string supporting replacements in the expected output.'''
> +    expect = _make_replacements(expect)
> +
> +    if not actual == expect:
> +        _error_unexpect_value(path, actual, expect)
> +        return False
> +    else:
> +        print('%s is ok: %s' % (_print_path(path), actual))
> +    return True
> +
> +
> +def _compare_number(path, actual, expect):
> +    '''Compare a JSON integer.'''
> +    if not actual == expect:
> +        _error_unexpect_value(path, actual, expect)
> +        return False
> +    else:
> +        print('%s is ok: %s' % (_print_path(path), actual))
> +    return True
> +
> +
> +def _inspect_ordering(arr):
> +    req_ordering = True
> +
> +    if not arr:
> +        return arr, req_ordering
> +
> +    if arr[0] == '__P1689_unordered__':
> +        arr.pop(0)
> +        req_ordering = False
> +
> +    return arr, req_ordering
> +
> +
> +def compare_json(path, actual, expect):
> +    actual_type = type(actual)
> +    expect_type = type(expect)
> +
> +    is_ok = True
> +
> +    if not actual_type == expect_type:
> +        _error_type_mismatch(path, actual_type, expect_type)
> +        is_ok = False
> +    elif actual_type == dict:
> +        is_ok = _compare_object(path, actual, expect)
> +    elif actual_type == list:
> +        expect, req_ordering = _inspect_ordering(expect)
> +        if not req_ordering:
> +            actual = set(actual)
> +            expect = set(expect)
> +        is_ok = _compare_array(path, actual, expect)
> +    elif actual_type == str:
> +        is_ok = _compare_string(path, actual, expect)
> +    elif actual_type == float:
> +        is_ok = _compare_number(path, actual, expect)
> +    elif actual_type == int:
> +        is_ok = _compare_number(path, actual, expect)
> +    elif actual_type == bool:
> +        is_ok = _compare_number(path, actual, expect)
> +    elif actual_type == type(None):
> +        pass
> +    else:
> +        _error_unknown_type(path, actual_type)
> +        is_ok = False
> +
> +    return is_ok
> +
> +
> +def validate_p1689(actual, expect):
> +    '''Validate a P1689 file against an expected output file.
> +
> +    Returns `False` if it fails, `True` if they are the same.
> +    '''
> +    with open(actual, 'r') as fin:
> +        actual_content = fin.read()
> +    with open(expect, 'r') as fin:
> +        expect_content = fin.read()
> +
> +    actual_json = json.loads(actual_content)
> +    expect_json = json.loads(expect_content)
> +
> +    return compare_json([], actual_json, expect_json)
> +
> +
> +if __name__ == '__main__':
> +    import sys
> +
> +    actual = None
> +    expect = None
> +
> +    # Parse arguments.
> +    args = sys.argv[1:]
> +    while args:
> +        # Take an argument.
> +        arg = args.pop(0)
> +
> +        # Parse out replacement expressions.
> +        if arg == '-r' or arg == '--replace':
> +            replacement = args.pop(0)
> +            (key, value) = replacement.split('=', maxsplit=1)
> +            REPLACEMENTS[key] = value
> +        # Flag to change how errors are reported.
> +        elif arg == '-A' or arg == '--all':
> +            ALL_ERRORS = True
> +        # Required arguments.
> +        elif arg == '-a' or arg == '--actual':
> +            actual = args.pop(0)
> +        elif arg == '-e' or arg == '--expect':
> +            expect = args.pop(0)
> +
> +    # Validate that we have the required arguments.
> +    if actual is None:
> +        raise RuntimeError('missing "actual" file')
> +    if expect is None:
> +        raise RuntimeError('missing "expect" file')
> +
> +    # Do the actual work.
> +    is_ok = validate_p1689(actual, expect)
> +
> +    # Fail if errors are found.
> +    if not is_ok:
> +        sys.exit(1)
> diff --git a/gcc/testsuite/lib/modules.exp b/gcc/testsuite/lib/modules.exp
> new file mode 100644
> index 00000000000..c7cfda6aae4
> --- /dev/null
> +++ b/gcc/testsuite/lib/modules.exp
> @@ -0,0 +1,71 @@
> +#   Copyright (C) 1997-2022 Free Software Foundation, Inc.
> +
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with GCC; see the file COPYING3.  If not see
> +# <http://www.gnu.org/licenses/>.
> +
> +# Verify various kinds of gcov output: line counts, branch percentages,
> +# and call return percentages.  None of this is language-specific.
> +
> +load_lib "target-supports.exp"
> +
> +#
> +# clean-p1689-file -- delete a working file the compiler creates for p1689
> +#
> +# TESTCASE is the name of the test.
> +# SUFFIX is file suffix
> +
> +proc clean-p1689-file { testcase suffix } {
> +    set basename [file tail $testcase]
> +    set base [file rootname $basename]
> +    remote_file host delete $base.$suffix
> +}
> +
> +#
> +# clean-p1689 -- delete the working files the compiler creates for p1689
> +#
> +# TESTCASE is the name of the test.
> +#
> +proc clean-p1689 { testcase } {
> +    clean-p1689-file $testcase "d"
> +    clean-p1689-file $testcase "json"
> +}
> +
> +# Call by dg-final to check a P1689 dependency file
> +
> +proc run-check-p1689-valid { depfile template } {
> +    global srcdir subdir
> +    # Extract the test file name from the arguments.
> +    set testcase [file rootname [file tail $depfile]]
> +
> +    verbose "Running P1689 validation for $testcase in $srcdir/$subdir" 2
> +    set testcase [remote_download host $testcase]
> +
> +    set pytest_script "test-p1689.py"
> +    if { ![check_effective_target_recent_python3] } {
> +      unsupported "$pytest_script python3 is missing"
> +      return
> +    }
> +
> +    verbose "running script" 1
> +    spawn -noecho python3 $srcdir/$subdir/$pytest_script --all --actual $depfile --expect $srcdir/$subdir/$template
> +
> +    expect {
> +      -re "ERROR: (\[^\r\n\]*)" {
> +       fail $expect_out(0,string)
> +       exp_continue
> +      }
> +    }
> +
> +    clean-p1689 $testcase
> +}
> diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
> index 8df071e1587..aa1c065104a 100644
> --- a/libcpp/include/cpplib.h
> +++ b/libcpp/include/cpplib.h
> @@ -302,6 +302,9 @@ typedef CPPCHAR_SIGNED_T cppchar_signed_t;
>   /* Style of header dependencies to generate.  */
>   enum cpp_deps_style { DEPS_NONE = 0, DEPS_USER, DEPS_SYSTEM };
>   
> +/* Format of header dependencies to generate.  */
> +enum cpp_deps_format { DEPS_FMT_NONE = 0, DEPS_FMT_P1689R5 };

 From the earlier review:

> > Why not add this to cpp_deps_style?
>
> It is orthogonal. The `-M` flags and `-fdeps-*` flags are similar, but
> `-fdeps-*` dependencies are fundamentally different than `-M`
> dependencies. The comment does need updated though.

Makes sense, but "cpp_deps_style" and "cpp_deps_format" don't sound 
orthogonal, they sound overlapping.  Maybe just call it 
"cpp_fdeps_format" to match all the other fdeps, and fix the comment; 
maybe just add "structured" here, too.

> `-M` is about discovered dependencies: those that you find out while
> doing work. `-fdep-*` is about ordering dependencies: extracting
> information from file content in order to even order future work around.

I'm not sure I see the distinction; Makefiles also express ordering 
dependencies.  In both cases, you want to find out from the files what 
order you will want to process them in when building the project.

For modules, there's currently a problem arising from the lack of either 
on-demand BMI generation (apart from Nathan's P1602 prototype) or 
fallback to consuming modules as source directly, so indeed it's 
currently important to support a separate discovery phase.  But that 
seems like a separate question from the output file format; the 
discovery phase could emit Makefile dependencies and not the p1689 
format, if that makes more sense for the individual project.

For some projects, I imagine the discovery commands could also be the 
same as the build commands, and you just need to keep trying until the 
build succeeds.  I wonder if there's a way to automatically retry if 
dependency files got updated...

>   /* The possible normalization levels, from most restrictive to least.  */
>   enum cpp_normalize_level {
>     /* In NFKC.  */
> @@ -589,6 +592,9 @@ struct cpp_options
>       /* Style of header dependencies to generate.  */
>       enum cpp_deps_style style;
>   
> +    /* Format of header dependencies to generate.  */
> +    enum cpp_deps_format format;

And this should probably be fdeps_format.

>       /* Assume missing files are generated files.  */
>       bool missing_files;
>   
> @@ -1112,9 +1118,9 @@ extern void cpp_post_options (cpp_reader *);
>   extern void cpp_init_iconv (cpp_reader *);
>   
>   /* Call this to finish preprocessing.  If you requested dependency
> -   generation, pass an open stream to write the information to,
> -   otherwise NULL.  It is your responsibility to close the stream.  */
> -extern void cpp_finish (cpp_reader *, FILE *deps_stream);
> +   generation, pass open stream(s) to write the information to,
> +   otherwise NULL.  It is your responsibility to close the stream(s).  */
> +extern void cpp_finish (cpp_reader *, FILE *deps_stream, FILE *fdeps_stream = NULL);
>   
>   /* Call this to release the handle at the end of preprocessing.  Any
>      use of the handle after this function returns is invalid.  */
> diff --git a/libcpp/include/mkdeps.h b/libcpp/include/mkdeps.h
> index 920e2791334..33c7437a481 100644
> --- a/libcpp/include/mkdeps.h
> +++ b/libcpp/include/mkdeps.h
> @@ -53,20 +53,29 @@ extern void deps_add_default_target (class mkdeps *, const char *);
>   
>   /* Adds a module target.  The module name and cmi name are copied.  */
>   extern void deps_add_module_target (struct mkdeps *, const char *module,
> -				    const char *cmi, bool is_header);
> +				    const char *cmi, bool is_header,
> +				    bool is_exported);
>   
>   /* Adds a module dependency.  The module name is copied.  */
>   extern void deps_add_module_dep (struct mkdeps *, const char *module);
>   
> +/* Add an output.  */
> +extern void deps_add_output (struct mkdeps *, const char *, bool);
> +
>   /* Add a dependency (appears on the right side of the colon) to the
>      deps list.  Dependencies will be printed in the order that they
>      were entered with this function.  By convention, the first
>      dependency entered should be the primary source file.  */
>   extern void deps_add_dep (class mkdeps *, const char *);
>   
> -/* Write out a deps buffer to a specified file.  The last argument
> -   is the number of columns to word-wrap at (0 means don't wrap).  */
> -extern void deps_write (const cpp_reader *, FILE *, unsigned int);
> +/* Write out a deps buffer to a specified file.  The third argument
> +   is the number of columns to word-wrap at (0 means don't wrap).
> +   The last argument indicates whether to output extra information
> +   (namely modules).  */
> +extern void deps_write (const struct cpp_reader *, FILE *, unsigned int);

Seems like this change isn't needed anymore.

> +/* Write out a deps buffer to a specified file in P1689R5 format.  */
> +extern void deps_write_p1689r5 (const struct mkdeps *, FILE *);
>   
>   /* Write out a deps buffer to a file, in a form that can be read back
>      with deps_restore.  Returns nonzero on error, in which case the
> diff --git a/libcpp/init.cc b/libcpp/init.cc
> index c508f06112a..d34fd6fdeef 100644
> --- a/libcpp/init.cc
> +++ b/libcpp/init.cc
> @@ -855,7 +855,7 @@ read_original_directory (cpp_reader *pfile)
>      Maybe it should also reset state, such that you could call
>      cpp_start_read with a new filename to restart processing.  */
>   void
> -cpp_finish (cpp_reader *pfile, FILE *deps_stream)
> +cpp_finish (struct cpp_reader *pfile, FILE *deps_stream, FILE *fdeps_stream)
>   {
>     /* Warn about unused macros before popping the final buffer.  */
>     if (CPP_OPTION (pfile, warn_unused_macros))
> @@ -869,8 +869,15 @@ cpp_finish (cpp_reader *pfile, FILE *deps_stream)
>     while (pfile->buffer)
>       _cpp_pop_buffer (pfile);
>   
> -  if (deps_stream)
> -    deps_write (pfile, deps_stream, 72);
> +  cpp_deps_format deps_format = CPP_OPTION (pfile, deps.format);
> +  if (deps_format == DEPS_FMT_P1689R5 && fdeps_stream)
> +    deps_write_p1689r5 (pfile->deps, fdeps_stream);
> +
> +  if (CPP_OPTION (pfile, deps.style) != DEPS_NONE
> +      && deps_stream)

Why does the Make handling need to change?  deps_stream should be null 
if we aren't emitting Make deps, right?

> +    {
> +      deps_write (pfile, deps_stream, 72);
> +    }
>   
>     /* Report on headers that could use multiple include guards.  */
>     if (CPP_OPTION (pfile, print_include_names))
> diff --git a/libcpp/mkdeps.cc b/libcpp/mkdeps.cc
> index 8f9585c3c0a..081287c94ba 100644
> --- a/libcpp/mkdeps.cc
> +++ b/libcpp/mkdeps.cc
> @@ -81,7 +81,8 @@ public:
>     };
>   
>     mkdeps ()
> -    : module_name (NULL), cmi_name (NULL), is_header_unit (false), quote_lwm (0)
> +    : primary_output (NULL), module_name (NULL), cmi_name (NULL)
> +    , is_header_unit (false), is_exported (false), quote_lwm (0)
>     {
>     }
>     ~mkdeps ()
> @@ -90,6 +91,9 @@ public:
>   
>       for (i = targets.size (); i--;)
>         free (const_cast <char *> (targets[i]));
> +    free (const_cast <char *> (primary_output));
> +    for (i = outputs.size (); i--;)
> +      free (const_cast <char *> (outputs[i]));
>       for (i = deps.size (); i--;)
>         free (const_cast <char *> (deps[i]));
>       for (i = vpath.size (); i--;)
> @@ -103,6 +107,8 @@ public:
>   public:
>     vec<const char *> targets;
>     vec<const char *> deps;
> +  const char * primary_output;
> +  vec<const char *> outputs;

fdeps_targets, I think

>     vec<velt> vpath;
>     vec<const char *> modules;
>   
> @@ -110,6 +116,7 @@ public:
>     const char *module_name;
>     const char *cmi_name;
>     bool is_header_unit;
> +  bool is_exported;
>     unsigned short quote_lwm;
>   };
>   
> @@ -288,6 +295,21 @@ deps_add_default_target (class mkdeps *d, const char *tgt)
>       }
>   }
>   
> +/* Adds an output O.  We make a copy, so it need not be a permanent
> +   string.  */
> +void
> +deps_add_output (struct mkdeps *d, const char *o, bool is_primary)

As mentioned above, I think this should be fdeps_add_target.  And please 
add more commentary about why this wouldn't just be the same as -MT.

> +{
> +  o = apply_vpath (d, o);
> +  if (is_primary)
> +  {
> +    if (d->primary_output)
> +      d->outputs.push (d->primary_output);
> +    d->primary_output = xstrdup (o);
> +  } else
> +    d->outputs.push (xstrdup (o));
> +}
> +
>   void
>   deps_add_dep (class mkdeps *d, const char *t)
>   {
> @@ -325,12 +347,13 @@ deps_add_vpath (class mkdeps *d, const char *vpath)
>   
>   void
>   deps_add_module_target (struct mkdeps *d, const char *m,
> -			const char *cmi, bool is_header_unit)
> +			const char *cmi, bool is_header_unit, bool is_exported)
>   {
>     gcc_assert (!d->module_name);
>     
>     d->module_name = xstrdup (m);
>     d->is_header_unit = is_header_unit;
> +  d->is_exported = is_exported;
>     d->cmi_name = xstrdup (cmi);
>   }
>   
> @@ -395,10 +418,15 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
>     if (colmax && colmax < 34)
>       colmax = 34;
>   
> +  /* Write out C++ modules information if no other `-fdeps-format=`
> +   * option is given. */

Stray *

> +  cpp_deps_format deps_format = CPP_OPTION (pfile, deps.format);
> +  bool write_make_modules_deps = deps_format == DEPS_FMT_NONE;

Maybe && CPP_OPTION (pfile, deps.modules)?

> +
>     if (d->deps.size ())
>       {
>         column = make_write_vec (d->targets, fp, 0, colmax, d->quote_lwm);
> -      if (CPP_OPTION (pfile, deps.modules) && d->cmi_name)
> +      if (write_make_modules_deps && CPP_OPTION (pfile, deps.modules) && d->cmi_name)

So you don't need to check it here.

>   	column = make_write_name (d->cmi_name, fp, column, colmax);
>         fputs (":", fp);
>         column++;
> @@ -412,7 +440,7 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
>     if (!CPP_OPTION (pfile, deps.modules))

And then this can be if (!write_make_modules_deps) and you don't need to 
check the flag in the rest of the ifs.

>       return;
>   
> -  if (d->modules.size ())
> +  if (write_make_modules_deps && d->modules.size ())
>       {
>         column = make_write_vec (d->targets, fp, 0, colmax, d->quote_lwm);
>         if (d->cmi_name)
> @@ -423,7 +451,7 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
>         fputs ("\n", fp);
>       }
>   
> -  if (d->module_name)
> +  if (write_make_modules_deps && d->module_name)
>       {
>         if (d->cmi_name)
>   	{
> @@ -455,7 +483,7 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
>   	}
>       }
>     
> -  if (d->modules.size ())
> +  if (write_make_modules_deps && d->modules.size ())
>       {
>         column = fprintf (fp, "CXX_IMPORTS +=");
>         make_write_vec (d->modules, fp, column, colmax, 0, ".c++m");
> @@ -468,11 +496,118 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
>   /* Really we should be opening fp here.  */
>   
>   void
> -deps_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
> +deps_write (const struct cpp_reader *pfile, FILE *fp, unsigned int colmax)

As with the prototype, this change seems unneeded.

>   {
>     make_write (pfile, fp, colmax);
>   }
>   
> +static void
> +p1689r5_write_filepath (const char *name, FILE *fp)
> +{
> +  if (_cpp_valid_utf8_str (name))
> +    {
> +      fputc ('"', fp);
> +      for (const char* c = name; *c; c++)
> +	{
> +	  // Escape control characters.
> +	  if (ISCNTRL (*c))
> +	    fprintf (fp, "\\u%04x", *c);
> +	  // JSON escape characters.
> +	  else if (*c == '"' || *c == '\\')
> +	    {
> +	      fputc ('\\', fp);
> +	      fputc (*c, fp);
> +	    }
> +	  // Everything else.
> +	  else
> +	    fputc (*c, fp);
> +	}
> +      fputc ('"', fp);
> +    }
> +  else
> +    {
> +      // TODO: print an error
> +    }
> +}
> +
> +static void
> +p1689r5_write_vec (const mkdeps::vec<const char *> &vec, FILE *fp)
> +{
> +  for (unsigned ix = 0; ix != vec.size (); ix++)
> +    {
> +      p1689r5_write_filepath (vec[ix], fp);
> +      if (ix < vec.size () - 1)
> +	fputc (',', fp);
> +      fputc ('\n', fp);
> +    }
> +}
> +
> +void
> +deps_write_p1689r5 (const struct mkdeps *d, FILE *fp)
> +{
> +  fputs ("{\n", fp);
> +
> +  fputs ("\"rules\": [\n", fp);
> +  fputs ("{\n", fp);
> +
> +  if (d->primary_output)
> +    {
> +      fputs ("\"primary-output\": ", fp);
> +      p1689r5_write_filepath (d->primary_output, fp);
> +      fputs (",\n", fp);
> +    }
> +
> +  if (d->outputs.size ())
> +    {
> +      fputs ("\"outputs\": [\n", fp);
> +      p1689r5_write_vec (d->outputs, fp);
> +      fputs ("],\n", fp);
> +    }
> +
> +  if (d->module_name)
> +    {
> +      fputs ("\"provides\": [\n", fp);
> +      fputs ("{\n", fp);
> +
> +      fputs ("\"logical-name\": ", fp);
> +      p1689r5_write_filepath (d->module_name, fp);
> +      fputs (",\n", fp);
> +
> +      fprintf (fp, "\"is-interface\": %s\n", d->is_exported ? "true" : "false");
> +
> +      // TODO: header-unit information
> +
> +      fputs ("}\n", fp);
> +      fputs ("],\n", fp);
> +    }
> +
> +  fputs ("\"requires\": [\n", fp);
> +  for (size_t i = 0; i < d->modules.size (); i++)
> +    {
> +      if (i != 0)
> +	fputs (",\n", fp);
> +      fputs ("{\n", fp);
> +
> +      fputs ("\"logical-name\": ", fp);
> +      p1689r5_write_filepath (d->modules[i], fp);
> +      fputs ("\n", fp);
> +
> +      // TODO: header-unit information
> +
> +      fputs ("}\n", fp);
> +    }
> +  fputs ("]\n", fp);
> +
> +  fputs ("}\n", fp);
> +
> +  fputs ("],\n", fp);
> +
> +  fputs ("\"version\": 0,\n", fp);
> +  fputs ("\"revision\": 0\n", fp);
> +
> +  fputs ("}\n", fp);
> +}

Is there a reason not to use the gcc/json.h interface for JSON output?

Jason