[PATCH RFC] c++: add module extensions

[PATCH RFC] c++: add module extensions

[Jason Merrill]

Tested x86_64-pc-linux-gnu.  Any thoughts about the mkdeps output?

-- 8< --

There is a trend in the broader C++ community to use a different extension
for module interface units, even though they are compiled in the same way as
other source files.  Let's also support these extensions.

.ixx is the MSVC standard, while the .c*m are supported by Clang.  libc++
standard headers use .cppm, as their other source files use .cpp.
Perhaps libstdc++ will use .ccm for parallel consistency?

One issue with .c++m is that libcpp/mkdeps.cc uses it for the phony
dependencies to express module dependencies, so I'm disabling that one for
now.  We probably want to change the extension that mkdeps uses to something
less likely to be an actual file, say .module? .c++-module?

gcc/cp/ChangeLog:

	* lang-specs.h: Add module interface extensions.
---
 gcc/cp/lang-specs.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/gcc/cp/lang-specs.h b/gcc/cp/lang-specs.h
index 7a7f5ff0ab5..74b450fd66e 100644
--- a/gcc/cp/lang-specs.h
+++ b/gcc/cp/lang-specs.h
@@ -39,6 +39,14 @@ along with GCC; see the file COPYING3.  If not see
   {".HPP", "@c++-header", 0, 0, 0},
   {".tcc", "@c++-header", 0, 0, 0},
   {".hh",  "@c++-header", 0, 0, 0},
+  /* Module interface unit.  Do we also want a .C counterpart?
+     Skipping .c++m for now at least to avoid conflicts with .PHONY .c++m
+     files in mkdeps.cc output.  */
+  {".ixx", "@c++", 0, 0, 0}, /* MSVC */
+  {".cppm", "@c++", 0, 0, 0}, /* Clang/libc++ */
+  {".cxxm", "@c++", 0, 0, 0},
+  /* {".c++m", "@c++", 0, 0, 0}, */
+  {".ccm", "@c++", 0, 0, 0},
   {"@c++-header",
       "%{E|M|MM:cc1plus -E %{fmodules-ts:-fdirectives-only -fmodule-header}"
       "  %(cpp_options) %2 %(cpp_debug_options)}"

base-commit: 1a5e4dd83788ea4c049d354d83ad58a6a3d747e6
prerequisite-patch-id: 3c000c95725bc74cff0b0e33fac97055caa64e7e
-- 
2.44.0

[pushed] c++: add module extensions

[Jason Merrill]

Revised to change mkdeps and the docs.

Tested x86_64-pc-linux-gnu, applying to trunk.

-- 8< --

There is a trend in the broader C++ community to use a different extension
for module interface units, even though (in GCC) they are compiled in the
same way as other source files.  Let's recognize these extensions as C++.

.ixx is the MSVC standard, while the .c*m are supported by Clang.  libc++
standard headers use .cppm, as their other source files use .cpp.
Perhaps libstdc++ might use .ccm for parallel consistency?

One issue with .c++m is that libcpp/mkdeps.cc has been using it for the
phony dependencies to express module dependencies, so I'm changing mkdeps to
something less likely to be an actual file, ".c++-module".

gcc/cp/ChangeLog:

	* lang-specs.h: Add module interface extensions.

gcc/ChangeLog:

	* doc/invoke.texi: Update module extension docs.

libcpp/ChangeLog:

	* mkdeps.cc (make_write): Change .c++m to .c++-module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dep-1_a.C
	* g++.dg/modules/dep-1_b.C
	* g++.dg/modules/dep-2.C: Change .c++m to .c++-module.
---
 gcc/doc/invoke.texi                    | 20 ++++++++++----------
 gcc/cp/lang-specs.h                    |  6 ++++++
 gcc/testsuite/g++.dg/modules/dep-1_a.C |  4 ++--
 gcc/testsuite/g++.dg/modules/dep-1_b.C |  8 ++++----
 gcc/testsuite/g++.dg/modules/dep-2.C   |  4 ++--
 libcpp/mkdeps.cc                       | 13 ++++++-------
 6 files changed, 30 insertions(+), 25 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2cba380718b..517a782987d 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -2317,9 +2317,12 @@ other language.
 C++ source files conventionally use one of the suffixes @samp{.C},
 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
 @samp{.cxx}; C++ header files often use @samp{.hh}, @samp{.hpp},
-@samp{.H}, or (for shared template code) @samp{.tcc}; and
-preprocessed C++ files use the suffix @samp{.ii}.  GCC recognizes
-files with these names and compiles them as C++ programs even if you
+@samp{.H}, or (for shared template code) @samp{.tcc};
+preprocessed C++ files use the suffix @samp{.ii}; and C++20 module interface
+units sometimes use @samp{.ixx}, @samp{.cppm}, @samp{.cxxm}, @samp{.c++m},
+or @samp{.ccm}.
+
+GCC recognizes files with these names and compiles them as C++ programs even if you
 call the compiler the same way as for compiling C programs (usually
 with the name @command{gcc}).
 
@@ -37705,13 +37708,10 @@ Modular compilation is @emph{not} enabled with just the
 version selected, although in pre-C++20 versions, it is of course an
 extension.
 
-No new source file suffixes are required or supported.  If you wish to
-use a non-standard suffix (@pxref{Overall Options}), you also need
-to provide a @option{-x c++} option too.@footnote{Some users like to
-distinguish module interface files with a new suffix, such as naming
-the source @code{module.cppm}, which involves
-teaching all tools about the new suffix.  A different scheme, such as
-naming @code{module-m.cpp} would be less invasive.}
+No new source file suffixes are required.  A few suffixes preferred
+for module interface units by other compilers (e.g. @samp{.ixx},
+@samp{.cppm}) are supported, but files with these suffixes are treated
+the same as any other C++ source file.
 
 Compiling a module interface unit produces an additional output (to
 the assembly or object file), called a Compiled Module Interface
diff --git a/gcc/cp/lang-specs.h b/gcc/cp/lang-specs.h
index 7a7f5ff0ab5..e5651567a2d 100644
--- a/gcc/cp/lang-specs.h
+++ b/gcc/cp/lang-specs.h
@@ -39,6 +39,12 @@ along with GCC; see the file COPYING3.  If not see
   {".HPP", "@c++-header", 0, 0, 0},
   {".tcc", "@c++-header", 0, 0, 0},
   {".hh",  "@c++-header", 0, 0, 0},
+  /* Module interface unit.  Should there also be a .C counterpart?  */
+  {".ixx", "@c++", 0, 0, 0}, /* MSVC */
+  {".cppm", "@c++", 0, 0, 0}, /* Clang/libc++ */
+  {".cxxm", "@c++", 0, 0, 0},
+  {".c++m", "@c++", 0, 0, 0},
+  {".ccm", "@c++", 0, 0, 0},
   {"@c++-header",
       "%{E|M|MM:cc1plus -E %{fmodules-ts:-fdirectives-only -fmodule-header}"
       "  %(cpp_options) %2 %(cpp_debug_options)}"
diff --git a/gcc/testsuite/g++.dg/modules/dep-1_a.C b/gcc/testsuite/g++.dg/modules/dep-1_a.C
index 5ec5dd30f6d..3e92eeaef9f 100644
--- a/gcc/testsuite/g++.dg/modules/dep-1_a.C
+++ b/gcc/testsuite/g++.dg/modules/dep-1_a.C
@@ -4,6 +4,6 @@ export module m:part;
 // { dg-module-cmi m:part }
 
 // All The Backslashes!
-// { dg-final { scan-file dep-1_a.d {\nm:part\.c\+\+m: gcm.cache/m-part\.gcm} } }
+// { dg-final { scan-file dep-1_a.d {\nm:part\.c\+\+-module: gcm.cache/m-part\.gcm} } }
 // { dg-final { scan-file dep-1_a.d {\ngcm.cache/m-part\.gcm:| dep-1_a\.o} } }
-// { dg-final { scan-file dep-1_a.d {\n\.PHONY: m:part\.c\+\+m} } }
+// { dg-final { scan-file dep-1_a.d {\n\.PHONY: m:part\.c\+\+-module} } }
diff --git a/gcc/testsuite/g++.dg/modules/dep-1_b.C b/gcc/testsuite/g++.dg/modules/dep-1_b.C
index 7a0794eda51..265ebfcda64 100644
--- a/gcc/testsuite/g++.dg/modules/dep-1_b.C
+++ b/gcc/testsuite/g++.dg/modules/dep-1_b.C
@@ -3,8 +3,8 @@ export module m;
 // { dg-module-cmi m }
 
 export import :part;
-// { dg-final { scan-file dep-1_b.d {\ndep-1_b\.s gcm.cache/m\.gcm: m:part\.c\+\+m} } }
-// { dg-final { scan-file dep-1_b.d {\nm\.c\+\+m: gcm.cache/m\.gcm} } }
-// { dg-final { scan-file dep-1_b.d {\n\.PHONY: m\.c\+\+m} } }
+// { dg-final { scan-file dep-1_b.d {\ndep-1_b\.s gcm.cache/m\.gcm: m:part\.c\+\+-module} } }
+// { dg-final { scan-file dep-1_b.d {\nm\.c\+\+-module: gcm.cache/m\.gcm} } }
+// { dg-final { scan-file dep-1_b.d {\n\.PHONY: m\.c\+\+-module} } }
 // { dg-final { scan-file dep-1_b.d {\ngcm.cache/m\.gcm:| dep-1_b.o} } }
-// { dg-final { scan-file dep-1_b.d {\nCXX_IMPORTS \+= m:part\.c\+\+m} } }
+// { dg-final { scan-file dep-1_b.d {\nCXX_IMPORTS \+= m:part\.c\+\+-module} } }
diff --git a/gcc/testsuite/g++.dg/modules/dep-2.C b/gcc/testsuite/g++.dg/modules/dep-2.C
index 096243096e3..2dccab3554d 100644
--- a/gcc/testsuite/g++.dg/modules/dep-2.C
+++ b/gcc/testsuite/g++.dg/modules/dep-2.C
@@ -5,8 +5,8 @@ module m:part;
 // { dg-module-cmi !m:part }
 
 // All The Backslashes!
-// { dg-final { scan-file dep-2.d {\nm:part\.c\+\+m: gcm.cache/m-part\.gcm} } }
+// { dg-final { scan-file dep-2.d {\nm:part\.c\+\+-module: gcm.cache/m-part\.gcm} } }
 // { dg-final { scan-file dep-2.d {\ngcm.cache/m:part\.gcm:| dep-2\.o} } }
-// { dg-final { scan-file dep-2.d {\n\.PHONY: m:part\.c\+\+m} } }
+// { dg-final { scan-file dep-2.d {\n\.PHONY: m:part\.c\+\+-module} } }
 
 // { dg-final { scan-file dep-2.i {\nmodule  m:part;\n} } }
diff --git a/libcpp/mkdeps.cc b/libcpp/mkdeps.cc
index 4cf0cf09178..622d0dc2ef8 100644
--- a/libcpp/mkdeps.cc
+++ b/libcpp/mkdeps.cc
@@ -411,8 +411,7 @@ make_write_vec (const mkdeps::vec<const char *> &vec, FILE *fp,
   return col;
 }
 
-/* Write the dependencies to a Makefile.  If PHONY is true, add
-   .PHONY targets for all the dependencies too.  */
+/* Write the dependencies to a Makefile.  */
 
 static void
 make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
@@ -453,7 +452,7 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
 	column = make_write_name (d->cmi_name, fp, column, colmax);
       fputs (":", fp);
       column++;
-      column = make_write_vec (d->modules, fp, column, colmax, 0, ".c++m");
+      column = make_write_vec (d->modules, fp, column, colmax, 0, ".c++-module");
       fputs ("\n", fp);
     }
 
@@ -463,7 +462,7 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
 	{
 	  /* module-name : cmi-name */
 	  column = make_write_name (d->module_name, fp, 0, colmax,
-				    true, ".c++m");
+				    true, ".c++-module");
 	  fputs (":", fp);
 	  column++;
 	  column = make_write_name (d->cmi_name, fp, column, colmax);
@@ -471,7 +470,7 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
 
 	  column = fprintf (fp, ".PHONY:");
 	  column = make_write_name (d->module_name, fp, column, colmax,
-				    true, ".c++m");
+				    true, ".c++-module");
 	  fputs ("\n", fp);
 	}
 
@@ -488,11 +487,11 @@ make_write (const cpp_reader *pfile, FILE *fp, unsigned int colmax)
 	  fputs ("\n", fp);
 	}
     }
-  
+
   if (d->modules.size ())
     {
       column = fprintf (fp, "CXX_IMPORTS +=");
-      make_write_vec (d->modules, fp, column, colmax, 0, ".c++m");
+      make_write_vec (d->modules, fp, column, colmax, 0, ".c++-module");
       fputs ("\n", fp);
     }
 }

base-commit: 19c491d1848a8410559247183597096778967edf
-- 
2.44.0