[C++20] [Modules] Enable reduced BMI by default

As documented in 20.x, we'd like to keep reduced BMI off by default for
1~2 versions. And now we're in 22.x.

I rarely receive bug reports for reduced BMI. I am not sure about the
reason. Maybe not a lot of people are using it. Or it is really stable
enough.

And also, we've been enabling the reduced BMI internally for roughly half a
year.

So I think it's the time to move on. See the document changes for other
information.

Author: ChuanqiXu9

clang/docs/ReleaseNotes.rst

@@ -77,6 +77,10 @@ C++ Specific Potentially Breaking Changes
   whose nested-name-specifier doesn't refer to a base class such as
   ``using CurrentClass::Foo;`` is now rejected in C++98 mode.
 
+- For C++20 modules, the Reduced BMI mode will be the default option. This may introduce
+  regressions if your build system supports two-phase compilation model but haven't support
+  reduced BMI or it is a compiler bug or a bug in users code.
+
 ABI Changes in This Version
 ---------------------------
 

clang/docs/StandardCPlusPlusModules.rst

@@ -687,16 +687,12 @@ fails to instantiate. For such issues, users can add references to ``N::g`` in
 the `module purview <https://eel.is/c++draft/module.unit#5>`_ of ``M.cppm`` to
 ensure it is reachable, e.g. ``using N::g;``.
 
-Support for Reduced BMIs is still experimental, but it may become the default
-in the future. The expected roadmap for Reduced BMIs as of Clang 19.x is:
-
-1. ``-fexperimental-modules-reduced-bmi`` was introduced in v19.x
-2. For v20.x, ``-fmodules-reduced-bmi`` is introduced as an equivalent non-experimental
-   option. It is expected to stay opt-in for 1~2 releases, though the period depends
-   on user feedback and may be extended.
-3. Finally, ``-fmodules-reduced-bmi`` will be the default. When that time
-   comes, the term BMI will refer to the Reduced BMI and the Full BMI will only
-   be meaningful to build systems which elect to support two-phase compilation.
+As of Clang 22.x, the Reduced BMI is enabled by default. You may still want to
+use Full BMI with ``-fno-modules-reduced-bmi`` in the following case:
+1. Your build system uses two-phase compilation but it haven't adjusted the
+   implementation for reduced BMI.
+2. You meet a regression with Reduced BMI that you cannot work around. Please
+   report an issue for this case.
 
 Experimental Non-Cascading Changes
 ----------------------------------

clang/include/clang/Driver/Options.td

@@ -3257,13 +3257,14 @@ defm skip_odr_check_in_gmf : BoolOption<"f", "skip-odr-check-in-gmf",
           "Perform ODR checks for decls in the global module fragment.">>,
   Group<f_Group>;
 
-def modules_reduced_bmi : Flag<["-"], "fmodules-reduced-bmi">,
-  Group<f_Group>, Visibility<[ClangOption, CC1Option]>,
-  HelpText<"Generate the reduced BMI">,
-  MarshallingInfoFlag<FrontendOpts<"GenReducedBMI">>;
+defm modules_reduced_bmi : BoolOption<"f", "modules-reduced-bmi",
+  FrontendOpts<"GenReducedBMI">, DefaultFalse,
+  NegFlag<SetFalse>,
+  PosFlag<SetTrue, [], [ClangOption, CC1Option],
+          "Generate the reduced BMI">>;
 
 def experimental_modules_reduced_bmi : Flag<["-"], "fexperimental-modules-reduced-bmi">,
-  Group<f_Group>, Visibility<[ClangOption, CC1Option]>, Alias<modules_reduced_bmi>;
+  Group<f_Group>, Visibility<[ClangOption, CC1Option]>, Alias<fmodules_reduced_bmi>;
 
 def fmodules_embed_all_files : Joined<["-"], "fmodules-embed-all-files">,
   Visibility<[ClangOption, CC1Option, CLOption]>,

clang/lib/Driver/Driver.cpp

@@ -5131,11 +5131,13 @@ Action *Driver::ConstructPhaseAction(
     if (Args.hasArg(options::OPT_extract_api))
       return C.MakeAction<ExtractAPIJobAction>(Input, types::TY_API_INFO);
 
-    // With 'fexperimental-modules-reduced-bmi', we don't want to run the
+    // With 'fmodules-reduced-bmi', we don't want to run the
     // precompile phase unless the user specified '--precompile'. In the case
     // the '--precompile' flag is enabled, we will try to emit the reduced BMI
     // as a by product in GenerateModuleInterfaceAction.
-    if (Args.hasArg(options::OPT_modules_reduced_bmi) &&
+    if (!Args.hasArg(options::OPT_fno_modules_reduced_bmi) &&
+        (Input->getType() == driver::types::TY_CXXModule ||
+         Input->getType() == driver::types::TY_PP_CXXModule) &&
         !Args.getLastArg(options::OPT__precompile))
       return Input;
 
@@ -6323,7 +6325,7 @@ const char *Driver::GetNamedOutputPath(Compilation &C, const JobAction &JA,
   // `-fmodule-output`.
   if (!AtTopLevel && isa<PrecompileJobAction>(JA) &&
       JA.getType() == types::TY_ModuleFile && SpecifiedModuleOutput) {
-    assert(!C.getArgs().hasArg(options::OPT_modules_reduced_bmi));
+    assert(C.getArgs().hasArg(options::OPT_fno_modules_reduced_bmi));
     return GetModuleOutputPath(C, JA, BaseInput);
   }
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4085,31 +4085,34 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
   // module fragment.
   CmdArgs.push_back("-fskip-odr-check-in-gmf");
 
-  if (Args.hasArg(options::OPT_modules_reduced_bmi) &&
+  if (!Args.hasArg(options::OPT_fno_modules_reduced_bmi) &&
       (Input.getType() == driver::types::TY_CXXModule ||
-       Input.getType() == driver::types::TY_PP_CXXModule)) {
+       Input.getType() == driver::types::TY_PP_CXXModule) &&
+      !Args.hasArg(options::OPT__precompile)) {
     CmdArgs.push_back("-fmodules-reduced-bmi");
 
     if (Args.hasArg(options::OPT_fmodule_output_EQ))
       Args.AddLastArg(CmdArgs, options::OPT_fmodule_output_EQ);
-    else {
-      if (Args.hasArg(options::OPT__precompile) &&
-          (!Args.hasArg(options::OPT_o) ||
-           Args.getLastArg(options::OPT_o)->getValue() ==
-               getCXX20NamedModuleOutputPath(Args, Input.getBaseInput()))) {
-        D.Diag(diag::err_drv_reduced_module_output_overrided);
-      }
-
+    else
       CmdArgs.push_back(Args.MakeArgString(
           "-fmodule-output=" +
           getCXX20NamedModuleOutputPath(Args, Input.getBaseInput())));
-    }
   }
 
-  // Noop if we see '-fmodules-reduced-bmi' with other translation
-  // units than module units. This is more user friendly to allow end uers to
-  // enable this feature without asking for help from build systems.
-  Args.ClaimAllArgs(options::OPT_modules_reduced_bmi);
+  if (Args.hasArg(options::OPT_fmodules_reduced_bmi) &&
+      Args.hasArg(options::OPT__precompile) &&
+      (!Args.hasArg(options::OPT_o) ||
+       Args.getLastArg(options::OPT_o)->getValue() ==
+           getCXX20NamedModuleOutputPath(Args, Input.getBaseInput()))) {
+    D.Diag(diag::err_drv_reduced_module_output_overrided);
+  }
+
+  // Noop if we see '-fmodules-reduced-bmi' or `-fno-modules-reduced-bmi` with
+  // other translation units than module units. This is more user friendly to
+  // allow end uers to enable this feature without asking for help from build
+  // systems.
+  Args.ClaimAllArgs(options::OPT_fmodules_reduced_bmi);
+  Args.ClaimAllArgs(options::OPT_fno_modules_reduced_bmi);
 
   // We need to include the case the input file is a module file here.
   // Since the default compilation model for C++ module interface unit will

clang/test/ClangScanDeps/modules-full-named-modules.cppm

@@ -92,14 +92,7 @@ export void Hello();
 // CHECK-NEXT:             ]
 // CHECK:                 "command-line": [
 // CHECK:                   "-o",
-// CHECK-NEXT:              "{{.*}}/M-{{.*}}.pcm"
-// CHECK:                 ]
-// CHECK:                 "input-file": "[[PREFIX]]/M.cppm"
-// CHECK:               },
-// CHECK-NEXT:          {
-// CHECK:                 "command-line": [
-// CHECK:                   "-o",
-// CHECK-NEXT:              "[[PREFIX]]/M.o"
+// CHECK-NEXT:              "{{.*}}/M.o"
 // CHECK:                 ]
 // CHECK:                 "input-file": "[[PREFIX]]/M.cppm"
 // CHECK:               }
@@ -160,18 +153,7 @@ void World() {
 // CHECK-NEXT:             ]
 // CHECK:                 "command-line": [
 // CHECK:                   "-o",
-// CHECK-NEXT:              "{{.*}}/impl_part-{{.*}}.pcm",
-// CHECK:                 ]
-// CHECK:                 "input-file": "[[PREFIX]]/impl_part.cppm"
-// CHECK:               },
-// CHECK-NEXT:          {
-// CHECK:                 "named-module": "M:impl_part"
-// CHECK-NEXT:            "named-module-deps": [
-// CHECK-NEXT:              "M:interface_part"
-// CHECK-NEXT:             ]
-// CHECK:                 "command-line": [
-// CHECK:                   "-o",
-// CHECK-NEXT:              "[[PREFIX]]/impl_part.o",
+// CHECK-NEXT:              "{{.*}}/impl_part.o",
 // CHECK:                 ]
 // CHECK:                 "input-file": "[[PREFIX]]/impl_part.cppm"
 // CHECK:               }
@@ -194,16 +176,7 @@ export void World();
 // CHECK-NOT:             "named-module-deps": []
 // CHECK:                 "command-line": [
 // CHECK:                   "-o",
-// CHECK-NEXT:              "{{.*}}/interface_part-{{.*}}.pcm",
-// CHECK:                 ]
-// CHECK:                 "input-file": "[[PREFIX]]/interface_part.cppm"
-// CHECK:               },
-// CHECK-NEXT:          {
-// CHECK:                 "named-module": "M:interface_part"
-// CHECK-NOT:             "named-module-deps": []
-// CHECK:                 "command-line": [
-// CHECK:                   "-o",
-// CHECK-NEXT:              "[[PREFIX]]/interface_part.o",
+// CHECK-NEXT:              "{{.*}}/interface_part.o",
 // CHECK:                 ]
 // CHECK:                 "input-file": "[[PREFIX]]/interface_part.cppm"
 // CHECK:               }
@@ -259,14 +232,7 @@ int main() {
 // CHECK-NEXT:             ]
 // CHECK:                 "command-line": [
 // CHECK:                   "-o",
-// CHECK-NEXT:              "{{.*}}/M-{{.*}}.pcm"
-// CHECK:                 ]
-// CHECK:                 "input-file": "[[PREFIX]]/M.cppm"
-// CHECK:               },
-// CHECK-NEXT:          {
-// CHECK:                 "command-line": [
-// CHECK:                   "-o",
-// CHECK-NEXT:              "[[PREFIX]]/M.o"
+// CHECK-NEXT:              "{{.*}}/M.o"
 // CHECK:                 ]
 // CHECK:                 "input-file": "[[PREFIX]]/M.cppm"
 // CHECK:               },
@@ -292,18 +258,7 @@ int main() {
 // CHECK-NEXT:             ]
 // CHECK:                 "command-line": [
 // CHECK:                   "-o",
-// CHECK-NEXT:              "{{.*}}/impl_part-{{.*}}.pcm",
-// CHECK:                 ]
-// CHECK:                 "input-file": "[[PREFIX]]/impl_part.cppm"
-// CHECK:               },
-// CHECK-NEXT:          {
-// CHECK:                 "named-module": "M:impl_part"
-// CHECK-NEXT:            "named-module-deps": [
-// CHECK-NEXT:              "M:interface_part"
-// CHECK-NEXT:             ]
-// CHECK:                 "command-line": [
-// CHECK:                   "-o",
-// CHECK-NEXT:              "[[PREFIX]]/impl_part.o",
+// CHECK-NEXT:              "{{.*}}/impl_part.o",
 // CHECK:                 ]
 // CHECK:                 "input-file": "[[PREFIX]]/impl_part.cppm"
 // CHECK:               }
@@ -316,16 +271,7 @@ int main() {
 // CHECK-NOT:             "named-module-deps": []
 // CHECK:                 "command-line": [
 // CHECK:                   "-o",
-// CHECK-NEXT:              "{{.*}}/interface_part-{{.*}}.pcm",
-// CHECK:                 ]
-// CHECK:                 "input-file": "[[PREFIX]]/interface_part.cppm"
-// CHECK:               },
-// CHECK-NEXT:          {
-// CHECK:                 "named-module": "M:interface_part"
-// CHECK-NOT:             "named-module-deps": []
-// CHECK:                 "command-line": [
-// CHECK:                   "-o",
-// CHECK-NEXT:              "[[PREFIX]]/interface_part.o",
+// CHECK-NEXT:              "{{.*}}/interface_part.o",
 // CHECK:                 ]
 // CHECK:                 "input-file": "[[PREFIX]]/interface_part.cppm"
 // CHECK:               }

clang/test/Driver/module-fgen-reduced-bmi.cppm

@@ -64,7 +64,8 @@
 // RUN:     -Wno-missing-reduced-bmi -### 2>&1 | FileCheck Hello.cppm -check-prefix=NO_WARN
 //
 // RUN: %clang -std=c++20 Hello.cppm --precompile -o Hello.pcm \
-// RUN:     -Wno-missing-reduced-bmi -### 2>&1 | FileCheck Hello.cppm -check-prefix=NO_WARN
+// RUN:     -fno-modules-reduced-bmi -Wno-missing-reduced-bmi -### 2>&1 | \
+// RUN:     FileCheck Hello.cppm -check-prefix=NO_WARN
 
 //--- Hello.cppm
 export module Hello;

clang/test/Driver/module-output.cppm

@@ -13,28 +13,29 @@
 // Tests that the .pcm file will be generated in the same directory with the specified
 // output and the name of the .pcm file should be the same with the input file.
 // RUN: %clang -std=c++20 %t/Hello.cppm -fmodule-output -c -o %t/output/Hello.o \
-// RUN:   -### 2>&1 | FileCheck %t/Hello.cppm
+// RUN:   -fno-modules-reduced-bmi -### 2>&1 | FileCheck %t/Hello.cppm
 //
 // Tests that the output file will be generated in the input directory if the output
 // file is not the corresponding object file.
 // RUN: %clang -std=c++20 %t/Hello.cppm %t/AnotherModule.cppm -fmodule-output -o \
-// RUN:   %t/output/a.out -### 2>&1 | FileCheck  %t/AnotherModule.cppm
+// RUN:   %t/output/a.out -fno-modules-reduced-bmi -### 2>&1 | FileCheck  %t/AnotherModule.cppm
 //
 // Tests that clang will reject the command line if it specifies -fmodule-output with
 // multiple archs.
 // RUN: not %clang %t/Hello.cppm -fmodule-output -arch i386 -arch x86_64 -### \
-// RUN:   --target=x86_64-apple-darwin 2>&1 | FileCheck %t/Hello.cppm -check-prefix=MULTIPLE-ARCH
+// RUN:   -fno-modules-reduced-bmi --target=x86_64-apple-darwin 2>&1 | FileCheck %t/Hello.cppm \
+// RUN:   -check-prefix=MULTIPLE-ARCH
 
 // Tests that the .pcm file will be generated in the same path with the specified one
 // in the comamnd line.
 // RUN: %clang -std=c++20 %t/Hello.cppm -fmodule-output=%t/pcm/Hello.pcm -o %t/Hello.o \
-// RUN:   -c -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-SPECIFIED
+// RUN:   -fno-modules-reduced-bmi  -c -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-SPECIFIED
 //
 // RUN: %clang -std=c++20 %t/Hello.cppm -fmodule-output=%t/Hello.pcm -fmodule-output -c -fsyntax-only \
-// RUN:   -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-NOT-USED
+// RUN:   -fno-modules-reduced-bmi  -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-NOT-USED
 
 // Test that we can emit a warning if the type of the input file is not a module interface unit.
-// RUN: %clang -std=c++20 %t/a.cpp -fmodule-output -c -o %t/a.o -### 2>&1 | FileCheck %t/a.cpp
+// RUN: %clang -std=c++20 %t/a.cpp -fmodule-output -fno-modules-reduced-bmi  -c -o %t/a.o -### 2>&1 | FileCheck %t/a.cpp
 
 //--- Hello.cppm
 export module Hello;

clang/test/Driver/modules.cpp

@@ -34,7 +34,7 @@
 
 // Check combining precompile and compile steps works.
 //
-// RUN: %clang -std=c++2a -x c++-module %t/foo.cpp -S -o %t/foo2.pcm.o -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE --check-prefix=CHECK-COMPILE
+// RUN: %clang -std=c++2a -x c++-module -fno-modules-reduced-bmi %t/foo.cpp -S -o %t/foo2.pcm.o -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE --check-prefix=CHECK-COMPILE
 
 // Check that .cppm is treated as a module implicitly.
 //

clang/test/Modules/mingw-exceptions.cppm

@@ -1,5 +1,6 @@
 // REQUIRES: x86-registered-target
-// RUN: %clang -target x86_64-windows-gnu -x c++-module -std=gnu++23 -c -o /dev/null -Xclang -disable-llvm-passes %s
+// RUN: %clang -target x86_64-windows-gnu -x c++-module -std=gnu++23 -fno-modules-reduced-bmi \
+// RUN:     -c -o /dev/null -Xclang -disable-llvm-passes %s
 
 // Make sure the command succeeds and doesn't break on the -exception-model flag in cc1.
 export module empty;