[Driver] [C++20] [Modules] Fix --precompile with -fmodule-output

Close llvm#159780

Author: ChuanqiXu9

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4051,15 +4051,24 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
   // module fragment.
   CmdArgs.push_back("-fskip-odr-check-in-gmf");
 
-  if (!Args.hasArg(options::OPT_fno_modules_reduced_bmi) &&
-      (Input.getType() == driver::types::TY_CXXModule ||
-       Input.getType() == driver::types::TY_PP_CXXModule) &&
-      !Args.hasArg(options::OPT__precompile)) {
-    CmdArgs.push_back("-fmodules-reduced-bmi");
+  if (Input.getType() == driver::types::TY_CXXModule ||
+      Input.getType() == driver::types::TY_PP_CXXModule) {
+    if (!Args.hasArg(options::OPT_fno_modules_reduced_bmi))
+      CmdArgs.push_back("-fmodules-reduced-bmi");
 
     if (Args.hasArg(options::OPT_fmodule_output_EQ))
       Args.AddLastArg(CmdArgs, options::OPT_fmodule_output_EQ);
-    else
+    else if (!Args.hasArg(options::OPT__precompile) ||
+             Args.hasArg(options::OPT_fmodule_output))
+      // If --precompile is specified, we will always generate a module file if
+      // we're compiling an importable module unit. This is fine even if the
+      // compilation process won't reach the point of generating the module file
+      // (e.g., in the preprocessing mode), since the attached flag
+      // '-fmodule-output' is useless.
+      //
+      // But if '--precompile' is specified, it might be annoying to always
+      // generate the module file as '--precompile' will generate the module
+      // file anyway.
       CmdArgs.push_back(Args.MakeArgString(
           "-fmodule-output=" +
           getCXX20NamedModuleOutputPath(Args, Input.getBaseInput())));

clang/test/Driver/modules.cpp

@@ -7,7 +7,7 @@
 // RUN: %clang -std=c++2a -x c++-module --precompile %t/foo.cpp -o %t/foo.pcm -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE
 // RUN: %clang -std=gnu++2a -x c++-module --precompile %t/foo.cpp -o %t/foo-gnu.pcm -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE
 //
-// CHECK-PRECOMPILE: -cc1 {{.*}} -emit-module-interface
+// CHECK-PRECOMPILE: -cc1 {{.*}} -emit-module-interface 
 // CHECK-PRECOMPILE-SAME: -o {{.*}}.pcm
 // CHECK-PRECOMPILE-SAME: -x c++
 // CHECK-PRECOMPILE-SAME: foo.cpp
@@ -41,6 +41,12 @@
 // RUN: cp %t/foo.cpp %t/foo.cppm
 // RUN: %clang -std=c++2a --precompile %t/foo.cppm -o %t/foo.pcm -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE
 
+// RUN: %clang -std=c++20 -x c++-module %t/foo.cpp --precompile -o %t/foo.pcm -### 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE-WO-MODULE-OUTPUT
+// CHECK-PRECOMPILE-WO-MODULE-OUTPUT-NOT: -fmodule-output=
+
+// RUN: %clang -std=c++20 -x c++-module %t/foo.cpp --precompile -fmodule-output=%t/foo.reduced.pcm -o %t/foo.pcm -### 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE-WT-MODULE-OUTPUT
+// CHECK-PRECOMPILE-WT-MODULE-OUTPUT: -fmodule-output=
+
 //--- foo.cpp
 export module foo;