[NFC] [C++20] [Modules] Pulling out getCXX20NamedModuleOutputPath into a seperate function

Required in the review process of
#85050.

Author: ChuanqiXu9

clang/lib/Driver/Driver.cpp

@@ -5814,19 +5814,9 @@ static const char *GetModuleOutputPath(Compilation &C, const JobAction &JA,
          (C.getArgs().hasArg(options::OPT_fmodule_output) ||
           C.getArgs().hasArg(options::OPT_fmodule_output_EQ)));
 
-  if (Arg *ModuleOutputEQ =
-          C.getArgs().getLastArg(options::OPT_fmodule_output_EQ))
-    return C.addResultFile(ModuleOutputEQ->getValue(), &JA);
+  SmallString<256> OutputPath =
+      tools::getCXX20NamedModuleOutputPath(C.getArgs(), BaseInput);
 
-  SmallString<64> OutputPath;
-  Arg *FinalOutput = C.getArgs().getLastArg(options::OPT_o);
-  if (FinalOutput && C.getArgs().hasArg(options::OPT_c))
-    OutputPath = FinalOutput->getValue();
-  else
-    OutputPath = BaseInput;
-
-  const char *Extension = types::getTypeTempSuffix(JA.getType());
-  llvm::sys::path::replace_extension(OutputPath, Extension);
   return C.addResultFile(C.getArgs().MakeArgString(OutputPath.c_str()), &JA);
 }
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3839,6 +3839,24 @@ bool Driver::getDefaultModuleCachePath(SmallVectorImpl<char> &Result) {
   return false;
 }
 
+llvm::SmallString<256>
+clang::driver::tools::getCXX20NamedModuleOutputPath(const ArgList &Args,
+                                                    const char *BaseInput) {
+  if (Arg *ModuleOutputEQ = Args.getLastArg(options::OPT_fmodule_output_EQ))
+    return StringRef(ModuleOutputEQ->getValue());
+
+  SmallString<256> OutputPath;
+  if (Arg *FinalOutput = Args.getLastArg(options::OPT_o);
+      FinalOutput && Args.hasArg(options::OPT_c))
+    OutputPath = FinalOutput->getValue();
+  else
+    OutputPath = BaseInput;
+
+  const char *Extension = types::getTypeTempSuffix(types::TY_ModuleFile);
+  llvm::sys::path::replace_extension(OutputPath, Extension);
+  return OutputPath;
+}
+
 static bool RenderModulesOptions(Compilation &C, const Driver &D,
                                  const ArgList &Args, const InputInfo &Input,
                                  const InputInfo &Output, bool HaveStd20,

clang/lib/Driver/ToolChains/Clang.h

@@ -193,6 +193,21 @@ DwarfFissionKind getDebugFissionKind(const Driver &D,
                                      const llvm::opt::ArgList &Args,
                                      llvm::opt::Arg *&Arg);
 
+// Calculate the output path of the module file when compiling a module unit
+// with the `-fmodule-output` option or `-fmodule-output=` option specified.
+// The behavior is:
+// - If `-fmodule-output=` is specfied, then the module file is
+//   writing to the value.
+// - Otherwise if the output object file of the module unit is specified, the
+// output path
+//   of the module file should be the same with the output object file except
+//   the corresponding suffix. This requires both `-o` and `-c` are specified.
+// - Otherwise, the output path of the module file will be the same with the
+//   input with the corresponding suffix.
+llvm::SmallString<256>
+getCXX20NamedModuleOutputPath(const llvm::opt::ArgList &Args,
+                              const char *BaseInput);
+
 } // end namespace tools
 
 } // end namespace driver