[Flang][Driver] Add -print-resource-dir command line flag to emit Flang's resource directory (#90886)

This should be a NFC change for all drivers, but Flang.

Author: mjklemm

clang/include/clang/Driver/Driver.h

@@ -747,6 +747,9 @@ class Driver {
   /// option.
   void setDriverMode(StringRef DriverModeValue);
 
+  /// Set the resource directory, depending on which driver is being used.
+  void setResourceDirectory();
+
   /// Parse the \p Args list for LTO options and record the type of LTO
   /// compilation based on which -f(no-)?lto(=.*)? option occurs last.
   void setLTOMode(const llvm::opt::ArgList &Args);

clang/include/clang/Driver/Options.td

@@ -5491,7 +5491,10 @@ def print_prog_name_EQ : Joined<["-", "--"], "print-prog-name=">,
   Visibility<[ClangOption, CLOption]>;
 def print_resource_dir : Flag<["-", "--"], "print-resource-dir">,
   HelpText<"Print the resource directory pathname">,
-  Visibility<[ClangOption, CLOption]>;
+  HelpTextForVariants<[FlangOption],
+                      "Print the resource directory pathname that contains lib and "
+                      "include directories with the runtime libraries and MODULE files.">,
+  Visibility<[ClangOption, CLOption, FlangOption]>;
 def print_search_dirs : Flag<["-", "--"], "print-search-dirs">,
   HelpText<"Print the paths used for finding libraries and programs">,
   Visibility<[ClangOption, CLOption]>;

clang/lib/Driver/Driver.cpp

@@ -229,9 +229,6 @@ Driver::Driver(StringRef ClangExecutable, StringRef TargetTriple,
     UserConfigDir = static_cast<std::string>(P);
   }
 #endif
-
-  // Compute the path to the resource directory.
-  ResourceDir = GetResourcesPath(ClangExecutable, CLANG_RESOURCE_DIR);
 }
 
 void Driver::setDriverMode(StringRef Value) {
@@ -250,6 +247,24 @@ void Driver::setDriverMode(StringRef Value) {
     Diag(diag::err_drv_unsupported_option_argument) << OptName << Value;
 }
 
+void Driver::setResourceDirectory() {
+  // Compute the path to the resource directory, depending on the driver mode.
+  switch (Mode) {
+  case GCCMode:
+  case GXXMode:
+  case CPPMode:
+  case CLMode:
+  case DXCMode:
+    ResourceDir = GetResourcesPath(ClangExecutable, CLANG_RESOURCE_DIR);
+    break;
+  case FlangMode:
+    SmallString<64> customResourcePathRelativeToDriver{".."};
+    ResourceDir =
+        GetResourcesPath(ClangExecutable, customResourcePathRelativeToDriver);
+    break;
+  }
+}
+
 InputArgList Driver::ParseArgStrings(ArrayRef<const char *> ArgStrings,
                                      bool UseDriverMode, bool &ContainsError) {
   llvm::PrettyStackTraceString CrashInfo("Command line argument parsing");
@@ -1202,6 +1217,7 @@ Compilation *Driver::BuildCompilation(ArrayRef<const char *> ArgList) {
   if (!DriverMode.empty())
     setDriverMode(DriverMode);
 
+  setResourceDirectory();
   // FIXME: What are we going to do with -V and -b?
 
   // Arguments specified in command line.

flang/test/Driver/print-resource-dir.F90

@@ -0,0 +1,4 @@
+! DEFINE: %{resource_dir} = %S/Inputs/resource_dir
+! RUN: %flang -print-resource-dir -resource-dir=%{resource_dir}.. \
+! RUN:  | FileCheck -check-prefix=PRINT-RESOURCE-DIR -DFILE=%{resource_dir} %s
+! PRINT-RESOURCE-DIR: [[FILE]]