[flang] Revisions for review comments.

Original-commit: flang-compiler/f18@1a03f7d5bb
Reviewed-on: https://github.com/flang-compiler/f18/pull/324

flang/lib/FIR/program.h

@@ -28,6 +28,13 @@ namespace Fortran::FIR {
 class Procedure;
 class GraphWriter;
 
+// FIR is a composable hierarchy of owned objects meant to represent a Fortran
+// compilation unit operationally.  At this point, the top-level object is a
+// Program.  A Program owns a list of Procedures and a list of data objects, all
+// with process lifetimes (to-do).  These objects are referenced by pointers.  A
+// Procedure owns a list of BasicBlocks.  A BasicBlock is referenced by a
+// pointer.  A BasicBlock owns a list of Statements.  A Statement is referenced
+// by a pointer.
 class Program final {
 public:
   friend GraphWriter;

flang/tools/f18/f18.cc

@@ -468,6 +468,8 @@ int main(int argc, char *const argv[]) {
           << "  -fdebug-resolve-names\n"
           << "  -fdebug-instrumented-parse\n"
           << "  -fdebug-semantics    perform semantic checks\n"
+          << "  -fdotty              print FIR as a dotty graph\n"
+          << "  -fdebug-dump-linear-ir dump the flat linear FIR for debug\n"
           << "  -v -c -o -I -D -U    have their usual meanings\n"
           << "  -help                print this again\n"
           << "Other options are passed through to the compiler.\n";