[flang] Make the frontend driver error out when requesting multiple actions

With this change, the following invocations will be treated as errors
(multiple actions are specified):
```
$ flang-new -fc1 -E -fsyntax-only file.95
$ flang-new -fc1 -fsyntax-only -fdebug-dump-symbols file.95
```
In the examples above it is not clear whether it is `-fsyntax-only` or
the other action that is run (i.e. `-E` or `-fdebug-dump-symbols`). It
makes sense to disallow such usage. This should also lead to cleaner and
clearer tests (the `RUN` lines using `%flang_fc1` will only allow one
action).

This change means that `flang-new -fc1` and `clang -cc1` will behave
differently when multiple action options are specified. As frontend
drivers are mostly used by compiler developers, this shouldn't affect or
confuse the compiler end-users. Also, `flang-new` and `clang` remain
consistent.

Tests are updated accordingly. More specifically, I've made sure that
every test specifies only one action. I've also taken the opportunity to
simplify "multiple-input-files.f90" a bit.

Differential Revision: https://reviews.llvm.org/D111781

1 files changed, 10 insertions, 0 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index acdfcb8..159c563 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -107,6 +107,16 @@ static bool ParseFrontendArgs(FrontendOptions &opts, llvm::opt::ArgList &args,
   // By default the frontend driver creates a ParseSyntaxOnly action.
   opts.programAction = ParseSyntaxOnly;
 
+  // Treat multiple action options as an invocation error. Note that `clang
+  // -cc1` does accept multiple action options, but will only consider the
+  // rightmost one.
+  if (args.hasMultipleArgs(clang::driver::options::OPT_Action_Group)) {
+    const unsigned diagID = diags.getCustomDiagID(
+        clang::DiagnosticsEngine::Error, "Only one action option is allowed");
+    diags.Report(diagID);
+    return false;
+  }
+
   // Identify the action (i.e. opts.ProgramAction)
   if (const llvm::opt::Arg *a =
           args.getLastArg(clang::driver::options::OPT_Action_Group)) {