[flang][driver] Add the new flang compiler and frontend drivers

Summary:

This is the first patch implementing the new Flang driver as outlined in [1],
[2] & [3]. It creates Flang driver (`flang-new`) and Flang frontend driver
(`flang-new -fc1`). These will be renamed as `flang` and `flang -fc1` once the
current Flang throwaway driver, `flang`, can be replaced with `flang-new`.

Currently only 2 options are supported: `-help` and `--version`.

`flang-new` is implemented in terms of libclangDriver, defaulting the driver
mode to `FlangMode` (added to libclangDriver in [4]). This ensures that the
driver runs in Flang mode regardless of the name of the binary inferred from
argv[0].

The design of the new Flang compiler and frontend drivers is inspired by it
counterparts in Clang [3]. Currently, the new Flang compiler and frontend
drivers re-use Clang libraries: clangBasic, clangDriver and clangFrontend.

To identify Flang options, this patch adds FlangOption/FC1Option enums.
Driver::printHelp is updated so that `flang-new` prints only Flang options.
The new Flang driver is disabled by default. To enable it, set
`-DBUILD_FLANG_NEW_DRIVER=ON` when configuring CMake and add clang to
`LLVM_ENABLE_PROJECTS` (e.g. -DLLVM_ENABLE_PROJECTS=“clang;flang;mlir”).

[1] “RFC: new Flang driver - next steps”
http://lists.llvm.org/pipermail/flang-dev/2020-July/000470.html
[2] “RFC: Adding a fortran mode to the clang driver for flang”
http://lists.llvm.org/pipermail/cfe-dev/2019-June/062669.html
[3] “RFC: refactoring libclangDriver/libclangFrontend to share with Flang”
http://lists.llvm.org/pipermail/cfe-dev/2020-July/066393.html
[4] https://reviews.llvm.org/rG6bf55804924d5a1d902925ad080b1a2b57c5c75c

co-authored-by: Andrzej Warzynski <andrzej.warzynski@arm.com>

Reviewed By: richard.barton.arm, sameeranjoshi

Differential Revision: https://reviews.llvm.org/D86089

2 files changed, 50 insertions, 0 deletions

diff --git a/flang/lib/FrontendTool/CMakeLists.txt b/flang/lib/FrontendTool/CMakeLists.txt
new file mode 100644
index 0000000..eda040f
--- /dev/null
+++ b/flang/lib/FrontendTool/CMakeLists.txt
@@ -0,0 +1,11 @@
+add_flang_library(flangFrontendTool
+  ExecuteCompilerInvocation.cpp
+
+  LINK_LIBS
+  clangBasic
+  clangDriver
+
+  LINK_COMPONENTS
+  Option
+  Support
+)
diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
new file mode 100644
index 0000000..ab773c9
--- /dev/null
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -0,0 +1,39 @@
+//===--- ExecuteCompilerInvocation.cpp ------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file holds ExecuteCompilerInvocation(). It is split into its own file to
+// minimize the impact of pulling in essentially everything else in Flang.
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Frontend/CompilerInstance.h"
+#include "clang/Driver/Options.h"
+#include "llvm/Option/OptTable.h"
+#include "llvm/Support/CommandLine.h"
+
+namespace Fortran::frontend {
+bool ExecuteCompilerInvocation(CompilerInstance *flang) {
+  // Honor -help.
+  if (flang->GetFrontendOpts().showHelp_) {
+    clang::driver::getDriverOptTable().PrintHelp(llvm::outs(),
+        "flang-new -fc1 [options] file...", "LLVM 'Flang' Compiler",
+        /*Include=*/clang::driver::options::FlangOption,
+        /*Exclude=*/0, /*ShowAllAliases=*/false);
+    return true;
+  }
+
+  // Honor -version.
+  if (flang->GetFrontendOpts().showVersion_) {
+    llvm::cl::PrintVersionMessage();
+    return true;
+  }
+
+  return true;
+}
+
+} // namespace Fortran::frontend