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

Authored by CarolineConcatto on Sep 11 2020, 2:17 AM.

Description

[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