[flang][driver] Add support for `-mmlir`

The semantics of `-mmlir` are identical to `-mllvm`. The only notable
difference is that `-mmlir` options should be forwarded to MLIR rather
than LLVM.

Note that MLIR llvm::cl options are lazily constructed on demand (see
the definition of options in PassManagerOptions.cpp). This means that:
  * MLIR global options are only visible when explicitly initialised and
    displayed only when using `-mmlir --help`,
  * Flang and LLVM global options are always visible and displayed when
    using either `-mllvm -help` or `-mmlir --help`.

In other words, `-mmlir --help` is a superset of `-mllvm --help`. This is not
ideal, but we'd need to refactor all option definitions in Flang and
LLVM to improve this. I suggesting leaving this for later.

Differential Revision: https://reviews.llvm.org/D123297

2 files changed, 18 insertions, 0 deletions

diff --git a/flang/lib/FrontendTool/CMakeLists.txt b/flang/lib/FrontendTool/CMakeLists.txt
index 1a29f4a..0753313 100644
--- a/flang/lib/FrontendTool/CMakeLists.txt
+++ b/flang/lib/FrontendTool/CMakeLists.txt
@@ -11,6 +11,7 @@ add_flang_library(flangFrontendTool
   flangFrontend
   clangBasic
   clangDriver
+  MLIRPass
 
   LINK_COMPONENTS
   Option
diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index 04e00e5..bc2bf1d 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -19,6 +19,8 @@
 #include "llvm/Option/Option.h"
 #include "llvm/Support/BuryPointer.h"
 #include "llvm/Support/CommandLine.h"
+#include "mlir/IR/MLIRContext.h"
+#include "mlir/Pass/PassManager.h"
 
 namespace Fortran::frontend {
 
@@ -150,6 +152,21 @@ bool ExecuteCompilerInvocation(CompilerInstance *flang) {
     llvm::cl::ParseCommandLineOptions(numArgs + 1, args.get());
   }
 
+  // Honor -mmlir. This should happen AFTER plugins have been loaded!
+  if (!flang->frontendOpts().mlirArgs.empty()) {
+    mlir::registerMLIRContextCLOptions();
+    mlir::registerPassManagerCLOptions();
+    unsigned numArgs = flang->frontendOpts().mlirArgs.size();
+    auto args = std::make_unique<const char *[]>(numArgs + 2);
+    args[0] = "flang (MLIR option parsing)";
+
+    for (unsigned i = 0; i != numArgs; ++i)
+      args[i + 1] = flang->frontendOpts().mlirArgs[i].c_str();
+
+    args[numArgs + 1] = nullptr;
+    llvm::cl::ParseCommandLineOptions(numArgs + 1, args.get());
+  }
+
   // If there were errors in processing arguments, don't do anything else.
   if (flang->diagnostics().hasErrorOccurred()) {
     return false;