[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

clang/include/clang/Driver/Options.td

@@ -3264,6 +3264,8 @@ def mlinker_version_EQ : Joined<["-"], "mlinker-version=">,
 def mllvm : Separate<["-"], "mllvm">,Flags<[CC1Option,CC1AsOption,CoreOption,FC1Option,FlangOption]>,
   HelpText<"Additional arguments to forward to LLVM's option processing">,
   MarshallingInfoStringVector<FrontendOpts<"LLVMArgs">>;
+def mmlir : Separate<["-"], "mmlir">, Flags<[CoreOption,FC1Option,FlangOption]>,
+  HelpText<"Additional arguments to forward to MLIR's option processing">;
 def ffuchsia_api_level_EQ : Joined<["-"], "ffuchsia-api-level=">,
   Group<m_Group>, Flags<[CC1Option]>, HelpText<"Set Fuchsia API level">,
   MarshallingInfoInt<LangOpts<"FuchsiaAPILevel">>;

clang/lib/Driver/ToolChains/Flang.cpp

@@ -113,6 +113,11 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
     A->render(Args, CmdArgs);
   }
 
+  for (const Arg *A : Args.filtered(options::OPT_mmlir)) {
+    A->claim();
+    A->render(Args, CmdArgs);
+  }
+
   if (Output.isFilename()) {
     CmdArgs.push_back("-o");
     CmdArgs.push_back(Output.getFilename());

flang/include/flang/Frontend/FrontendOptions.h

@@ -273,6 +273,10 @@ struct FrontendOptions {
   /// should only be used for debugging and experimental features.
   std::vector<std::string> llvmArgs;
 
+  /// A list of arguments to forward to MLIR's option processing; this
+  /// should only be used for debugging and experimental features.
+  std::vector<std::string> mlirArgs;
+
   // Return the appropriate input kind for a file extension. For example,
   /// "*.f" would return Language::Fortran.
   ///

flang/lib/Frontend/CompilerInvocation.cpp

@@ -597,6 +597,9 @@ bool CompilerInvocation::CreateFromArgs(CompilerInvocation &res,
   res.frontendOpts_.llvmArgs =
       args.getAllArgValues(clang::driver::options::OPT_mllvm);
 
+  res.frontendOpts_.mlirArgs =
+      args.getAllArgValues(clang::driver::options::OPT_mmlir);
+
   return success;
 }
 

flang/lib/Frontend/FrontendActions.cpp

@@ -427,6 +427,7 @@ void CodeGenAction::GenerateLLVMIR() {
 
   // Create the pass pipeline
   fir::createMLIRToLLVMPassPipeline(pm);
+  mlir::applyPassManagerCLOptions(pm);
 
   // Run the pass manager
   if (!mlir::succeeded(pm.run(*mlirModule))) {

flang/lib/FrontendTool/CMakeLists.txt

@@ -11,6 +11,7 @@ add_flang_library(flangFrontendTool
   flangFrontend
   clangBasic
   clangDriver
+  MLIRPass
 
   LINK_COMPONENTS
   Option

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;

flang/test/Driver/driver-help-hidden.f90

@@ -48,6 +48,7 @@
 ! CHECK-NEXT: -help     Display available options
 ! CHECK-NEXT: -I <dir>               Add directory to the end of the list of include search paths
 ! CHECK-NEXT: -mllvm <value>         Additional arguments to forward to LLVM's option processing
+! CHECK-NEXT: -mmlir <value>         Additional arguments to forward to MLIR's option processing
 ! CHECK-NEXT: -module-dir <dir>      Put MODULE files in <dir>
 ! CHECK-NEXT: -nocpp                 Disable predefined and command line preprocessor macros
 ! CHECK-NEXT: -o <file> Write output to <file>

flang/test/Driver/driver-help.f90

@@ -48,6 +48,7 @@
 ! HELP-NEXT: -help                  Display available options
 ! HELP-NEXT: -I <dir>               Add directory to the end of the list of include search paths
 ! HELP-NEXT: -mllvm <value>         Additional arguments to forward to LLVM's option processing
+! HELP-NEXT: -mmlir <value>         Additional arguments to forward to MLIR's option processing
 ! HELP-NEXT: -module-dir <dir>      Put MODULE files in <dir>
 ! HELP-NEXT: -nocpp                 Disable predefined and command line preprocessor macros
 ! HELP-NEXT: -o <file>              Write output to <file>
@@ -124,6 +125,7 @@
 ! HELP-FC1-NEXT: -I <dir>               Add directory to the end of the list of include search paths
 ! HELP-FC1-NEXT: -load <dsopath>        Load the named plugin (dynamic shared object)
 ! HELP-FC1-NEXT: -mllvm <value>         Additional arguments to forward to LLVM's option processing
+! HELP-FC1-NEXT: -mmlir <value>         Additional arguments to forward to MLIR's option processing
 ! HELP-FC1-NEXT: -module-dir <dir>      Put MODULE files in <dir>
 ! HELP-FC1-NEXT: -module-suffix <suffix> Use <suffix> as the suffix for module files (the default value is `.mod`)
 ! HELP-FC1-NEXT: -nocpp                 Disable predefined and command line preprocessor macros

flang/test/Driver/mllvm_vs_mmlir.f90

@@ -0,0 +1,19 @@
+! Verify that `-mllvm` options are forwarded to LLVM and `-mmlir` to MLIR.
+
+! In practice, '-mmlir --help' is a super-set of '-mllvm --help' and that limits what we can test here. With a better seperation of
+! LLVM, MLIR and Flang global options, we should be able to write a stricter test.
+
+!------------
+! RUN COMMAND
+!------------
+! RUN: %flang_fc1  -mmlir --help | FileCheck %s --check-prefix=MLIR
+! RUN: %flang_fc1  -mllvm --help | FileCheck %s --check-prefix=MLLVM
+
+!----------------
+! EXPECTED OUTPUT
+!----------------
+! MLIR: flang (MLIR option parsing) [options]
+! MLIR: --mlir-{{.*}}
+
+! MLLVM: flang (LLVM option parsing) [options]
+! MLLVM-NOT: --mlir-{{.*}}