[flang][Driver] Add support for -mllvm -print-pipeline-passes

The behavior deliberately mimics that of clang. Ideally, -print-pipeline-passes
should be a first-class driver option. Notes to this effect have been added in 
the appropriate places in both flang and clang.

---------

Co-authored-by: Tarun Prabhu <tarun.prabhu@gmail.com>

clang/lib/CodeGen/BackendUtil.cpp

@@ -1095,6 +1095,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
       TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
   }
 
+  // FIXME: This should eventually be replaced by a first-class driver option.
+  // This should be done for both clang and flang simultaneously.
   // Print a textual, '-passes=' compatible, representation of pipeline if
   // requested.
   if (PrintPipelinePasses) {

flang/lib/Frontend/FrontendActions.cpp

@@ -79,6 +79,10 @@
 
 #include "flang/Tools/CLOptions.inc"
 
+namespace llvm {
+extern cl::opt<bool> PrintPipelinePasses;
+} // namespace llvm
+
 using namespace Fortran::frontend;
 
 // Declare plugin extension function declarations.
@@ -1016,6 +1020,20 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {
   else if (action == BackendActionTy::Backend_EmitLL)
     mpm.addPass(llvm::PrintModulePass(os));
 
+  // FIXME: This should eventually be replaced by a first-class driver option.
+  // This should be done for both flang and clang simultaneously.
+  // Print a textual, '-passes=' compatible, representation of pipeline if
+  // requested. In this case, don't run the passes. This mimics the behavior of
+  // clang.
+  if (llvm::PrintPipelinePasses) {
+    mpm.printPipeline(llvm::outs(), [&pic](llvm::StringRef className) {
+      auto passName = pic.getPassNameForClassName(className);
+      return passName.empty() ? className : passName;
+    });
+    llvm::outs() << "\n";
+    return;
+  }
+
   // Run the passes.
   mpm.run(*llvmModule, mam);
 }

flang/test/Driver/print-pipeline-passes.f90

@@ -0,0 +1,7 @@
+! RUN: %flang_fc1 -mllvm -print-pipeline-passes -emit-llvm-bc -o /dev/null -O0 %s 2>&1 | FileCheck %s
+
+! Just check a few passes to ensure that something reasonable is being printed.
+! CHECK: always-inline
+! CHECK-SAME: annotation-remarks
+
+end program