[MLIR][Flang][DebugInfo] Set debug info format in MLIR->IR translation (#95098)

MLIR's LLVM dialect does not internally support debug records, only
converting to/from debug intrinsics. To smooth the transition from
intrinsics to records, there is a step prior to IR->MLIR translation
that switches the IR module to intrinsic-form; this patch adds the
equivalent conversion to record-form at MLIR->IR translation, and also
modifies the flang front end to use the WriteNewDbgInfoFormat flag when
it is emitting LLVM IR.

Author: SLTozer

flang/lib/Frontend/FrontendActions.cpp

@@ -50,6 +50,7 @@
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/Bitcode/BitcodeWriterPass.h"
 #include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
+#include "llvm/IR/DebugProgramInstruction.h"
 #include "llvm/IR/LLVMRemarkStreamer.h"
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/IR/Verifier.h"
@@ -81,6 +82,8 @@ using namespace Fortran::frontend;
   llvm::PassPluginLibraryInfo get##Ext##PluginInfo();
 #include "llvm/Support/Extension.def"
 
+extern llvm::cl::opt<bool> WriteNewDbgInfoFormat;
+
 /// Save the given \c mlirModule to a temporary .mlir file, in a location
 /// decided by the -save-temps flag. No files are produced if the flag is not
 /// specified.
@@ -1271,6 +1274,12 @@ void CodeGenAction::executeAction() {
   runOptimizationPipeline(ci.isOutputStreamNull() ? *os : ci.getOutputStream());
 
   if (action == BackendActionTy::Backend_EmitLL) {
+    // When printing LLVM IR, we should convert the module to the debug info
+    // format that LLVM expects us to print.
+    llvm::ScopedDbgInfoFormatSetter FormatSetter(*llvmModule,
+                                                 WriteNewDbgInfoFormat);
+    if (WriteNewDbgInfoFormat)
+      llvmModule->removeDebugIntrinsicDeclarations();
     llvmModule->print(ci.isOutputStreamNull() ? *os : ci.getOutputStream(),
                       /*AssemblyAnnotationWriter=*/nullptr);
     return;

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -64,6 +64,8 @@ using namespace mlir;
 using namespace mlir::LLVM;
 using namespace mlir::LLVM::detail;
 
+extern llvm::cl::opt<bool> UseNewDbgInfoFormat;
+
 #include "mlir/Dialect/LLVMIR/LLVMConversionEnumsToLLVM.inc"
 
 namespace {
@@ -1789,6 +1791,9 @@ prepareLLVMModule(Operation *m, llvm::LLVMContext &llvmContext,
                   StringRef name) {
   m->getContext()->getOrLoadDialect<LLVM::LLVMDialect>();
   auto llvmModule = std::make_unique<llvm::Module>(name, llvmContext);
+  // ModuleTranslation can currently only construct modules in the old debug
+  // info format, so set the flag accordingly.
+  llvmModule->setNewDbgInfoFormatFlag(false);
   if (auto dataLayoutAttr =
           m->getDiscardableAttr(LLVM::LLVMDialect::getDataLayoutAttrName())) {
     llvmModule->setDataLayout(cast<StringAttr>(dataLayoutAttr).getValue());
@@ -1867,6 +1872,11 @@ mlir::translateModuleToLLVMIR(Operation *module, llvm::LLVMContext &llvmContext,
   if (failed(translator.convertFunctions()))
     return nullptr;
 
+  // Once we've finished constructing elements in the module, we should convert
+  // it to use the debug info format desired by LLVM.
+  // See https://llvm.org/docs/RemoveDIsDebugInfo.html
+  translator.llvmModule->setIsNewDbgInfoFormat(UseNewDbgInfoFormat);
+
   if (!disableVerification &&
       llvm::verifyModule(*translator.llvmModule, &llvm::errs()))
     return nullptr;