[clang] [CodeGen] Implement fallback path for backend NPM run (PR #210248)

[Vikram Hegde via cfe-commits]

https://github.com/vikramRH updated https://github.com/llvm/llvm-project/pull/210248

>From b66c997367e2ca52c6736fcb3675e0a844642384 Mon Sep 17 00:00:00 2001
From: vikhegde <vikram.hegde at amd.com>
Date: Thu, 16 Jul 2026 16:12:34 +0530
Subject: [PATCH] [CodeGen] Implement fallback path for backend NPM run

---
 .../clang/Basic/DiagnosticFrontendKinds.td    |  3 +
 clang/include/clang/Basic/DiagnosticGroups.td |  1 +
 clang/lib/CodeGen/BackendUtil.cpp             | 55 ++++++++++++++-----
 3 files changed, 45 insertions(+), 14 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticFrontendKinds.td b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
index cef2fc32a1642..21e230e6177e0 100644
--- a/clang/include/clang/Basic/DiagnosticFrontendKinds.td
+++ b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -129,6 +129,9 @@ def err_fe_unable_to_create_subtarget : Error<
     "unable to create subtarget: '%0'%select{ with features '%2'|}1">;
 def err_fe_unable_to_interface_with_target : Error<
     "unable to interface with target machine">;
+def warn_fe_failed_new_pass_manager : Warning<
+    "new pass manager failed, retrying codegen with legacy PM">,
+    InGroup<NewPassManager>;
 def err_fe_unable_to_open_output : Error<
     "unable to open output file '%0': '%1'">;
 def warn_fe_macro_contains_embedded_newline : Warning<
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 79583534b9bbd..35ed6d82652f9 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1793,6 +1793,7 @@ Results can be filtered by function name by passing
 // Compatibility flag name from old versions of Clang.
 def : DiagGroup<"frame-larger-than=", [BackendFrameLargerThan]>;
 def BackendPlugin : DiagGroup<"backend-plugin">;
+def NewPassManager : DiagGroup<"new-pass-manager">;
 def RemarkBackendPlugin : DiagGroup<"remark-backend-plugin">;
 def BackendOptimizationRemark : DiagGroup<"pass">;
 def BackendOptimizationRemarkMissed : DiagGroup<"pass-missed">;
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 2b755fa916e55..95a73a793898e 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -194,10 +194,10 @@ class EmitAssemblyHelper {
                                 std::unique_ptr<raw_pwrite_stream> &OS,
                                 std::unique_ptr<llvm::ToolOutputFile> &DwoOS,
                                 CodeGenFileType CGFT);
-  void RunCodegenPipelineNewPM(BackendAction Action,
-                               std::unique_ptr<raw_pwrite_stream> &OS,
-                               std::unique_ptr<llvm::ToolOutputFile> &DwoOS,
-                               CodeGenFileType CGFT);
+  Error RunCodegenPipelineNewPM(BackendAction Action,
+                                std::unique_ptr<raw_pwrite_stream> &OS,
+                                std::unique_ptr<llvm::ToolOutputFile> &DwoOS,
+                                CodeGenFileType CGFT);
   void TimeCodegenPasses(llvm::function_ref<void()> RunPasses);
 
   /// Check whether we should emit a module summary for regular LTO.
@@ -1249,10 +1249,14 @@ void EmitAssemblyHelper::RunCodegenPipeline(
   }
 
   if (CodeGenOpts.EnableNewPMCodeGen) {
-    RunCodegenPipelineNewPM(Action, OS, DwoOS, CGFT);
-  } else {
-    RunCodegenPipelineLegacy(Action, OS, DwoOS, CGFT);
+    if (!RunCodegenPipelineNewPM(Action, OS, DwoOS, CGFT))
+      return;
+
+    Diags.Report(diag::warn_fe_failed_new_pass_manager);
   }
+
+  // NPM path (if enabled) failed to construct pipeline. retry with legacy PM.
+  RunCodegenPipelineLegacy(Action, OS, DwoOS, CGFT);
 }
 
 void EmitAssemblyHelper::RunCodegenPipelineLegacy(
@@ -1292,7 +1296,7 @@ void EmitAssemblyHelper::RunCodegenPipelineLegacy(
   TimeCodegenPasses([&] { CodeGenPasses.run(*TheModule); });
 }
 
-void EmitAssemblyHelper::RunCodegenPipelineNewPM(
+Error EmitAssemblyHelper::RunCodegenPipelineNewPM(
     BackendAction Action, std::unique_ptr<raw_pwrite_stream> &OS,
     std::unique_ptr<llvm::ToolOutputFile> &DwoOS, CodeGenFileType CGFT) {
   ModulePassManager MPM;
@@ -1309,6 +1313,12 @@ void EmitAssemblyHelper::RunCodegenPipelineNewPM(
   TargetMachine *TMPointer = TM.get();
   PassBuilder PB(TMPointer, PTOptions, std::nullopt, &PIC,
                  CI.getVirtualFileSystemPtr());
+
+  StandardInstrumentations SI(TheModule->getContext(),
+                              CodeGenOpts.DebugPassManager,
+                              CodeGenOpts.VerifyEach);
+  SI.registerCallbacks(PIC, &MAM);
+
   PB.registerModuleAnalyses(MAM);
   PB.registerCGSCCAnalyses(CGAM);
   PB.registerFunctionAnalyses(FAM);
@@ -1316,17 +1326,34 @@ void EmitAssemblyHelper::RunCodegenPipelineNewPM(
   PB.registerMachineFunctionAnalyses(MFAM);
   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM, &MFAM);
 
+  TargetLibraryInfoImpl TLII(TheModule->getTargetTriple());
+  FAM.registerPass([&] { return TargetLibraryAnalysis(TLII); });
   MAM.registerPass([&] { return MachineModuleAnalysis(MMI); });
+  MAM.registerPass([&] {
+    const llvm::TargetOptions &Options = TM->Options;
+    return RuntimeLibraryAnalysis(TargetTriple, Options.ExceptionModel,
+                                  Options.FloatABIType, Options.EABIVersion,
+                                  Options.MCOptions.ABIName, Options.VecLib);
+  });
+
+  if (Error BuildPipelineError = TM->buildCodeGenPipeline(
+          MPM, MAM, *OS, DwoOS ? &DwoOS->os() : nullptr, CGFT, Opt,
+          MMI.getContext(), &PIC))
+    return BuildPipelineError;
 
-  Error BuildPipelineError =
-      TM->buildCodeGenPipeline(MPM, MAM, *OS, DwoOS ? &DwoOS->os() : nullptr,
-                               CGFT, Opt, MMI.getContext(), &PIC);
-  if (BuildPipelineError) {
-    Diags.Report(diag::err_fe_unable_to_interface_with_target);
-    return;
+  if (PrintPipelinePasses) {
+    std::string PipelineStr;
+    raw_string_ostream OutS(PipelineStr);
+    MPM.printPipeline(OutS, [&PIC](StringRef ClassName) {
+      auto PassName = PIC.getPassNameForClassName(ClassName);
+      return PassName.empty() ? ClassName : PassName;
+    });
+    outs() << PipelineStr << '\n';
+    return Error::success();
   }
 
   TimeCodegenPasses([&] { MPM.run(*TheModule, MAM); });
+  return Error::success();
 }
 
 void EmitAssemblyHelper::TimeCodegenPasses(