[clang] Note that -debug-pass only works with legacy PM when using new PM

[aeubanks]

Summary

Follow-up to https://reviews.llvm.org/D97810.

[ychen]

Sorry, I forgot that codegen is still legacy PM, so -debug-pass=Structure does give the correct output. Then users need to specify both -debug-pass-manager and -debug-pass=x to get the pass information. Hmm, not a big deal but seems hassles. Maybe we should bridge -debug-pass=Structure to -debug-pass-manager ? Or actually, this could be a feature where users can specify either pipeline they want the pass information for. WDYT?

clang/lib/CodeGen/BackendUtil.cpp

 ///
 /// This API is planned to have its functionality finished and then to replace
 /// `EmitAssembly` at some point in the future when the default switches.
 void EmitAssemblyHelper::EmitAssemblyWithNewPassManager(
     BackendAction Action, std::unique_ptr<raw_pwrite_stream> OS) {
   TimeRegion Region(CodeGenOpts.TimePasses ? &CodeGenerationTime : nullptr);
   setCommandLineOpts(CodeGenOpts);
 
+  if (llvm::legacy::debugPassSpecified()) {
+    llvm::errs()
+        << "-debug-pass/-fdebug-pass-* does not work with the new pass manager "
+           "for the middle-end optimization pipeline, either use "
+           "-fdebug-pass-manager, or use the legacy PM "
+           "(-flegacy-pass-manager)\n";
+  }
+
   bool RequiresCodeGen = (Action != Backend_EmitNothing &&
                           Action != Backend_EmitBC &&
                           Action != Backend_EmitLL);
   CreateTargetMachine(RequiresCodeGen);
 
   if (RequiresCodeGen && !TM)
     return;
   if (TM)

clang/test/Misc/debug-pass-npm.c

+// RUN: %clang -fexperimental-new-pass-manager %s -c -o /dev/null -mllvm -debug-pass=Structure 2>&1 | FileCheck %s
+// RUN: %clang -fexperimental-new-pass-manager %s -c -o /dev/null -fdebug-pass-arguments 2>&1 | FileCheck %s
+// RUN: %clang -flegacy-pass-manager %s -c -o /dev/null -fdebug-pass-arguments 2>&1 | FileCheck %s --check-prefix=LPM
+
+void foo() {}
+
+// CHECK: does not work
+// LPM-NOT: does not work