[ThinLTO][NewPM] Clean up dead code under -O0

[aeubanks]

Summary

We're running into undefined references using ThinLTO with -O0 on
Windows/Chrome. This fixes that.

This matches the legacy PM.

[tejohnson]

llvm/lib/Passes/PassBuilder.cpp

1542

Looking at the old PM, we also call:
MPM.add(createLowerTypeTestsPass(nullptr, nullptr, true));

to drop any leftover type tests. Thinking that should probably get added here too (it's normally done below in the ModuleSimplificationPipeline, but of course that doesn't get reached for O0).

[aeubanks]

llvm/lib/Passes/PassBuilder.cpp

1542

It's already added right above, but only when we have an ImportSummary. Should another be added when we don't have an ImportSummary?

[tejohnson]

llvm/lib/Passes/PassBuilder.cpp

1542

Note the args are different - LTT needs to be called a second time with the third parameter set to true to remove all the type tests that the first call leaves behind for potential use in optimization.

[aeubanks]

add lowertypetests

llvm/lib/Passes/PassBuilder.cpp

1542

ah I didn't notice that, done

[tejohnson]

lgtm

llvm/lib/Passes/PassBuilder.cpp

1542

Can you add a comment? Just use the one used where we do the same thing for O0 in the regular LTO pipeline:

// Run a second time to clean up any type tests left behind by WPD for use
// in ICP.

[aeubanks]

add comment

llvm/lib/Passes/PassBuilder.cpp

if (ImportSummary) {
     // devirtualize more effectively, so it should operate on the IR first.
     //
     // The WPD and LowerTypeTest passes need to run at -O0 to lower type
     // metadata and intrinsics.
     MPM.addPass(WholeProgramDevirtPass(nullptr, ImportSummary));
     MPM.addPass(LowerTypeTestsPass(nullptr, ImportSummary));
   }
 
-  if (Level == OptimizationLevel::O0)
+  if (Level == OptimizationLevel::O0) {
+    // Drop available_externally and unreferenced globals. This is necessary
+    // with ThinLTO in order to avoid leaving undefined references to dead
+    // globals in the object file.
+    MPM.addPass(EliminateAvailableExternallyPass());
+    MPM.addPass(GlobalDCEPass());
     return MPM;
+  }
 
   // Force any function attributes we want the rest of the pipeline to observe.
   MPM.addPass(ForceFunctionAttrsPass());
 
   // Add the core simplification pipeline.
   MPM.addPass(buildModuleSimplificationPipeline(
       Level, ThinOrFullLTOPhase::ThinLTOPostLink));
 

llvm/test/Other/new-pm-O0-defaults.ll

 ; CHECK-DEFAULT-NEXT: Running pass: AlwaysInlinerPass
 ; CHECK-DEFAULT-NEXT: Running analysis: InnerAnalysisManagerProxy
 ; CHECK-DEFAULT-NEXT: Running analysis: ProfileSummaryAnalysis
 ; CHECK-MATRIX-NEXT: Running pass: LowerMatrixIntrinsicsPass
 ; CHECK-MATRIX-NEXT: Running analysis: TargetIRAnalysis
 ; CHECK-PRE-LINK-NEXT: Running pass: CanonicalizeAliasesPass
 ; CHECK-PRE-LINK-NEXT: Running pass: NameAnonGlobalPass
 ; CHECK-THINLTO-NEXT: Running pass: Annotation2MetadataPass
+; CHECK-THINLTO-NEXT: Running pass: EliminateAvailableExternallyPass
+; CHECK-THINLTO-NEXT: Running pass: GlobalDCEPass
 ; CHECK-LTO-NEXT: Running pass: Annotation2MetadataPass
 ; CHECK-LTO-NEXT: Running pass: WholeProgramDevirtPass
 ; CHECK-LTO-NEXT: Running analysis: InnerAnalysisManagerProxy
 ; CHECK-LTO-NEXT: Running pass: LowerTypeTestsPass
 ; CHECK-LTO-NEXT: Running pass: LowerTypeTestsPass
 ; CHECK-LTO-NEXT: Running pass: AnnotationRemarksPass
 ; CHECK-NEXT: Running pass: PrintModulePass