[ThinLTO] Ensure anonymous globals renamed even at -O0

[tejohnson]

Summary

This fixes an issue when files are compiled with -flto=thin
at default -O0. We need to rename anonymous globals before attempting
to write the module summary because all values need names for
the summary. This was happening at -O1 and above, but not before
the early exit when constructing the pipeline for -O0.

Also add an internal -prepare-for-thinlto option to enable this
to be tested via opt.

Fixes PR30419.

[mehdi_amini]

LGTM.

llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp

static cl::opt<bool> EnableNonLTOGlobalsModRef(
     "enable-non-lto-gmr", cl::init(true), cl::Hidden,
     cl::desc(
         "Enable the GlobalsModRef AliasAnalysis outside of the LTO pipeline."));
 
 static cl::opt<bool> EnableLoopLoadElim(
     "enable-loop-load-elim", cl::init(true), cl::Hidden,
     cl::desc("Enable the LoopLoadElimination Pass"));
 
+static cl::opt<bool>
+    EnablePrepareForThinLTO("prepare-for-thinlto", cl::init(false), cl::Hidden,
+                            cl::desc("Enable preparation for ThinLTO."));
+
 static cl::opt<bool> RunPGOInstrGen(
     "profile-generate", cl::init(false), cl::Hidden,
     cl::desc("Enable PGO instrumentation."));
 
 static cl::opt<std::string>
     PGOOutputFile("profile-generate-file", cl::init(""), cl::Hidden,
                       cl::desc("Specify the path of profile data file."));
 
PassManagerBuilder::PassManagerBuilder() {
     DisableGVNLoadPRE = false;
     VerifyInput = false;
     VerifyOutput = false;
     MergeFunctions = false;
     PrepareForLTO = false;
     EnablePGOInstrGen = RunPGOInstrGen;
     PGOInstrGen = PGOOutputFile;
     PGOInstrUse = RunPGOInstrUse;
-    PrepareForThinLTO = false;
+    PrepareForThinLTO = EnablePrepareForThinLTO;
     PerformThinLTO = false;
 }
 
 PassManagerBuilder::~PassManagerBuilder() {
   delete LibraryInfo;
   delete Inliner;
 }
 
if (OptLevel == 0) {
     // that pass manager. To prevent this we insert a no-op module pass to reset
     // the pass manager to get the same behavior as EP_OptimizerLast in non-O0
     // builds. The function merging pass is
     if (MergeFunctions)
       MPM.add(createMergeFunctionsPass());
     else if (!GlobalExtensions->empty() || !Extensions.empty())
       MPM.add(createBarrierNoopPass());
 
+    if (PrepareForThinLTO)
+      // Rename anon globals to be able to export them in the summary.
+      MPM.add(createNameAnonGlobalPass());
+
     addExtensionsToPM(EP_EnabledOnOptLevel0, MPM);
     return;
   }
 
   // Add LibraryInfo if we have some.
   if (LibraryInfo)
     MPM.add(new TargetLibraryInfoWrapperPass(*LibraryInfo));
 

llvm/trunk/test/Transforms/NameAnonGlobals/rename.ll

 ; RUN: opt -S -name-anon-globals < %s | FileCheck %s
+; RUN: opt -prepare-for-thinlto -O0 -module-summary -o %t.bc < %s
 
 
 ; foo contribute to the unique hash for the module
 define void @foo() {
     ret void
 }
 
 ; bar is internal, and does not contribute to the unique hash for the module