[BitcodeReader] Infer the correct runtime preemption for GlobalValue

[steven_wu]

Summary

To allow bitcode built by old compiler to pass the current verifer,
BitcodeReader needs to auto infer the correct runtime preemption from
linkage and visibility for GlobalValues.

Since llvm-6.0 bitcode already contains the new field but can be
incorrect in some cases, the attribute needs to be recomputed all the
time in BitcodeReader. This will make all the GVs has dso_local marked
correctly if read from bitcode, and it should still allow the verifier
to catch mistakes in optimization passes.

This should fix PR38009.

[steven_wu]

oops, accidentally opened a new review. closing.

lib/Bitcode/Reader/BitcodeReader.cpp

if (!UseStrtab) {
     Name = OldFormatName;
   }
   Comdat *C = TheModule->getOrInsertComdat(Name);
   C->setSelectionKind(SK);
   ComdatList.push_back(C);
   return Error::success();
 }
 
+static void inferDSOLocal(GlobalValue *GV) {
+  // infer dso_local from linkage and visibility if it is not encoded.
+  if (GV->hasLocalLinkage() ||
+      (!GV->hasDefaultVisibility() && !GV->hasExternalWeakLinkage()))
+    GV->setDSOLocal(true);
+}
+
 Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
   // v1: [pointer type, isconst, initid, linkage, alignment, section,
   // visibility, threadlocal, unnamed_addr, externally_initialized,
   // dllstorageclass, comdat, attributes, preemption specifier] (name in VST)
   // v2: [strtab_offset, strtab_size, v1]
   StringRef Name;
   std::tie(Name, Record) = readNameFromStrtab(Record);
 
Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
   if (Record.size() > 12) {
     auto AS = getAttributes(Record[12]).getFnAttributes();
     NewGV->setAttributes(AS);
   }
 
   if (Record.size() > 13) {
     NewGV->setDSOLocal(getDecodedDSOLocal(Record[13]));
   }
+  inferDSOLocal(NewGV);
 
   return Error::success();
 }
 
 Error BitcodeReader::parseFunctionRecord(ArrayRef<uint64_t> Record) {
   // v1: [type, callingconv, isproto, linkage, paramattr, alignment, section,
   // visibility, gc, unnamed_addr, prologuedata, dllstorageclass, comdat,
   // prefixdata,  personalityfn, preemption specifier] (name in VST)
if (Record.size() > 13 && Record[13] != 0)
     FunctionPrefixes.push_back(std::make_pair(Func, Record[13] - 1));
 
   if (Record.size() > 14 && Record[14] != 0)
     FunctionPersonalityFns.push_back(std::make_pair(Func, Record[14] - 1));
 
   if (Record.size() > 15) {
     Func->setDSOLocal(getDecodedDSOLocal(Record[15]));
   }
+  inferDSOLocal(Func);
 
   ValueList.push_back(Func);
 
   // If this is a function with a body, remember the prototype we are
   // creating now, so that we can match up the body with them later.
   if (!isProto) {
     Func->setIsMaterializable(true);
     FunctionsWithBodies.push_back(Func);
else
       upgradeDLLImportExportLinkage(NewGA, Linkage);
     if (OpNum != Record.size())
       NewGA->setThreadLocalMode(getDecodedThreadLocalMode(Record[OpNum++]));
     if (OpNum != Record.size())
       NewGA->setUnnamedAddr(getDecodedUnnamedAddrType(Record[OpNum++]));
   }
   if (OpNum != Record.size())
     NewGA->setDSOLocal(getDecodedDSOLocal(Record[OpNum++]));
+  inferDSOLocal(NewGA);
+
   ValueList.push_back(NewGA);
   IndirectSymbolInits.push_back(std::make_pair(NewGA, Val));
   return Error::success();
 }
 
 Error BitcodeReader::parseModule(uint64_t ResumeBit,
                                  bool ShouldLazyLoadMetadata) {
   if (ResumeBit)