Correctly report modified status for DataFlowSanitizer

[serge-sans-paille]

Summary

Related to https://reviews.llvm.org/D80916

[vitalybuka]

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

1047

there is a global inserted in the beginning

[vitalybuka]

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

814

maybe always true after this point?

[serge-sans-paille]

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

814

only if the globals don't already exist.

[vitalybuka]

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

814

initializeCallbackFunctions and initializeRuntimeFunctions also changes the module
and even without inserting globals it change setThreadLocalMode

[serge-sans-paille]

Take review into account.

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

void DataFlowSanitizer::initializeCallbackFunctions(Module &M) {
   DFSanCmpCallbackFn = Mod->getOrInsertFunction("__dfsan_cmp_callback",
                                                 DFSanLoadStoreCmpCallbackFnTy);
 }
 
 bool DataFlowSanitizer::runOnModule(Module &M) {
   if (ABIList.isIn(M, "skip"))
     return false;
 
   if (!GetArgTLSPtr) {
     Type *ArgTLSTy = ArrayType::get(ShadowTy, 64);
     ArgTLS = Mod->getOrInsertGlobal("__dfsan_arg_tls", ArgTLSTy);
     if (GlobalVariable *G = dyn_cast<GlobalVariable>(ArgTLS))
       G->setThreadLocalMode(GlobalVariable::InitialExecTLSModel);
   }
   if (!GetRetvalTLSPtr) {
     RetvalTLS = Mod->getOrInsertGlobal("__dfsan_retval_tls", ShadowTy);
     if (GlobalVariable *G = dyn_cast<GlobalVariable>(RetvalTLS))
if (ClDebugNonzeroLabels) {
         BranchInst *BI = cast<BranchInst>(SplitBlockAndInsertIfThen(
             Ne, Pos, /*Unreachable=*/false, ColdCallWeights));
         IRBuilder<> ThenIRB(BI);
         ThenIRB.CreateCall(DFSF.DFS.DFSanNonzeroLabelFn, {});
       }
     }
   }
 
-  return false;
+  return !FnsToInstrument.empty();
 }
 
 Value *DFSanFunction::getArgTLSPtr() {
   if (ArgTLSPtr)
     return ArgTLSPtr;
   if (DFS.ArgTLS)
     return ArgTLSPtr = DFS.ArgTLS;