Add a flag to check whether dt exists

Author: paperchalice

llvm/include/llvm/CodeGen/MachineDominators.h

@@ -266,6 +266,7 @@ class MachineDominatorTree : public DomTreeBase<MachineBasicBlock> {
 /// \brief Analysis pass which computes a \c MachineDominatorTree.
 class MachineDominatorTreeWrapperPass : public MachineFunctionPass {
   MachineDominatorTree DT;
+  bool IsDomTreeEmpty = true;
 
 public:
   static char ID;

llvm/lib/CodeGen/MachineDominators.cpp

@@ -70,6 +70,7 @@ MachineDominatorTreeWrapperPass::MachineDominatorTreeWrapperPass()
 char &llvm::MachineDominatorsID = MachineDominatorTreeWrapperPass::ID;
 
 bool MachineDominatorTreeWrapperPass::runOnMachineFunction(MachineFunction &F) {
+  IsDomTreeEmpty = false;
   DT.calculate(F);
   return false;
 }
@@ -83,10 +84,11 @@ void MachineDominatorTree::calculate(MachineFunction &F) {
 void MachineDominatorTreeWrapperPass::releaseMemory() {
   DT.CriticalEdgesToSplit.clear();
   DT.reset();
+  IsDomTreeEmpty = true;
 }
 
 void MachineDominatorTreeWrapperPass::verifyAnalysis() const {
-  if (VerifyMachineDomInfo)
+  if (VerifyMachineDomInfo && !IsDomTreeEmpty)
     if (!DT.verify(MachineDominatorTree::VerificationLevel::Basic))
       report_fatal_error("MachineDominatorTree verification failed!");
 }