Fix infinite loop in getFirstNonMarker

hpvm/projects/hetero-c++/lib/HCCVerifier.cpp

@@ -460,15 +460,19 @@ static Instruction* getFirstNonDebug(BasicBlock* block) {
   return inst;
 }
 
+// Finds the first instruction in the basic block which is not a marker and no
+// instruction after it in the block is either
 Instruction* HCCVerifier::getFirstNonMarker(BasicBlock* block) {
-  Instruction* result = block->getFirstNonPHIOrDbg();
-  while (!result->isTerminator()) {
-    CallInst* call = dyn_cast<CallInst>(result);
-    if (!call) break;
-    if (markerFunctions.find(call->getCalledFunction())
-        != markerFunctions.end()) {
-      result = result->getNextNonDebugInstruction();
+  // Traverse from the end, stopping when we encounter a marker
+  Instruction* current = block->getTerminator();
+  Instruction* result = current;
+  while (current) {
+    if (CallInst* call = dyn_cast<CallInst>(current)) {
+      if (markerFunctions.find(call->getCalledFunction())
+          != markerFunctions.end()) break;
     }
+    result = current;
+    current = current->getPrevNonDebugInstruction();
   }
   return result;
 }