diff --git a/contributors.txt b/contributors.txt
index ccd9304934..b249c21b75 100644
--- a/contributors.txt
+++ b/contributors.txt
@@ -318,4 +318,4 @@ YYYY/MM/DD, github id, Full name, email
 2021/09/23, skalt, Steven Kalt, kalt.steven@gmail.com
 2021/10/10, tools4origins, Erwan Guyomarc'h, contact@erwan-guyomarch.fr
 2021/10/19, jcking, Justin King, jcking@google.com
-2021/10/31, skef, Skef Iterum, github@skef.org
\ No newline at end of file
+2021/10/31, skef, Skef Iterum, github@skef.org
diff --git a/runtime/Cpp/runtime/src/atn/LexerATNSimulator.cpp b/runtime/Cpp/runtime/src/atn/LexerATNSimulator.cpp
index 381351a95a..023ce8d224 100755
--- a/runtime/Cpp/runtime/src/atn/LexerATNSimulator.cpp
+++ b/runtime/Cpp/runtime/src/atn/LexerATNSimulator.cpp
@@ -80,10 +80,13 @@ size_t LexerATNSimulator::match(CharStream *input, size_t mode) {
   _startIndex = input->index();
   _prevAccept.reset();
   const dfa::DFA &dfa = _decisionToDFA[mode];
-  if (dfa.s0 == nullptr) {
+  _stateLock.readLock();
+  dfa::DFAState* s0 = dfa.s0;
+  _stateLock.readUnlock();
+  if (s0 == nullptr) {
     return matchATN(input);
   } else {
-    return execATN(input, dfa.s0);
+    return execATN(input, s0);
   }
 }
 
@@ -111,10 +114,7 @@ size_t LexerATNSimulator::matchATN(CharStream *input) {
   bool suppressEdge = s0_closure->hasSemanticContext;
   s0_closure->hasSemanticContext = false;
 
-  dfa::DFAState *next = addDFAState(s0_closure.release());
-  if (!suppressEdge) {
-    _decisionToDFA[_mode].s0 = next;
-  }
+  dfa::DFAState *next = addDFAState(s0_closure.release(), suppressEdge);
 
   size_t predict = execATN(input, next);
 
@@ -536,6 +536,10 @@ void LexerATNSimulator::addDFAEdge(dfa::DFAState *p, size_t t, dfa::DFAState *q)
 }
 
 dfa::DFAState *LexerATNSimulator::addDFAState(ATNConfigSet *configs) {
+  return addDFAState(configs, true);
+}
+
+dfa::DFAState *LexerATNSimulator::addDFAState(ATNConfigSet *configs, bool suppressEdge) {
   /* the lexer evaluates predicates on-the-fly; by this point configs
    * should not contain any configurations with unevaluated predicates.
    */
@@ -563,6 +567,9 @@ dfa::DFAState *LexerATNSimulator::addDFAState(ATNConfigSet *configs) {
     auto iterator = dfa.states.find(proposed);
     if (iterator != dfa.states.end()) {
       delete proposed;
+      if (!suppressEdge) {
+        _decisionToDFA[_mode].s0 = *iterator;
+      }
       _stateLock.writeUnlock();
       return *iterator;
     }
@@ -572,6 +579,9 @@ dfa::DFAState *LexerATNSimulator::addDFAState(ATNConfigSet *configs) {
   proposed->configs->setReadonly(true);
 
   dfa.states.insert(proposed);
+  if (!suppressEdge) {
+    _decisionToDFA[_mode].s0 = proposed;
+  }
   _stateLock.writeUnlock();
 
   return proposed;
diff --git a/runtime/Cpp/runtime/src/atn/LexerATNSimulator.h b/runtime/Cpp/runtime/src/atn/LexerATNSimulator.h
index cfe2d913be..e0b8407d89 100755
--- a/runtime/Cpp/runtime/src/atn/LexerATNSimulator.h
+++ b/runtime/Cpp/runtime/src/atn/LexerATNSimulator.h
@@ -199,6 +199,8 @@ namespace atn {
     /// </summary>
     virtual dfa::DFAState *addDFAState(ATNConfigSet *configs);
 
+    virtual dfa::DFAState *addDFAState(ATNConfigSet *configs, bool suppressEdge);
+
   public:
     dfa::DFA& getDFA(size_t mode);
 
diff --git a/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp b/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp
index 01c83c15e6..7200fa2ba6 100755
--- a/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp
+++ b/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp
@@ -94,14 +94,18 @@ size_t ParserATNSimulator::adaptivePredict(TokenStream *input, size_t decision,
   });
 
   dfa::DFAState *s0;
+  _stateLock.readLock();
   if (dfa.isPrecedenceDfa()) {
     // the start state for a precedence DFA depends on the current
     // parser precedence, and is provided by a DFA method.
+    _edgeLock.readLock();
     s0 = dfa.getPrecedenceStartState(parser->getPrecedence());
+    _edgeLock.readUnlock();
   } else {
     // the start state for a "regular" DFA is just s0
     s0 = dfa.s0;
   }
+  _stateLock.readUnlock();
 
   if (s0 == nullptr) {
     bool fullCtx = false;
@@ -109,6 +113,7 @@ size_t ParserATNSimulator::adaptivePredict(TokenStream *input, size_t decision,
                                                                  &ParserRuleContext::EMPTY, fullCtx);
 
     _stateLock.writeLock();
+    dfa::DFAState* ds0 = dfa.s0;
     if (dfa.isPrecedenceDfa()) {
       /* If this is a precedence DFA, we use applyPrecedenceFilter
        * to convert the computed start state to a precedence start
@@ -116,8 +121,8 @@ size_t ParserATNSimulator::adaptivePredict(TokenStream *input, size_t decision,
        * appropriate start state for the precedence level rather
        * than simply setting DFA.s0.
        */
-      dfa.s0->configs = std::move(s0_closure); // not used for prediction but useful to know start configs anyway
-      dfa::DFAState *newState = new dfa::DFAState(applyPrecedenceFilter(dfa.s0->configs.get())); /* mem-check: managed by the DFA or deleted below */
+      ds0->configs = std::move(s0_closure); // not used for prediction but useful to know start configs anyway
+      dfa::DFAState *newState = new dfa::DFAState(applyPrecedenceFilter(ds0->configs.get())); /* mem-check: managed by the DFA or deleted below */
       s0 = addDFAState(dfa, newState);
       dfa.setPrecedenceStartState(parser->getPrecedence(), s0, _edgeLock);
       if (s0 != newState) {
@@ -127,8 +132,9 @@ size_t ParserATNSimulator::adaptivePredict(TokenStream *input, size_t decision,
       dfa::DFAState *newState = new dfa::DFAState(std::move(s0_closure)); /* mem-check: managed by the DFA or deleted below */
       s0 = addDFAState(dfa, newState);
 
-      if (dfa.s0 != s0) {
-        delete dfa.s0; // Delete existing s0 DFA state, if there's any.
+      if (ds0 != s0) {
+        delete ds0; // Delete existing s0 DFA state, if there's any.
+        ds0 = nullptr;
         dfa.s0 = s0;
       }
       if (s0 != newState) {
diff --git a/runtime/Cpp/runtime/src/dfa/DFA.cpp b/runtime/Cpp/runtime/src/dfa/DFA.cpp
index 3f83180afa..6bcad630ae 100755
--- a/runtime/Cpp/runtime/src/dfa/DFA.cpp
+++ b/runtime/Cpp/runtime/src/dfa/DFA.cpp
@@ -32,13 +32,12 @@ DFA::DFA(atn::DecisionState *atnStartState, size_t decision)
   }
 }
 
-DFA::DFA(DFA &&other) : atnStartState(other.atnStartState), decision(other.decision) {
+DFA::DFA(DFA &&other) : atnStartState(other.atnStartState), s0(other.s0), decision(other.decision) {
   // Source states are implicitly cleared by the move.
   states = std::move(other.states);
 
   other.atnStartState = nullptr;
   other.decision = 0;
-  s0 = other.s0;
   other.s0 = nullptr;
   _precedenceDfa = other._precedenceDfa;
   other._precedenceDfa = false;
@@ -52,8 +51,9 @@ DFA::~DFA() {
     delete state;
   }
 
-  if (!s0InList)
+  if (!s0InList) {
     delete s0;
+  }
 }
 
 bool DFA::isPrecedenceDfa() const {