diff --git a/lib/astutils.cpp b/lib/astutils.cpp
index 56b04edb4b0..ff4320be8c1 100644
--- a/lib/astutils.cpp
+++ b/lib/astutils.cpp
@@ -991,7 +991,7 @@ static const Token * getVariableInitExpression(const Variable * var)
 const Token* isInLoopCondition(const Token* tok)
 {
     const Token* top = tok->astTop();
-    return top && Token::Match(top->previous(), "for|while (") ? top : nullptr;
+    return Token::Match(top->previous(), "for|while (") ? top : nullptr;
 }
 
 /// If tok2 comes after tok1
diff --git a/lib/checkcondition.cpp b/lib/checkcondition.cpp
index a1a71c5675c..09a24157470 100644
--- a/lib/checkcondition.cpp
+++ b/lib/checkcondition.cpp
@@ -834,8 +834,6 @@ static std::string innerSmtString(const Token * tok)
 {
     if (!tok)
         return "if";
-    if (!tok->astTop())
-        return "if";
     const Token * top = tok->astTop();
     if (top->str() == "(" && top->astOperand1())
         return top->astOperand1()->str();
@@ -1119,7 +1117,7 @@ static std::string conditionString(const Token * tok)
 
 static bool isIfConstexpr(const Token* tok) {
     const Token* const top = tok->astTop();
-    return top && Token::simpleMatch(top->astOperand1(), "if") && top->astOperand1()->isConstexpr();
+    return Token::simpleMatch(top->astOperand1(), "if") && top->astOperand1()->isConstexpr();
 }
 
 void CheckCondition::checkIncorrectLogicOperator()
diff --git a/lib/checkother.cpp b/lib/checkother.cpp
index 9fa60ae233d..b89a5918c76 100644
--- a/lib/checkother.cpp
+++ b/lib/checkother.cpp
@@ -2307,8 +2307,6 @@ static const Token * getSingleExpressionInBlock(const Token * tok)
     if (!tok)
         return nullptr;
     const Token * top = tok->astTop();
-    if (!top)
-        return nullptr;
     const Token * nextExpression = nextAfterAstRightmostLeaf(top);
     if (!Token::simpleMatch(nextExpression, "; }"))
         return nullptr;
diff --git a/lib/checkstl.cpp b/lib/checkstl.cpp
index 07fb63ac8cb..866a450670e 100644
--- a/lib/checkstl.cpp
+++ b/lib/checkstl.cpp
@@ -1613,22 +1613,8 @@ static const Token* skipLocalVars(const Token* const tok)
     if (Token::simpleMatch(tok, "{"))
         return skipLocalVars(tok->next());
 
-    const Token *top = tok->astTop();
-    if (!top) {
-        const Token *semi = Token::findsimplematch(tok, ";");
-        if (!semi)
-            return tok;
-        if (!Token::Match(semi->previous(), "%var% ;"))
-            return tok;
-        const Token *varTok = semi->previous();
-        const Variable *var = varTok->variable();
-        if (!var)
-            return tok;
-        if (var->nameToken() != varTok)
-            return tok;
-        return skipLocalVars(semi->next());
-    }
     if (tok->isAssignmentOp()) {
+        const Token *top = tok->astTop();
         const Token *varTok = top->astOperand1();
         const Variable *var = varTok->variable();
         if (!var)
diff --git a/lib/config.h b/lib/config.h
index 0aecf2c52d5..ebd2ebbbe14 100644
--- a/lib/config.h
+++ b/lib/config.h
@@ -121,6 +121,16 @@
 #  define DEPRECATED
 #endif
 
+// TODO: GCC apparently also supports this but there is no documentation on it
+// returns_nonnull
+#if __has_cpp_attribute (gnu::returns_nonnull)
+#  define RET_NONNULL [[gnu::returns_nonnull]]
+#elif (defined(__clang__) && ((__clang_major__ > 3) || ((__clang_major__ == 3) && (__clang_minor__ >= 7))))
+#  define RET_NONNULL __attribute__((returns_nonnull))
+#else
+#  define RET_NONNULL
+#endif
+
 #define REQUIRES(msg, ...) class=typename std::enable_if<__VA_ARGS__::value>::type
 
 #include <string>
diff --git a/lib/forwardanalyzer.cpp b/lib/forwardanalyzer.cpp
index 17e340a5014..b779aac8e87 100644
--- a/lib/forwardanalyzer.cpp
+++ b/lib/forwardanalyzer.cpp
@@ -585,7 +585,7 @@ namespace {
                     return Break(Analyzer::Terminate::Bail);
                 } else if (tok->str() == ";" && tok->astParent()) {
                     Token* top = tok->astTop();
-                    if (top && Token::Match(top->previous(), "for|while (") && Token::simpleMatch(top->link(), ") {")) {
+                    if (Token::Match(top->previous(), "for|while (") && Token::simpleMatch(top->link(), ") {")) {
                         Token* endCond = top->link();
                         Token* endBlock = endCond->linkAt(1);
                         Token* condTok = getCondTok(top);
diff --git a/lib/programmemory.cpp b/lib/programmemory.cpp
index 97c1756a4c0..95fce1bb958 100644
--- a/lib/programmemory.cpp
+++ b/lib/programmemory.cpp
@@ -509,7 +509,7 @@ void ProgramMemoryState::assume(const Token* tok, bool b, bool isEmpty)
         programMemoryParseCondition(pm, tok, nullptr, *settings, b);
     const Token* origin = tok;
     const Token* top = tok->astTop();
-    if (top && Token::Match(top->previous(), "for|while|if (") && !Token::simpleMatch(tok->astParent(), "?")) {
+    if (Token::Match(top->previous(), "for|while|if (") && !Token::simpleMatch(tok->astParent(), "?")) {
         origin = top->link()->next();
         if (!b && origin->link()) {
             origin = origin->link();
@@ -1727,8 +1727,6 @@ namespace {
                 return {unknown()};
             for (const Token* tok = scope->bodyStart->next(); precedes(tok, scope->bodyEnd); tok = tok->next()) {
                 const Token* top = tok->astTop();
-                if (!top)
-                    return {unknown()};
 
                 if (Token::simpleMatch(top, "return") && top->astOperand1())
                     return {execute(top->astOperand1())};
diff --git a/lib/reverseanalyzer.cpp b/lib/reverseanalyzer.cpp
index 04aba5bc203..d8b2def1216 100644
--- a/lib/reverseanalyzer.cpp
+++ b/lib/reverseanalyzer.cpp
@@ -295,7 +295,7 @@ namespace {
                     if (!condTok)
                         break;
                     Analyzer::Action condAction = analyzeRecursive(condTok);
-                    const bool inLoop = condTok->astTop() && Token::Match(condTok->astTop()->previous(), "for|while (");
+                    const bool inLoop = Token::Match(condTok->astTop()->previous(), "for|while (");
                     // Evaluate condition of for and while loops first
                     if (inLoop) {
                         if (Token::findmatch(tok->link(), "goto|break", tok))
diff --git a/lib/token.h b/lib/token.h
index ca7e7ce1e03..38481ed6a99 100644
--- a/lib/token.h
+++ b/lib/token.h
@@ -919,9 +919,9 @@ class CPPCHECKLIB Token {
      * @param prepend Insert the new token before this token when it's not
      * the first one on the tokens list.
      */
-    Token* insertToken(const std::string& tokenStr, const std::string& originalNameStr = emptyString, const std::string& macroNameStr = emptyString, bool prepend = false);
+    RET_NONNULL Token* insertToken(const std::string& tokenStr, const std::string& originalNameStr = emptyString, const std::string& macroNameStr = emptyString, bool prepend = false);
 
-    Token* insertTokenBefore(const std::string& tokenStr, const std::string& originalNameStr = emptyString, const std::string& macroNameStr = emptyString)
+    RET_NONNULL Token* insertTokenBefore(const std::string& tokenStr, const std::string& originalNameStr = emptyString, const std::string& macroNameStr = emptyString)
     {
         return insertToken(tokenStr, originalNameStr, macroNameStr, true);
     }
@@ -1224,7 +1224,7 @@ class CPPCHECKLIB Token {
     /**
      * @return the first token of the next argument. Does only work on argument
      * lists. Requires that Tokenizer::createLinks2() has been called before.
-     * Returns 0, if there is no next argument.
+     * Returns nullptr, if there is no next argument.
      */
     const Token* nextArgument() const;
     Token *nextArgument();
@@ -1232,14 +1232,14 @@ class CPPCHECKLIB Token {
     /**
      * @return the first token of the next argument. Does only work on argument
      * lists. Should be used only before Tokenizer::createLinks2() was called.
-     * Returns 0, if there is no next argument.
+     * Returns nullptr, if there is no next argument.
      */
     const Token* nextArgumentBeforeCreateLinks2() const;
 
     /**
      * @return the first token of the next template argument. Does only work on template argument
      * lists. Requires that Tokenizer::createLinks2() has been called before.
-     * Returns 0, if there is no next argument.
+     * Returns nullptr, if there is no next argument.
      */
     const Token* nextTemplateArgument() const;
 
@@ -1477,14 +1477,14 @@ class CPPCHECKLIB Token {
         return nullptr;
 
     }
-    Token *astTop() {
+    RET_NONNULL Token *astTop() {
         Token *ret = this;
         while (ret->mImpl->mAstParent)
             ret = ret->mImpl->mAstParent;
         return ret;
     }
 
-    const Token *astTop() const {
+    RET_NONNULL const Token *astTop() const {
         const Token *ret = this;
         while (ret->mImpl->mAstParent)
             ret = ret->mImpl->mAstParent;
diff --git a/lib/valueflow.cpp b/lib/valueflow.cpp
index c19799ea412..5f88aa01b99 100644
--- a/lib/valueflow.cpp
+++ b/lib/valueflow.cpp
@@ -635,7 +635,7 @@ static bool isConditionKnown(const Token* tok, bool then)
     while (parent && (parent->str() == op || parent->str() == "!" || parent->isCast()))
         parent = parent->astParent();
     const Token* top = tok->astTop();
-    if (top && Token::Match(top->previous(), "if|while|for ("))
+    if (Token::Match(top->previous(), "if|while|for ("))
         return parent == top || Token::simpleMatch(parent, ";");
     return parent && parent->str() != op;
 }
@@ -2372,7 +2372,7 @@ const Token* ValueFlow::getEndOfExprScope(const Token* tok, const Scope* default
                     end = varEnd;
 
                 const Token* top = var->nameToken()->astTop();
-                if (top && Token::simpleMatch(top->tokAt(-1), "if (")) { // variable declared in if (...)
+                if (Token::simpleMatch(top->tokAt(-1), "if (")) { // variable declared in if (...)
                     const Token* elseTok = top->link()->linkAt(1);
                     if (Token::simpleMatch(elseTok, "} else {") && tok->scope()->isNestedIn(elseTok->tokAt(2)->scope()))
                         end = tok->scope()->bodyEnd;
@@ -4751,8 +4751,6 @@ struct ConditionHandler {
                     continue;
 
                 const Token* top = tok->astTop();
-                if (!top)
-                    continue;
 
                 if (!Token::Match(top->previous(), "if|while|for (") && !Token::Match(tok->astParent(), "&&|%oror%|?|!"))
                     continue;
@@ -4927,8 +4925,6 @@ struct ConditionHandler {
                         const Settings& settings,
                         const std::set<const Scope*>& skippedFunctions) const {
         traverseCondition(symboldatabase, settings, skippedFunctions, [&](const Condition& cond, Token* condTok, const Scope* scope) {
-            Token* top = condTok->astTop();
-
             const MathLib::bigint path = cond.getPath();
             const bool allowKnown = path == 0;
 
@@ -5003,8 +4999,7 @@ struct ConditionHandler {
                 }
             }
 
-            if (!top)
-                return;
+            Token* top = condTok->astTop();
 
             if (top->previous()->isExpandedMacro()) {
                 for (std::list<ValueFlow::Value>* values : {&thenValues, &elseValues}) {
@@ -7086,7 +7081,7 @@ static void valueFlowContainerSize(const TokenList& tokenlist,
                 !Token::Match(nameToken, "%name% ("))
                 continue;
         }
-        if (nameToken->astTop() && Token::Match(nameToken->astTop()->previous(), "for|while"))
+        if (Token::Match(nameToken->astTop()->previous(), "for|while"))
             known = !isVariableChanged(var, settings);
         std::vector<ValueFlow::Value> values{ValueFlow::Value{size}};
         values.back().valueType = ValueFlow::Value::ValueType::CONTAINER_SIZE;