[1/3] Use manual regex algorithm switching

diff --git a/libstdc++-v3/include/bits/regex.tcc b/libstdc++-v3/include/bits/regex.tcc
index 5fa1f01..0d737a0 100644
--- a/libstdc++-v3/include/bits/regex.tcc
+++ b/libstdc++-v3/include/bits/regex.tcc
@@ -28,12 +28,12 @@ 
  *  Do not attempt to use it directly. @headername{regex}
  */
 
-// See below __regex_algo_impl to get what this is talking about. The default
-// value 1 indicated a conservative optimization without giving up worst case
-// performance.
-#ifndef _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT
-#define _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT 1
-#endif
+// A non-standard switch to let the user pick the matching algorithm.
+// If _GLIBCXX_REGEX_USE_THOMPSON_NFA is defined, the thompson NFA
+// algorithm will be used. This algorithm is not enabled by default,
+// and cannot be used if the regex contains back-references, but has better
+// (polynomial instead of exponential) worst case performace.
+// See __regex_algo_impl below.
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -66,24 +66,15 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       for (auto& __it : __res)
 	__it.matched = false;
 
-      // This function decide which executor to use under given circumstances.
-      // The _S_auto policy now is the following: if a NFA has no
-      // back-references and has more than _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT
-      // quantifiers (*, +, ?), the BFS executor will be used, other wise
-      // DFS executor. This is because DFS executor has a exponential upper
-      // bound, but better best-case performace. Meanwhile, BFS executor can
-      // effectively prevent from exponential-long time matching (which must
-      // contains many quantifiers), but it's slower in average.
-      //
-      // For simple regex, BFS executor could be 2 or more times slower than
-      // DFS executor.
-      //
-      // Of course, BFS executor cannot handle back-references.
+      // __policy is used by testsuites so that they can use Thompson NFA
+      // without defining a macro. Users should define
+      // _GLIBCXX_REGEX_USE_THOMPSON_NFA if they need to use this approach.
       bool __ret;
       if (!__re._M_automaton->_M_has_backref
-	  && (__policy == _RegexExecutorPolicy::_S_alternate
-	      || __re._M_automaton->_M_quant_count
-		> _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT))
+#ifndef _GLIBCXX_REGEX_USE_THOMPSON_NFA
+	  && __policy == _RegexExecutorPolicy::_S_alternate
+#endif
+	  )
 	{
 	  _Executor<_BiIter, _Alloc, _TraitsT, false>
 	    __executor(__s, __e, __m, __re, __flags);
diff --git a/libstdc++-v3/include/bits/regex_automaton.h b/libstdc++-v3/include/bits/regex_automaton.h
index a442cfe..64ecd6d 100644
--- a/libstdc++-v3/include/bits/regex_automaton.h
+++ b/libstdc++-v3/include/bits/regex_automaton.h
@@ -74,8 +74,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       size_t _M_backref_index;  // for _S_opcode_backref
       struct
       {
-	// for _S_opcode_alternative.
-	_StateIdT  _M_quant_index;
 	// for _S_opcode_alternative or _S_opcode_subexpr_lookahead
 	_StateIdT  _M_alt;
 	// for _S_opcode_word_boundary or _S_opcode_subexpr_lookahead or
@@ -120,7 +118,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     explicit
     _NFA_base(_FlagT __f)
     : _M_flags(__f), _M_start_state(0), _M_subexpr_count(0),
-    _M_quant_count(0), _M_has_backref(false)
+    _M_has_backref(false)
     { }
 
     _NFA_base(_NFA_base&&) = default;
@@ -145,7 +143,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _FlagT                    _M_flags;
     _StateIdT                 _M_start_state;
     _SizeT                    _M_subexpr_count;
-    _SizeT                    _M_quant_count;
     bool                      _M_has_backref;
   };
 
@@ -175,7 +172,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_StateT __tmp(_S_opcode_alternative);
 	// It labels every quantifier to make greedy comparison easier in BFS
 	// approach.
-	__tmp._M_quant_index = this->_M_quant_count++;
 	__tmp._M_next = __next;
 	__tmp._M_alt = __alt;
 	__tmp._M_neg = __neg;
diff --git a/libstdc++-v3/include/bits/regex_automaton.tcc b/libstdc++-v3/include/bits/regex_automaton.tcc
index 1476ae2..38787fa 100644
--- a/libstdc++-v3/include/bits/regex_automaton.tcc
+++ b/libstdc++-v3/include/bits/regex_automaton.tcc
@@ -74,9 +74,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       case _S_opcode_alternative:
 	__ostr << __id << " [label=\"" << __id << "\\nALT\"];\n"
 	       << __id << " -> " << _M_next
-	       << " [label=\"epsilon\", tailport=\"s\"];\n"
+	       << " [label=\"next\", tailport=\"s\"];\n"
 	       << __id << " -> " << _M_alt
-	       << " [label=\"epsilon\", tailport=\"n\"];\n";
+	       << " [label=\"alt\", tailport=\"n\"];\n";
 	break;
       case _S_opcode_backref:
 	__ostr << __id << " [label=\"" << __id << "\\nBACKREF "