libstdc++: Use constexpr-if to slightly simplify <regex>

This will hardly make a dent in the very slow compile times for <regex>
but it seems worth doing anyway.

libstdc++-v3/ChangeLog:

	* include/bits/regex_compiler.h: Replace _GLIBCXX17_CONSTEXPR
	with constexpr and disable diagnostics with pragmas.
	(_AnyMatcher::operator()): Use constexpr-if instead of tag
	dispatching. Postpone calls to _M_translate until after checking
	result of earlier calls.
	(_AnyMatcher::_M_apply): Remove both overloads.
	(_BracketMatcher::operator(), _BracketMatcher::_M_ready):
	Replace tag dispatching with 'if constexpr'.
	(_BracketMatcher::_M_apply(_CharT, true_type)): Remove.
	(_BracketMatcher::_M_apply(_CharT, false_type)): Remove second
	parameter.
	(_BracketMatcher::_M_make_cache): Remove both overloads.
	* include/bits/regex_compiler.tcc (_BracketMatcher::_M_apply):
	Remove second parameter.
	* include/bits/regex_executor.tcc: Replace _GLIBCXX17_CONSTEXPR
	with constexpr and disable diagnostics with pragmas.
	(_Executor::_M_handle_backref): Replace __glibcxx_assert with
	static_assert.
	(_Executor::_M_handle_accept): Mark _S_opcode_backref case as
	unreachable for non-DFS mode and do not instantiate
	_M_handle_backref for that case.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

libstdc++-v3/include/bits/regex_compiler.h

@@ -38,6 +38,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 _GLIBCXX_END_NAMESPACE_CXX11
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
 namespace __detail
 {
   /**
@@ -221,9 +223,9 @@ namespace __detail
       _CharT
       _M_translate(_CharT __ch) const
       {
-	if _GLIBCXX17_CONSTEXPR (__icase)
+	if constexpr (__icase)
 	  return _M_traits.translate_nocase(__ch);
-	else if _GLIBCXX17_CONSTEXPR (__collate)
+	else if constexpr (__collate)
 	  return _M_traits.translate(__ch);
 	else
 	  return __ch;
@@ -285,7 +287,7 @@ namespace __detail
       bool
       _M_match_range(_CharT __first, _CharT __last, _CharT __ch) const
       {
-	if _GLIBCXX17_CONSTEXPR (!__icase)
+	if constexpr (!__icase)
 	  return __first <= __ch && __ch <= __last;
 	else
 	  return this->_M_in_range_icase(__first, __last, __ch);
@@ -376,26 +378,20 @@ namespace __detail
 
       bool
       operator()(_CharT __ch) const
-      { return _M_apply(__ch, typename is_same<_CharT, char>::type()); }
-
-      bool
-      _M_apply(_CharT __ch, true_type) const
       {
-	auto __c = _M_translator._M_translate(__ch);
-	auto __n = _M_translator._M_translate('\n');
-	auto __r = _M_translator._M_translate('\r');
-	return __c != __n && __c != __r;
-      }
-
-      bool
-      _M_apply(_CharT __ch, false_type) const
-      {
-	auto __c = _M_translator._M_translate(__ch);
-	auto __n = _M_translator._M_translate('\n');
-	auto __r = _M_translator._M_translate('\r');
-	auto __u2028 = _M_translator._M_translate(u'\u2028');
-	auto __u2029 = _M_translator._M_translate(u'\u2029');
-	return __c != __n && __c != __r && __c != __u2028 && __c != __u2029;
+	const auto __c = _M_translator._M_translate(__ch);
+	if (__c == _M_translator._M_translate('\n'))
+	  return false;
+	if (__c == _M_translator._M_translate('\r'))
+	  return false;
+	if constexpr (!is_same<_CharT, char>::value)
+	  {
+	    if (__c == _M_translator._M_translate(u'\u2028')) // line sep
+	      return false;
+	    if (__c == _M_translator._M_translate(u'\u2029')) // para sep
+	      return false;
+	  }
+	return true;
       }
 
       _TransT _M_translator;
@@ -441,7 +437,10 @@ namespace __detail
       operator()(_CharT __ch) const
       {
 	_GLIBCXX_DEBUG_ASSERT(_M_is_ready);
-	return _M_apply(__ch, _UseCache());
+	if constexpr (_UseCache::value)
+	  if (!(__ch & 0x80)) [[__likely__]]
+	    return _M_cache[static_cast<_UnsignedCharT>(__ch)];
+	return _M_apply(__ch);
       }
 
       void
@@ -512,7 +511,9 @@ namespace __detail
 	std::sort(_M_char_set.begin(), _M_char_set.end());
 	auto __end = std::unique(_M_char_set.begin(), _M_char_set.end());
 	_M_char_set.erase(__end, _M_char_set.end());
-	_M_make_cache(_UseCache());
+	if constexpr (_UseCache::value)
+	  for (unsigned __i = 0; __i < 128; __i++) // Only cache 7-bit chars
+	    _M_cache[__i] = _M_apply(static_cast<_CharT>(__i));
 	_GLIBCXX_DEBUG_ONLY(_M_is_ready = true);
       }
 
@@ -531,22 +532,7 @@ namespace __detail
       using _UnsignedCharT = typename std::make_unsigned<_CharT>::type;
 
       bool
-      _M_apply(_CharT __ch, false_type) const;
-
-      bool
-      _M_apply(_CharT __ch, true_type) const
-      { return _M_cache[static_cast<_UnsignedCharT>(__ch)]; }
-
-      void
-      _M_make_cache(true_type)
-      {
-	for (unsigned __i = 0; __i < _M_cache.size(); __i++)
-	  _M_cache[__i] = _M_apply(static_cast<_CharT>(__i), false_type());
-      }
-
-      void
-      _M_make_cache(false_type)
-      { }
+      _M_apply(_CharT __ch) const;
 
     private:
       _GLIBCXX_STD_C::vector<_CharT>            _M_char_set;
@@ -565,6 +551,7 @@ namespace __detail
 
  ///@} regex-detail
 } // namespace __detail
+#pragma GCC diagnostic pop
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 

libstdc++-v3/include/bits/regex_compiler.tcc

@@ -598,7 +598,7 @@ namespace __detail
   template<typename _TraitsT, bool __icase, bool __collate>
     bool
     _BracketMatcher<_TraitsT, __icase, __collate>::
-    _M_apply(_CharT __ch, false_type) const
+    _M_apply(_CharT __ch) const
     {
       return [this, __ch]
       {

libstdc++-v3/include/bits/regex_executor.tcc

@@ -32,6 +32,8 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
 namespace __detail
 {
   template<typename _BiIter, typename _Alloc, typename _TraitsT,
@@ -217,7 +219,7 @@ namespace __detail
 	}
       else // Non-greedy mode
 	{
-	  if (__dfs_mode)
+	  if constexpr (__dfs_mode)
 	    {
 	      // vice-versa.
 	      _M_dfs(__match_mode, __state._M_next);
@@ -322,7 +324,7 @@ namespace __detail
 
       if (_M_current == _M_end)
 	return;
-      if (__dfs_mode)
+      if constexpr (__dfs_mode)
 	{
 	  if (__state._M_matches(*_M_current))
 	    {
@@ -393,7 +395,7 @@ namespace __detail
     void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
     _M_handle_backref(_Match_mode __match_mode, _StateIdT __i)
     {
-      __glibcxx_assert(__dfs_mode);
+      static_assert(__dfs_mode, "this should never be instantiated");
 
       const auto& __state = _M_nfa[__i];
       auto& __submatch = _M_cur_results[__state._M_backref_index];
@@ -426,7 +428,7 @@ namespace __detail
     void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
     _M_handle_accept(_Match_mode __match_mode, _StateIdT)
     {
-      if _GLIBCXX17_CONSTEXPR (__dfs_mode)
+      if constexpr (__dfs_mode)
 	{
 	  __glibcxx_assert(!_M_has_sol);
 	  if (__match_mode == _Match_mode::_Exact)
@@ -529,7 +531,11 @@ namespace __detail
 	case _S_opcode_match:
 	  _M_handle_match(__match_mode, __i); break;
 	case _S_opcode_backref:
-	  _M_handle_backref(__match_mode, __i); break;
+	  if constexpr (__dfs_mode)
+	    _M_handle_backref(__match_mode, __i);
+	  else
+	    __builtin_unreachable();
+	  break;
 	case _S_opcode_accept:
 	  _M_handle_accept(__match_mode, __i); break;
 	case _S_opcode_alternative:
@@ -564,6 +570,7 @@ namespace __detail
       return __left_is_word != __right_is_word;
     }
 } // namespace __detail
+#pragma GCC diagnostic pop
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace