[PATCH] libstdc++: Use 'if constexpr' to slightly simplify

[Jonathan Wakely]

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 (_AnyMatcher::operator()):
        Replace tag dispatching with 'if constexpr'.
        (_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.
---

Tested x86_64-linux.

 libstdc++-v3/include/bits/regex_compiler.h   | 59 +++++++++-----------
 libstdc++-v3/include/bits/regex_compiler.tcc |  2 +-
 2 files changed, 26 insertions(+), 35 deletions(-)

diff --git a/libstdc++-v3/include/bits/regex_compiler.h 
b/libstdc++-v3/include/bits/regex_compiler.h
index f24c7e3baa6..3931790091a 100644
--- a/libstdc++-v3/include/bits/regex_compiler.h
+++ b/libstdc++-v3/include/bits/regex_compiler.h
@@ -376,26 +376,21 @@ 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
       {
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
        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;
+       if constexpr (is_same<_CharT, char>::value)
+         return __c != __n && __c != __r;
+       else
+         {
+           auto __ls = _M_translator._M_translate(u'\u2028'); // line sep
+           auto __ps = _M_translator._M_translate(u'\u2029'); // para sep
+           return __c != __n && __c != __r && __c != __ls && __c != __ps;
+         }
+#pragma GCC diagnostic pop
       }
 
       _TransT _M_translator;
@@ -440,8 +435,14 @@ namespace __detail
       bool
       operator()(_CharT __ch) const
       {
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
        _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);
+#pragma GCC diagnostic pop
       }
 
       void
@@ -509,11 +510,16 @@ namespace __detail
       void
       _M_ready()
       {
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
        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);
+#pragma GCC diagnostic pop
       }
 
     private:
@@ -531,22 +537,7 @@ 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;
diff --git a/libstdc++-v3/include/bits/regex_compiler.tcc 
b/libstdc++-v3/include/bits/regex_compiler.tcc
index cd0db2761b5..59b79fdd106 100644
--- a/libstdc++-v3/include/bits/regex_compiler.tcc
+++ b/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]
       {
-- 
2.49.0