[Bug libstdc++/112925] New: Optimize std::string construction from std::istreambuf_iterator

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112925

            Bug ID: 112925
           Summary: Optimize std::string construction from
                    std::istreambuf_iterator
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Something like this would improve performance considerably. Instead of reading
a character at a time we can use streambuf::sgetn to copy directly into the
string's unused capacity:

--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -205,6 +205,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                _M_data(__another);
                _M_capacity(__capacity);
              }
+
+#if __cplusplus >= 201103L && 0
+           using _BufIter = istreambuf_iterator<_CharT, char_traits<_CharT>>;
+           if _GLIBCXX17_CONSTEXPR (is_same<_InIterator, _BufIter>::value)
+           {
+             if (!std::__is_constant_evaluated())
+               {
+                 auto __r = std::__copy_n_a(__beg, capacity() - size(),
+                                            data() + size(), false);
+                 __len = __r - data();
+                 _M_set_length(__len);
+                 continue;
+               }
+           }
+#endif
            traits_type::assign(_M_data()[__len++], *__beg);
            ++__beg;
          }