[Bug libstdc++/112682] New: More efficient std::basic_string move construction

[Bug libstdc++/112682] New: More efficient std::basic_string move construction

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112682

            Bug ID: 112682
           Summary: More efficient std::basic_string move construction
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

A few places in bits/basic_string.h have the following code:

```
        if (__str._M_is_local())
          {
            _M_init_local_buf();
            traits_type::copy(_M_local_buf, __str._M_local_buf,
                              __str.length() + 1);
          }
```

Despite the knowledge, that `__str.length()` is not greater than 15 the
compiler emits (and inlines) a memcpy call. That results in a quite big set of
instructions https://godbolt.org/z/j35MMfxzq

Replacing `__str.length() + 1` with `_S_local_capacity + 1` explicitly forces
the compiler to copy the whole `__str._M_local_buf`. As a result the assembly
becomes almost 5 times shorter and without any function calls or multiple
conditional jumps https://godbolt.org/z/bfq8bxra9


P.S.: not sure, if it is allowed to copy uninitialized data via
traits_type::copy and would the sanitizers be happy with such copy attempt.