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.
