https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66416
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2024-02-19
Keywords| |missed-optimization
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=23112
Status|UNCONFIRMED |NEW
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem is that string::find(char) uses std::char_traits::find, which is
optimized to use memchr, but there is no char-traits API for rfind, so
string::rfind(char) is a manual loop from the end of the string to the start.
We could add configure checks for memrchr and do something like:
--- a/libstdc++-v3/include/bits/string_view.tcc
+++ b/libstdc++-v3/include/bits/string_view.tcc
@@ -121,6 +121,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
if (--__size > __pos)
__size = __pos;
+#if _GLIBCXX_HAVE_MEMRCHR
+ if constexpr (is_same_v<_Traits, char_traits<char>>)
+ {
+ if (auto __ptr = memrchr(_M_str, __c, __size))
+ return __ptr - _M_str;
+ }
+ else
+#endif
for (++__size; __size-- > 0; )
if (traits_type::eq(this->_M_str[__size], __c))
return __size;
And then duplicate that in std::string::rfind, and ranges::find_last.
