https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107087
--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:4969dcd2b7a94ce6c0d07225b21b5f3c040a4902 commit r13-6962-g4969dcd2b7a94ce6c0d07225b21b5f3c040a4902 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Mar 31 13:44:04 2023 +0100 libstdc++: Teach optimizer that empty COW strings are empty [PR107087] The compiler doesn't know about the invariant that the _S_empty_rep() object is immutable and so _M_length and _M_refcount are always zero. This means that we get warnings about writing possibly-non-zero length strings into buffers that can't hold them. If we teach the compiler that the empty rep is always zero length, it knows it can be copied into any buffer. For Stage 1 we might want to also consider adding this to capacity(): if (_S_empty_rep()._M_capacity != 0) __builtin_unreachable(); And this to _Rep::_M_is_leaked() and _Rep::_M_is_shared(): if (_S_empty_rep()._M_refcount != 0) __builtin_unreachable(); libstdc++-v3/ChangeLog: PR tree-optimization/107087 * include/bits/cow_string.h (basic_string::size()): Add optimizer hint that _S_empty_rep()._M_length is always zero. (basic_string::length()): Call size().
