https://gcc.gnu.org/g:c1b4b71cfc958a622663ee5eb67a6acc5f9bb9b6
commit r14-12598-gc1b4b71cfc958a622663ee5eb67a6acc5f9bb9b6 Author: Jonathan Wakely <[email protected]> Date: Tue Mar 17 15:58:10 2026 +0000 libstdc++: Optimize fs::path::operator+=(const path&) alias check Instead of a loop that compares &p to the address of each path component, we can just do two pointer comparisons to see if &p is within the contiguous array of components. We don't need to make the same change to experimental::filesystem::path because as noted in r15-9709-gbeb0ffd36eedf0 the TS implementation doesn't attempt to optimize operator+= so doesn't care if the parameter aliases *this. libstdc++-v3/ChangeLog: * src/c++17/fs_path.cc (path::operator+=): Use pointer comparison to detect aliasing instead of a loop. Reviewed-by: Tomasz Kamiński <[email protected]> (cherry picked from commit 881b327773d3f366c2b638e3d07ef310b2a2dfe2) Diff: --- libstdc++-v3/src/c++17/fs_path.cc | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/libstdc++-v3/src/c++17/fs_path.cc b/libstdc++-v3/src/c++17/fs_path.cc index 857887ec58d8..94793da64105 100644 --- a/libstdc++-v3/src/c++17/fs_path.cc +++ b/libstdc++-v3/src/c++17/fs_path.cc @@ -886,9 +886,11 @@ path::operator+=(const path& p) return *this += p.native(); // Handle p += *i where i is in [p.begin(),p.end()), for the same reason. if (_M_type() == _Type::_Multi && p._M_type() != _Type::_Multi) - for (const path& cmpt : *this) - if (&cmpt == &p) [[unlikely]] + { + const auto first = _M_cmpts.begin(), last = first + _M_cmpts.size(); + if (!std::less<>()(&p, first) && std::less<>()(&p, last)) [[unlikely]] return *this += p.native(); + } #if _GLIBCXX_FILESYSTEM_IS_WINDOWS if (_M_type() == _Type::_Root_name
