https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105329
--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Richard Biener from comment #11) > (In reply to Richard Biener from comment #10) > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104336#c1 contains a workaround > > for libstdc++ > > Maybe > > _GLIBCXX20_CONSTEXPR > basic_string& > assign(const _CharT* __s) > { > __glibcxx_requires_string(__s); > return _M_replace(size_type(0), this->size(), __s, > traits_type::length(__s)); > } > > isn't the most efficient way to assign a string (constant) looking at > the complexity of _M_replace and its overlap checks (that we all expect > to be optimized away but which are not). In fact the _M_disjunct check > isn't optimized because we cannot tell if the std::string storage > overlaps the constant pool ... Maybe diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h index c3fbc53953c..e83f3e8afa4 100644 --- a/libstdc++-v3/include/bits/basic_string.h +++ b/libstdc++-v3/include/bits/basic_string.h @@ -407,7 +407,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 bool _M_disjunct(const _CharT* __s) const _GLIBCXX_NOEXCEPT { - return (less<const _CharT*>()(__s, _M_data()) + return (__builtin_constant_p (__s) + || less<const _CharT*>()(__s, _M_data()) || less<const _CharT*>()(_M_data() + this->size(), __s)); } but oddly we folds the __builtin_constant_p to false early but we treat __builtin_constant_p ("foo") as true. That's inconsistent in fold_builtin_constant_p: /* If this expression has side effects, show we don't know it to be a constant. Likewise if it's a pointer or aggregate type since in those case we only want literals, since those are only optimized when generating RTL, not later. And finally, if we are compiling an initializer, not code, we need to return a definite result now; there's not going to be any more optimization done. */ if (TREE_SIDE_EFFECTS (arg) || AGGREGATE_TYPE_P (TREE_TYPE (arg)) || POINTER_TYPE_P (TREE_TYPE (arg)) || cfun == 0 || folding_initializer || force_folding_builtin_constant_p) return integer_zero_node; fixing that (removing POINTER_TYPE_P) makes the workaround work (and the generated code simplifies greatly).
