https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100366
--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #2)
> The IL looks like the warning is justified:
The memcpy call is dead code, we just fail to notice it.
> <bb 13> [local count: 230225493]:
> # prephitmp_42 = PHI <_6(4), _7(3)>
This is always _6, because in bb 3 we have _6 == _7.
> pretmp_67 = vec_2(D)->D.33449._M_impl.D.32762._M_start;
> _69 = prephitmp_42 - pretmp_67;
Always 0.
> <bb 7> [local count: 220460391]:
> MEM <unsigned int> [(char * {ref-all})_155] = pretmp_72;
> _50 = vec_2(D)->D.33449._M_impl.D.32762._M_finish;
> _Num_51 = _50 - prephitmp_42;
Always 0, in bb 4 we copy _M_start in _M_finish if they are not already equal.
(sorry for the wrong FRE comment earlier)
Note that if I replace operator new/delete with malloc/free
inline void* operator new(std::size_t n){return __builtin_malloc(n);}
inline void operator delete(void*p)noexcept{__builtin_free(p);}
inline void operator delete(void*p,std::size_t)noexcept{__builtin_free(p);}
we optimize quite a bit more and the warning disappears.
