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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100366] spurious warning - std::vector::clear
 followed by std::vector::insert(vec.end(), ...) with -O2
Date: Wed, 05 May 2021 10:55:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100366

--- 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 =3D PHI <_6(4), _7(3)>

This is always _6, because in bb 3 we have _6 =3D=3D _7.

>   pretmp_67 =3D vec_2(D)->D.33449._M_impl.D.32762._M_start;
>   _69 =3D prephitmp_42 - pretmp_67;

Always 0.

>   <bb 7> [local count: 220460391]:
>   MEM <unsigned int> [(char * {ref-all})_155] =3D pretmp_72;
>   _50 =3D vec_2(D)->D.33449._M_impl.D.32762._M_finish;
>   _Num_51 =3D _50 - prephitmp_42;

Always 0, in bb 4 we copy _M_start in _M_finish if they are not already equ=
al.

(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.=