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From: "hubicka at ucw dot cz" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98925] Extend modref to handle return slot
 optimization
Date: Mon, 01 Feb 2021 19:11:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98925

--- Comment #1 from Jan Hubicka <hubicka at ucw dot cz> ---
> If I don't overstimate modref and alias analysis   destructor should disa=
ppear
> completely in the example below (from https://gcc.gnu.org/PR98499#c4):
>=20
> struct string {
>   char * _M_buf;
>   // local store
>   char _M_local_buf[16];
>=20
>   __attribute__((noinline)) string() : _M_buf(_M_local_buf) {}
>=20
>   ~string() {
>     if (_M_buf !=3D _M_local_buf)
>       __builtin_trap();
>   }
>=20
>   string(const string &__str); // no copies
> };

To see that _M_buf =3D=3D _M_local_buf at the destruction time would require
tracking the aggregate alue from the inlinined constructor to destructor
which is something fitting to ipa-prop.

Modref can helip in case _M_buf is initialized to NULL.  In that case
the address of return value would be non-escaping and we could optimize
the if condition to true (is we did incorrectly before).

I have WIP patch for this and will discuss with Martin Jambor the jump
functions for return values.

Thanks!
Honza=