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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110035] Missed optimization for dependent
 assignment statements
Date: Mon, 05 Jun 2023 07:16:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110035

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Pontakorn Prasertsuk from comment #7)
> For the LLVM IR code of the snippet I provided, Clang's alias analysis can
> prove that `new` call has no side effect to other memory location. This is
> indicated by `noalias` keyword at the return value of the `new` call (_Zn=
wm).
>=20
> According to Clang's Language Reference:
> "On function return values, the noalias attribute indicates that the
> function acts like a system memory allocation function, returning a point=
er
> to allocated storage disjoint from the storage for any other object
> accessible to the caller."
>=20
> Is this possible for GCC alias analysis pass?

>   MyClass c =3D a;
>   MyClass *b =3D new MyClass;
>   *b =3D c;

the point is that 'new' can alter the value of 'a', GCC already knows that
'b' is distinct from c and a but that's not the relevant thing.  It looks
like LLVM creates wrong-code here.=