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From: "rguenther at suse dot de" <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 08:11:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110035

--- Comment #10 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 5 Jun 2023, ptk.prasertsuk at gmail dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110035
>=20
> --- Comment #9 from Pontakorn Prasertsuk <ptk.prasertsuk at gmail dot com=
> ---
> (In reply to Richard Biener from comment #8)
> > (In reply to Pontakorn Prasertsuk from comment #7)
> > > For the LLVM IR code of the snippet I provided, Clang's alias analysi=
s can
> > > prove that `new` call has no side effect to other memory location. Th=
is is
> > > indicated by `noalias` keyword at the return value of the `new` call =
(_Znwm).
> > >=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 p=
ointer
> > > to allocated storage disjoint from the storage for any other object
> > > accessible to the caller."
> > >=20
> > > Is this possible for GCC alias analysis pass?
> >=20
> > >   MyClass c =3D a;
> > >   MyClass *b =3D new MyClass;
> > >   *b =3D c;
> >=20
> > the point is that 'new' can alter the value of 'a', GCC already knows t=
hat
> > 'b' is distinct from c and a but that's not the relevant thing.  It loo=
ks
> > like LLVM creates wrong-code here.
>=20
> In what case can 'new' alter 'a'? I thought memory allocation functions s=
uch as
> 'malloc, 'calloc' and 'new' cannot alias other memory locations than its =
return
> value.

'new' can be overridden by the user, you can declare your own=20
implementation that does fancy stuff behind the scenes, including
in the above case altering 'a'.  Welcome to C++ ...=