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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/107561] [13 Regression] g++.dg/pr71488.C  and
 [g++.dg/warn/Warray-bounds-16.C -m32] regression due to -Wstringop-overflow
 problem
Date: Wed, 01 Mar 2023 14:38:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107561

--- Comment #17 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #16)
> (In reply to Richard Biener from comment #15)
> > The compiler doesn't know that the allocation function cannot clobber *=
this.
> > The C++ frontend tries to communicate this by making 'this' restrict
> > qualified
> > and we make use of that info, but for calls we do not know how to use t=
he
> > info.
> >=20
> > Maybe we can special-case directly the actual parameter case and compute
> > the restrictness info for the call arguments.  The canonical example is
> >=20
> > void bar (void);
> > struct X {
> >   X (int);
> >   int i;
> >   int j;
> > };
> >=20
> > X::X(int k)
> > {
> >   i =3D k;
> >   bar ();
> >   j =3D i !=3D k;
> > }
> >=20
> > where if I understand you correctly, bar () is not allowed to modify *t=
his
> > (unless I pass it an argument to it, of course), even if *this is for
> > example
>=20
> Why?  Because it is a constructor and the object isn't fully constructed =
yet
> at that point?  For normal methods I certainly don't see anything that wo=
uld
> preclude such modifications.

The canonical C example would be

void bar (void);

void foo (struct X * restrict this, int k)
{
  this->i =3D k;
  bar ();
  this->j =3D i !=3D k;
}

where as I understand bar () cannot modify what *this points to since it
cannot build a proper derived access from 'this' (unless I pass it to bar).

The C++ frontend annotates 'this' with restrict.  For my example I get

void X::X (struct X * const this, int k)
{
  # PT =3D { D.2806 } (nonlocal, restrict)
  struct X * const this_5(D) =3D this;
  int k_7(D) =3D k;
  int _1;
  bool _2;
  int _3;

  <bb 2> :
  MEM[(struct X *)this_5(D) clique 1 base 1] =3D{v} {CLOBBER};
  MEM[(struct X *)this_5(D) clique 1 base 1].i =3D k_7(D);
  # USE =3D nonlocal escaped
  # CLB =3D nonlocal escaped
  bar ();
  _1 =3D MEM[(struct X *)this_5(D) clique 1 base 1].i;
  _2 =3D _1 !=3D k_7(D);
  # RANGE [irange] int [0, 1] NONZERO 0x1
  _3 =3D (int) _2;
  MEM[(struct X *)this_5(D) clique 1 base 1].j =3D _3;
  return;
}=