[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

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107561

--- 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 the
> > info.
> > 
> > Maybe we can special-case directly the actual parameter case and compute
> > the restrictness info for the call arguments.  The canonical example is
> > 
> > void bar (void);
> > struct X {
> >   X (int);
> >   int i;
> >   int j;
> > };
> > 
> > X::X(int k)
> > {
> >   i = k;
> >   bar ();
> >   j = i != k;
> > }
> > 
> > where if I understand you correctly, bar () is not allowed to modify *this
> > (unless I pass it an argument to it, of course), even if *this is for
> > example
> 
> 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 would
> preclude such modifications.

The canonical C example would be

void bar (void);

void foo (struct X * restrict this, int k)
{
  this->i = k;
  bar ();
  this->j = i != 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 = { D.2806 } (nonlocal, restrict)
  struct X * const this_5(D) = this;
  int k_7(D) = k;
  int _1;
  bool _2;
  int _3;

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