Re: [PATCH v4] c++: Implement -Wself-move warning [PR81159]

[Marek Polacek <polacek@redhat.com>]

On Thu, Aug 25, 2022 at 09:25:43AM -0400, Jason Merrill wrote:
> On 8/24/22 17:30, Marek Polacek wrote:
> > On Tue, Aug 23, 2022 at 05:27:00PM -0400, Jason Merrill wrote:
> > > On 8/23/22 09:39, Marek Polacek wrote:
> > > > +  tree arg = CALL_EXPR_ARG (fn, 0);
> > > > +  extract_op (arg);
> > > > +  if (TREE_CODE (arg) == ADDR_EXPR)
> > > > +    arg = TREE_OPERAND (arg, 0);
> > > > +  tree type = TREE_TYPE (lhs);
> > > > +  lhs = maybe_undo_parenthesized_ref (lhs);
> > > > +  STRIP_ANY_LOCATION_WRAPPER (lhs);
> > > > +  const bool print_var_p = (DECL_P (lhs)
> > > > +			    || REFERENCE_REF_P (lhs)
> > > > +			    || TREE_CODE (lhs) == COMPONENT_REF);
> > > 
> > > Why include REFERENCE_REF_P and COMPONENT_REF?  Reference refs should be
> > > stripped before this test, member refs aren't variables.
> > 
> > I'm checking REFERENCE_REF_P and COMPONENT_REF to say "moving a variable"
> > in #1 and #3.  The REFERENCE_REF_P check means that we also say "variable"
> > for #2.  Sure, "A variable is introduced by the declaration of a reference
> > other than a non-static data member", but I'm not sure if users care about
> > that here?
> > 
> > If I strip REFERENCE_REFs before the check then the result will be the
> > same.
> 
> That's what I was suggesting, yes: Strip the REFERENCE_REF so DECL_P can see
> the decl.

Ok, I've added the REFERENCE_REF stripping.  But I've still left the
COMPONENT_REF in.  Perhaps we could say "moving a member" to itself for
COMPONENT_REFs.  Or just say "moving 'x' of type 'int' to itself" and
avoid all of this.  :)

> I don't see where COMPONENT_REF comes in?

For #1 in the test below the COMPONENT_REF was created in finish_id_expression
-> finish_non_static_data_member -> build_class_member_access_expr and passed
down to maybe_warn_self_move from here:

#0  maybe_warn_self_move (loc=2147483652, lhs=<component_ref 0x7fffea380e10>,
    rhs=<indirect_ref 0x7fffea38a220>) at /home/mpolacek/src/gcc/gcc/cp/typeck.cc:8908
#1  0x0000000000f3d03e in cp_build_modify_expr (loc=2147483652, lhs=<component_ref 0x7fffea380e10>,
    modifycode=NOP_EXPR, rhs=<indirect_ref 0x7fffea38a220>, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck.cc:9161
#2  0x0000000000f3e461 in build_x_modify_expr (loc=2147483652, lhs=<component_ref 0x7fffea380e10>,
    modifycode=NOP_EXPR, rhs=<indirect_ref 0x7fffea38a220>, lookups=<tree 0x0>, complain=3)
    at /home/mpolacek/src/gcc/gcc/cp/typeck.cc:9446
#3  0x0000000000d92d4e in cp_parser_assignment_expression (parser=0x7fffea236850, pidk=0x0, cast_p=false,
    decltype_p=false) at /home/mpolacek/src/gcc/gcc/cp/parser.cc:10461
 
> > Or I could keep only the DECL_P check, but then we'll say "moving
> > an expression" for #1 and #2, which seems strange.
> > 
> > struct S {
> >    int x;
> >    int &r;
> >    void foo () {
> >      x = std::move (x); // #1
> >      r = std::move (r); // #2
> >    };
> > };
> > 
> > void
> > foo (int &r)
> > {
> >    r = std::move (r); // #3
> > }

Marek