Re: [PATCH] avoid warning on constant strncpy until next statement is reachable (PR 87028)

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Aug 29, 2018 at 2:12 AM Martin Sebor <msebor@gmail.com> wrote:
>
> >> Sadly, dstbase is the PARM_DECL for d.  That's where things are going
> >> "wrong".  Not sure why you're getting the PARM_DECL in that case.  I'd
> >> debug get_addr_base_and_unit_offset to understand what's going on.
> >> Essentially you're getting different results of
> >> get_addr_base_and_unit_offset in a case where they arguably should be
> >> the same.
> >
> > Probably get_attr_nonstring_decl has the same "mistake" and returns
> > the PARM_DECL instead of the SSA name pointer.  So we're comparing
> > apples and oranges here.
>
> Returning the SSA_NAME_VAR from get_attr_nonstring_decl() is
> intentional but the function need not (perhaps should not)
> also set *REF to it.
>
> >
> > Yeah:
> >
> > /* If EXPR refers to a character array or pointer declared attribute
> >    nonstring return a decl for that array or pointer and set *REF to
> >    the referenced enclosing object or pointer.  Otherwise returns
> >    null.  */
> >
> > tree
> > get_attr_nonstring_decl (tree expr, tree *ref)
> > {
> >   tree decl = expr;
> >   if (TREE_CODE (decl) == SSA_NAME)
> >     {
> >       gimple *def = SSA_NAME_DEF_STMT (decl);
> >
> >       if (is_gimple_assign (def))
> >         {
> >           tree_code code = gimple_assign_rhs_code (def);
> >           if (code == ADDR_EXPR
> >               || code == COMPONENT_REF
> >               || code == VAR_DECL)
> >             decl = gimple_assign_rhs1 (def);
> >         }
> >       else if (tree var = SSA_NAME_VAR (decl))
> >         decl = var;
> >     }
> >
> >   if (TREE_CODE (decl) == ADDR_EXPR)
> >     decl = TREE_OPERAND (decl, 0);
> >
> >   if (ref)
> >     *ref = decl;
> >
> > I see a lot of "magic" here again in the attempt to "propagate"
> > a nonstring attribute.
>
> That's the function's purpose: to look for the attribute.  Is
> there a better way to do this?

Well, the question is what "nonstring" is, semantically.  I read it
as sth like __restrinct - a pointer with "nonstring" attribute points
to a non-string.  So I suspect your function either computes
"may expr point to a nonstring" or "must expr point to a nonstring"
if it gets a pointer argument.  If it gets a (string) object it checks whether
that object is declared "nonstring" (thus, if you'd built a pointer to expr
whether that pointer _must_ point to a nonstring.  So I guess the first
one is "must".  Clearly looking at SSA_NAME_VAR isn't good here,
it would be semantically correct only for SSA_NAME_IS_DEFAULT_DEF
and SSA_NAME_VAR being a PARM_DECL.

I guess it would be nice to clearly separate the pointer vs. object case
by documentation in the function - all of the quoted parts above seem
to be for the address case so a gcc_assert (POINTER_TYPE_P (TREE_TYPE (decl))
inside the if (TREE_CODE (decl) == SSA_NAME) path should never trigger?

> > Note
> >
> > foo (char *p __attribute__(("nonstring")))
> > {
> >   p = "bar";
> >   strlen (p); // or whatever is necessary to call get_attr_nonstring_decl
> > }
> >
> > is perfectly valid and p as passed to strlen is _not_ nonstring(?).
>
> I don't know if you're saying that it should get a warning or
> shouldn't.  Right now it doesn't because the strlen() call is
> folded before we check for nonstring.

I say it shouldn't because I assign "bar" to p and after that p isn't
the original parameter anymore?

> I could see an argument for diagnosing it but I suspect you
> wouldn't like it because it would mean more warning from
> the folder.  I could also see an argument against it because,
> as you said, it's safe.
>
> If you take the assignment to p away then a warning is issued,
> and that's because p is declared with attribute nonstring.
> That's also why get_attr_nonstring_decl looks at SSA_NAME_VAR.
>
> > I think in your code comparing bases you want to look at the _original_
> > argument to the string function rather than what get_attr_nonstring_decl
> > returned as ref.
>
> I've adjusted get_attr_nonstring_decl() to avoid setting *REF
> to SSA_NAME_VAR.  That let me remove the GIMPLE_NOP code from
> the patch.  I've also updated the comment above SSA_NAME_VAR
> to clarify its purpose per Jeff's comments.
>
> Attached is an updated revision with these changes.
>
> Martin