On Fri, Apr 4, 2025 at 10:09 AM Martin Uecker <uec...@tugraz.at> wrote:
>
> Am Freitag, dem 04.04.2025 um 18:51 +0200 schrieb Michael Matz:
> > Hello,
> >
> > On Fri, 4 Apr 2025, Qing Zhao wrote:
> >
> > > So, a different attribute name “counted_by_exp” might be better?
> >
> > I would prefer Martins empty-decl idea to that: "counted_by(;len+0)"
> > (looks up 'len' normally, i.e. doesn't look into current struct). It
> > would naturally fit the either decl+expr or lone-ident parse.
> > It may look weird but empty declarations are okayish IMHO.
> >
> > But overall: I just don't know, it all looks a bit unsexy, there only seem
> > to be rocks and hard places :)
>
> I would not worry about this case too much, because I do expect this
> to be a common use case anyway. That it looks strange may even be
> an advantage here, as it alerts the reader that this is unusual.
>
I need to jump in here with a quick note. Once we get a reasonable
proposal that works for GCC, I need to write an RFC for the Clang
community too. So anything we decide here still needs to go through
that process before we implement anything permanent.
-bw
> Martin
>
> >
> >
> > Ciao,
> > Michael.
> >
> > >
> > > Qing
> > >
> > > > On Apr 4, 2025, at 11:55, Michael Matz <m...@suse.de> wrote:
> > > >
> > > > Hello,
> > > >
> > > > On Fri, 4 Apr 2025, Qing Zhao wrote:
> > > >
> > > > > A:
> > > > > constexpr int len = 20;
> > > > > struct s {
> > > > > int len;
> > > > > int *buf __attribute__ ((counted_by (len))); // this continues to be
> > > > > member ‘len’, not global ‘len'
> > > > > };
> > > > >
> > > > > B:
> > > > > constexpr int len = 20;
> > > > > struct s {
> > > > > int len;
> > > > > int *buf __attribute__ ((counted_by (len+0))); //this is an
> > > > > expression , ‘len' refers to the global;
> > > > > };
> > > > >
> > > > > When the parser is parsing the first identifier “len” inside the
> > > > > counted_by attribute, it cannot decide which syntax to use yet, it
> > > > > must
> > > > > look ahead at least one more word to decide, is this okay for the
> > > > > current C parser?
> > > >
> > > > As I understood Bills proposal (but see below) a full expression that
> > > > isn't a lone identifier would always require the decl+expression syntax,
> > > > so the above would lead to a syntax error and wouldn't require further
> > > > look-ahead. ('len' doesn't introduce a type, hence it can't be
> > > > decl+expr,
> > > > hence it must be lone-ident, which then generates the syntax error on
> > > > seeing '+', after having successfully looked up 'len' among
> > > > the struct members).
> > > >
> > > > But I now realize that I may have misunderstood the proposal in the cace
> > > > that the expression does not in fact contain references to any struct
> > > > members, e.g.
> > > >
> > > > enum {FOO=42};
> > > > struct s {
> > > > int len;
> > > > int *buf __attribute__((counted_by( /*???*/ FOO + 0))); // no use of
> > > > len
> > > > };
> > > >
> > > > The proposal doesn't specifically talk about this case. Clearly there
> > > > is
> > > > no need to locally declare anything (at ???), but that would have been
> > > > the
> > > > syntactic hint to differentiate between both branches in the proposal.
> > > > So, ... hmm, that would seem to again introduce the ambiguity between
> > > > 'lone-ident' and 'expression'. I'm not sure how Bill wants to handle
> > > > that. One could requre a useless dummy declaration, but that would be
> > > > meh.
> > > >
> > > >
> > > > Ciao,
> > > > Michael.
> > >
> > >
>
