Hi Qing, On Tue, Aug 06, 2024 at 08:15:50PM GMT, Qing Zhao wrote: > Some comments on the documentation part. > > (Hopefully, this time my quoting format is good, I checked the email > sent back to myself, no formatting issue, but when I checked the emails > in the archive, > https://gcc.gnu.org/pipermail/gcc-patches/2024-August/659593.html, > yst, I see the quoting format issue you mentioned, adjusted my mail client > setting, > hopefully this time it’s good)
Yup; it's good now. Thanks! ;)
> > On Aug 6, 2024, at 08:22, Alejandro Colomar <a...@kernel.org> wrote:
> >
> > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > index 0b572afca72..6e1d302150d 100644
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -10391,6 +10391,33 @@ If the operand of the @code{__alignof__}
> > expression is a function,
> > the expression evaluates to the alignment of the function which may
> > be specified by attribute @code{aligned} (@pxref{Common Function
> > Attributes}).
> >
> > +@node Length
> > +@section Determining the Length of Arrays
> > +@cindex lengthof
> > +@cindex length
> > +@cindex array length
> > +
> > +The keyword @code{__lengthof__} determines the length of an array operand,
> > +that is, the number of elements in the array.
> > +Its syntax is just like @code{sizeof}.
> > +The operand must be a complete array type.
>
> What’s the behavior when the array operand is a VLA,
The number of elements in the array is returned.
The value is a run-time value; not a constant expression.
The behavior is identical to sizeof(a)/sizeof(*a), except that obviously
the operand is only written once, so it's only evaluated once.
size_t n = arc4random();
assert(n == __lengthof__(int [n]));
I think this is covered by
+The keyword @code{__lengthof__} determines the length of an array operand,
+that is, the number of elements in the array.
> or a flexible array member?
A flexible array member is an incomplete type, and so it is a constraint
violation. It results in a compile-time error.
I think this is covered by
+The operand must be a complete array type.
> Should these be clarified in the documentation part?
I don't know. I think it's already documented. I prefer keeping it
concise, as long as it's not ambiguous.
> > +The operand is not evaluated
> > +if the top-level length designator is an integer constant expression;
> > +and it is evaluated
> > +if the top-level length designator is not an integer constant expression.
>
> Can you add small examples to clarify the above?
The examples would be rather confusing.
__lengthof__(int [4][n]); // constant expression
__lengthof__(int [n][4]); // run-time value
I think these would be interesting in the docs. I'll check what would
be the appropriate formatting for these. Thanks!
> > +XXX: Do we want to document the following? I think so.
> > +XXX: It would prevent users from relying on __lengthof__
> > +XXX: for distinguishing arrays from pointers.
> > +XXX: I don't want users to complain in the future
> > +XXX: if this doesn't report errors on function parameters anymore
> > +XXX: and that breaks their assumptions.
> > +In the future,
> > +it might also accept a function parameter with array notation,
> > +an incomplete array whose length is specified by other means,
> > +such as attributes,
> > +or other similar cases.
>
> For incomplete array (for example, flexible array members), as I
> mentioned in another email:
> https://gcc.gnu.org/pipermail/gcc-patches/2024-August/659478.html
>
> I think that it’s better to wait the Length information is finally
> integrated into TYPE.
Yes, I agree for a start. But I also don't want to allow any user to
rely on the fact that __lengthof__ rejects pointers to determine that
something is not a pointer, because we reserve the possibility of
extending the operator to work on certain pointers. (I'm not saying
we'll do it; but we reserve the right.)
Maybe we can just trust that nobody will do that, and complain if we see
anyone using it in that way.
I think I'll just remove that paragraph.
Have a lovely night!
Alex
>
> thanks.
>
> Qing
--
<https://www.alejandro-colomar.es/>
signature.asc
Description: PGP signature
