(corrected address)
> On Mon, 20 Nov 2023, Jakub Jelinek wrote:
>
> > On Mon, Nov 20, 2023 at 08:37:55AM +0000, Richard Biener wrote:
> > > > I'm not sure about that, it would be nice for them to be usable there,
> > >
> > > Btw, I think that {( .. )} should be made usable in sizeof () and
> > > possibly even in at least C++ constant expressions (not sure about C).
> >
> > I believe the problkem is having new VAR_DECLs in those which actually
> > aren't file scope/namespace scope variables but there is no function
> > DECL_CONTEXT to attach to them. So, it probably wouldn't be one afternoon
> > change to allow that.
There is an open bug about this:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93239
But the required feature is simpler than full statementÂ
expression, essentially
let x = y in z
where x is an identifier and y and z are expression, and
this should be much easier to implement.
I thought about an extension to _Generic which would be
useful here:
_GNU_Generic(y, int x1: z1, float x2: z2)
or even
_GNU_Generic(y, default x: z)
which would be useful in general.
> >
> > > > but I think e.g. none of Joseph's implementation of those macros
> > > > made them usable there (except inside of sizeof/typeof/typeof_unquall)
> > > > and I don't see a requirement in the C23 standard that they must be
> > > > usable
> > > > in constant expressions.
> > > > The versions I've posted on Thursday were usable there except for
> > > > stdc_has_single_bit (but that actually can be implemented that way too)
> > > > and stdc_bit_floor. And the version I haven't posted that used the 3
> > > > patches posted on Saturday would have all functions usable when the
> > > > argument to those macros is a constant expression.
> > > >
> > > > BTW, if we go route of implementing all of the stdc_ type-generic macros
> > > > as builtins, we could as well not implement that way the following 4
> > > > # define stdc_first_leading_one(x) (__builtin_clzg (x, -1) + 1U)
> > > > # define stdc_first_trailing_one(x) (__builtin_ctzg (x, -1) + 1U)
> > > > # define stdc_count_ones(x) ((unsigned int) __builtin_popcountg (x))
> > > > # define stdc_has_single_bit(x) ((_Bool) (__builtin_popcountg (x) == 1))
> > > > which are implementable without any new extensions.
> > >
> > > I'd rather do all of those necessary as builtins instead of hacking
> > > around limitations. If we don't want to solve those limitations in
> > > a more generic way.
> >
> > Ok, I can prepare a patch for that, shouldn't be that hard.
> > Do you want all 14, or just the 10 and leave the above 4 with the
> > above definitions?
>
> I'd say all of them for consistency, we can parse/gimplify them to
> the open-coded variants then.
For use of _Generic with _BitInt one would need some kind
of _BitInt_Width(x) macro/builtin that returns the width as an
constant expressions, which would also be useful in general.
Then one could write:
_Generic(x, int a: foo, _BitInt(_BitInt_Width(x)): bar);
With this and an extension as suggested above, I think one could
solve this in a generic way.
Martin
> > > And of course nobody would write
> > >
> > > const int x = sizeof (stdc_first_leading_one (5));
> > >
> > > that's just stupid ... (but oh well).
> >
> > Well, standard testsuite needs to include that at least.
> > But of course, if it is usable in constant expressions,
> > unsigned a = stdc_bit_width ((unsigned _BitInt(824))
> > 435987349856735489657489657468954768954674589674598uwb *
> > 49876558967549867548967548967548967549867548967456uwb);
> > etc. can be useful in constant expressions.
> >
> > Jakub
> >
> >
