On Mon, 20 Nov 2023, Jakub Jelinek wrote:
> On Mon, Nov 20, 2023 at 09:18:57AM +0100, Florian Weimer wrote:
> > * Richard Biener:
> >
> > > Ugh. First of all I don't like that the exception is applied during
> > > folding. As for the problem of multi evaluation can't consumers use
> > > stmt expressions for this, say
> > >
> > > {( auto __tem = value; __builtin_xyz (__tem, __typeof (__tem)); ... )}
> > >
> > > ? Thus use 'auto' to avoid spelling 'value' multiple times?
> >
> > {( ? )} cannot be used in a constant expression, but the new macros are
> > supposed to be usable there.
>
> 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).
> 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.
And of course nobody would write
const int x = sizeof (stdc_first_leading_one (5));
that's just stupid ... (but oh well).
Richard.
