Re: [PATCH] c-family, middle-end: Add __builtin_c[lt]zg (arg, 0ULL) exception

[Jakub Jelinek <jakub@redhat.com>]

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.

> > 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?

> 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