Re: Draft C2Y _BitInt related proposals

On Thu, Jul 09, 2026 at 08:25:12PM +0200, Martin Uecker wrote:
> Am Dienstag, dem 07.07.2026 um 16:47 +0200 schrieb Jakub Jelinek:
> > Hi!
> > 
> > As I wrote earlier, I'd like to see _BitInt I/O support in
> > *printf*/*scanf* and extending support of _BitInt in stdbit.h
> > in C2Y.
> > 
> > For those I wrote drafts of possible C2Y papers:
> > https://jakubjelinek.github.io/wg14/va_arg_bitint.html
> > https://jakubjelinek.github.io/wg14/stdbit_bitint.html
> > 
> > I'd appreciate any comments on this.
> > Haven't tried to acquire paper numbers for these yet.
> > 
> > 	Jakub
> 
> Looks already good to me. I noticed some minor editorial
> issues:
> 
> In the paragraph before proposal there is some closing
> tag missing for <code> after _BitInt.
> 
> In "Possible variants" there is a closing parenthesis
> missing in the last sentence and in the proposed wording 
> 
> 7.16.2.3 The va_arg_bitint macro
> 
> there is a closing parenthesis missing in the description
> after _BitInt(N)

Thanks, hopefully fixed now.

> (Otherwise I still think that allowing a variably-modified
> _BitInt in va_arg would be more elegant, because we
> might also want _BitInt(*) for matching in _Generic)

_BitInt(*) for _Generic looks fine.

Accepting _BitInt(n) for va_arg second argument is more problematic.
va_arg expression has the type specified by the second argument,
so either _BitInt(n) where n is non-constant would need to be
allowed everywhere where _BitInt(N) with constant N is allowed, but
if it is also supposed to be used for the scanf case, i.e.
*va_arg (ap, _BitInt(n) *) = something;, it would need to be treated
not just as some simple VLA, but stores to it would need to be
again
if (n <= 8) *(_BitInt(8) [[gnu::noalias]] *) addr = something;
else if (n <= 16) *(_BitInt(16) [[gnu::noalias]] *) addr = something;
else if (n <= 32) *(_BitInt(32) [[gnu::noalias]] *) addr = something;
else if (n <= 64) *(_BitInt(64) [[gnu::noalias]] *) addr = something;
...
else actual VLA-ish store;
and loads would need to be treated similarly.
Or, if _BitInt(n) would be only allowed in va_arg argument and
there would be some special case, say that va_arg in that case returns
_BitInt(BITINT_MAXWIDTH), then va_arg would work, although very
inefficiently if n is significantly smaller than BITINT_MAXWIDTH, and
there would be major other problems (GCC vs. Clang. vs other compilers
having different BITINT_MAXWIDTH, so C library would need to be
implemented using the compiler with the largest supported one in order
not to truncate bits, and the next decade or so transition problem
of being able to compile C library with older compiler), but how do
you handle the scanf case then?

Implementation-wise, supporting the VLA _BitInt everywhere would be a
nightmare, in GCC the _BitInt lowering is around 300KiB of code and
having to deal with VLAs could significantly grow that.

	Jakub