Re: [RFC] Add stdckdint.h header for C23

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Jun 13, 2023 at 03:10:48PM +0000, Joseph Myers wrote:
> > So why does C2X say
> > Recommended practice
> > It is recommended to produce a diagnostic message if type2 or type3 are
> > not suitable integer types, or if *result is not a modifiable lvalue of
> > a suitable integer type.
> > ?
> > Or is it meant that a suitable integer type doesn't need to be necessarily
> > one that is listed in the previous paragraph?
> > Perhaps the checking could be guarded on #ifdef __STRICT_ANSI__, sure...
> 
> Diagnostics are better than doing something completely random - but making 
> it conditional when there are sensible semantics also makes sense.
> 
> It seems likely a future standard version will support these operations 
> for bit-precise types, at least.  (Bit-precise types are generally tricky 
> for type-generic operations; there's no standard way to select on them 
> with _Generic beyond listing individual types with specific widths, and no 
> standard way to determine the width of the bit-precise type of an 
> argument.  So implementing some type-generic operations for such types may 
> need new language extensions, prompting WG14 caution about requiring such 
> support - but this also makes support for such types in standard 
> type-generic macros in headers particularly valuable, precisely because 
> they can't be implemented purely in user code using standard language 
> features.)

Yeah, having say __builtin_{clz,ctz,ffs,popcount,parity} variants which would
be typegeneric and would allow say any normal integral or _BitInt type
(or just unsigned versions thereof?) would be useful.  Even without _BitInt
we have the problem that we don't have builtins for __uint128_t.

One question is if we should keep them UB on zero input or hardcode some particular
behavior for clz/ctz.  The backend defaults might not be appropriate, I
think if we'd make it non-UB, using the precision of the type would be
reasonable, whatever it is (__builtin_clzb ((unsigned _BitInt(126)) 0)
might be 126 etc.).

	Jakub