Re: [PATCH] c: Add __builtin_stdc_bit_{width,floor,ceil} builtins

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Nov 20, 2023 at 02:55:33PM +0000, Joseph Myers wrote:
> On Sat, 18 Nov 2023, Jakub Jelinek wrote:
> 
> > +@defbuiltin{@var{type} __builtin_stdc_bit_ceil (@var{type} @var{arg})}
> > +The @code{__builtin_stdc_bit_ceil} function is available only
> > +in C.  It is type-generic, the argument can be any unsigned integer
> > +(standard, extended or bit-precise).  No integral argument promotions are
> > +performed on the argument.  It is equivalent to
> > +@code{@var{arg} <= 1 ? (@var{type}) 1
> > +: (@var{type}) 1 << (@var{prec} - __builtin_clzg ((@var{type}) (@var{arg} - 1)))}
> > +where @var{prec} is bit width of @var{type}, except that side-effects
> > +in @var{arg} are evaluated just once.
> > +@enddefbuiltin
> 
> Note that stdc_bit_ceil now has defined behavior (return 0) on overflow: 
> CD2 comment FR-135 was accepted for the DIS at the June WG14 meeting.  
> This affects both the documentation and the implementation, as they need 
> to avoid an undefined shift by the width of the type.  That's why my 
> stdbit.h implementations have two shifts (not claiming that's necessarily 
> the optimal way of ensuring the correct result in the overflow case).
> 
>   return __x <= 1 ? 1 : ((uint64_t) 1) << (__bw64_inline (__x - 1) - 1) << 1;

So
  return __x <= 1 ? 1 : ((uint64_t) 2) << (__bw64_inline (__x - 1) - 1);
then?

	Jakub