Re: stdc_bit_ceil(3) and wrapping

["Yann Droneaud" <ydroneaud@opteya.com>]

Hi,

30 décembre 2022 à 20:55 "Alejandro Colomar via Libc-alpha" <libc-alpha@sourceware.org> a écrit:
> 
> I'm implementing a small part of <stdbit.h> equivalent code for shadow. I need 
> stdc_bit_ceilul() for a random number generator limited to a range (you've seen 
> some of this in the glibc mailing list.
> 
> $ grepc -tfd shadow_random_uniform
> ./libmisc/random.c:76:
> unsigned long
> shadow_random_uniform(unsigned long upper_bound)
> {
>  unsigned long r;
> 
>  do {
>  r = shadow_random();
>  r &= bit_ceil_wrapul(upper_bound) - 1; // optimization
>  } while (r > upper_bound - 1);
> 
>  return r;
> }
> 

What's wrong with the following ?

    if (upper_bound < 2)
        return 0;

    unsigned long max = upper_bound - 1;
    unsigned long mask = ULONG_MAX >> __builtin_clzl(max);

    do {
        r = shadow_random();
        r &= mask;
    } while (r > max);

    return r;

> However, I need that it doesn't have undefined behavior if it doesn't fit the 
> type, but rather that it wraps around (as the simplest implementation would do, 
> BTW). I've done the following:
> 
> $ cat lib/bit.h
> #include <limits.h>
> 
> inline int leading_zerosul(unsigned long x);
> inline int bit_widthul(unsigned long x);
> inline int bit_ceil_wrapul(unsigned long x);
> 
> inline int
> leading_zerosul(unsigned long x)
> {
>  return (x == 0) ? ULONG_WIDTH : __builtin_clz(x);
> }
> 
> inline int
> bit_widthul(unsigned long x)
> {
>  return ULONG_WIDTH - leading_zerosul(x);
> }
> 
> /* Similar to stdc_bit_ceilul(), but wrap around instead of UB. */
> inline int
> bit_ceil_wrapul(unsigned long x)
> {
>  return 1 << bit_widthul(x - 1);
> }
> 
> I was wondering if there was any reason to make that UB in the standard, when 
> unsigned wrapping has always been well-defined, and this is a case that is 
> likely to be implemented with some operation that wraps around, right? I can't 
> imagine of an implementation that invokes UB. Moreover, as you can see, it is 
> useful to make it wrap around in a defined way.
> 
> Would you consider either or both of being more generous in the GNU 
> implementation and guarantee wrap around, and/or suggest that the standard 
> guarantees the wrap around?
> 
> And BTW, if any of this code helps you implement that for GNU, please feel free 
> to take it. :)
> 

-- 
Yann Droneaud
OPTEYA