Hi,
30 décembre 2022 à 20:55 "Alejandro Colomar via Libc-alpha"
<libc-al...@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
