From: "Yann Droneaud" <ydroneaud@opteya.com>
To: "Alejandro Colomar" <alx.manpages@gmail.com>,
"Joseph Myers" <joseph@codesourcery.com>
Cc: gcc@gcc.gnu.org, "GNU C Library" <libc-alpha@sourceware.org>
Subject: Re: stdc_bit_ceil(3) and wrapping
Date: Fri, 30 Dec 2022 20:18:10 +0000 [thread overview]
Message-ID: <ab35dac8a309f8a0f358cd9e3df382d760d5b125@opteya.com> (raw)
In-Reply-To: <d6a6bcc1-2b65-8449-9379-1990a0840c58@gmail.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
next prev parent reply other threads:[~2022-12-30 20:18 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-30 19:53 Alejandro Colomar
2022-12-30 20:18 ` Yann Droneaud [this message]
2022-12-30 20:33 ` Alejandro Colomar
2022-12-30 22:33 ` Alejandro Colomar
2023-01-06 20:08 ` Alejandro Colomar
2022-12-30 20:47 ` Alejandro Colomar
2022-12-30 20:38 ` Joseph Myers
2022-12-30 20:46 ` Alejandro Colomar
2022-12-30 20:56 ` Joseph Myers
2022-12-30 21:01 ` Alejandro Colomar
2022-12-30 21:06 ` Alejandro Colomar
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