From: Brian Inglis <Brian.Inglis@SystematicSw.ab.ca>
To: newlib@sourceware.org
Subject: Re: [PATCH] Workaround for ffs() on LP64 targets
Date: Thu, 27 Jul 2017 21:03:00 -0000 [thread overview]
Message-ID: <52ed828d-dd7b-0327-79ae-051b8187b095@SystematicSw.ab.ca> (raw)
In-Reply-To: <42255ff2-66cf-e021-788b-250d45693374@embedded-brains.de>
On 2017-07-27 05:24, Sebastian Huber wrote:
> On 27/07/17 13:13, Eric Blake wrote:
>> On 07/27/2017 03:06 AM, Sebastian Huber wrote:
>>> Signed-off-by: Sebastian Huber
>>> ---
>>> newlib/libc/misc/ffs.c | 11 +++++++++++
>>> 1 file changed, 11 insertions(+)
>>>
>>> diff --git a/newlib/libc/misc/ffs.c b/newlib/libc/misc/ffs.c
>>> index ba5700920..a09cbd3bb 100644
>>> --- a/newlib/libc/misc/ffs.c
>>> +++ b/newlib/libc/misc/ffs.c
>>> @@ -31,6 +31,17 @@ No supporting OS subroutines are required. */
>>> int
>>> ffs(int i)
>>> {
>>> +#ifdef __LP64__
>>> + /* GCC would expand the __builtin_ffs() to ffs() in this case */
>>> + int bit;
>>> +
>>> + if (i == 0)
>>> + return (0);
>>> + for (bit = 1; !(i & 1); bit++)
>>> + i = (unsigned int)i >> 1;
>>> + return (bit);
>> If we're going to open-code it to work around the compiler creating an
>> infloop recursion to ffs(), at least code a straight-line version
>> without branches, rather than the painfully slow bit-by-bit loop.
>> There's plenty of examples on the web of writing ffs() by using
>> bit-twiddling without branching.
Definitive twiddling reference is now Hacker's Delight 2nd ed, Henry S. Warren,
Jr., 2013, Pearson/InformIT/AW; available in ebook formats:
https://github.com/jyfc/ebook/blob/master/02_algorithm/Hacker's%20Delight%202nd%20Edition.pdf
https://www.safaribooksonline.com/library/view/hackers-delight-second/9780133084993/
https://en.wikipedia.org/wiki/Hacker%27s_Delight
http://www.hackersdelight.org/
> This is roughly the same implementation we had before. I do not intend to
> optimize this.
Programmers using these functions expect the usage cost to be trivial and fairly
constant ~ O(log2(bits)) not O(bits); if not, they may implement their own!
Try this one, seems decently short; adjust for different word sizes; with gcc
-O3 on x86-64 compiles to 32 instructions branch free: YMMV
int
ffsll( long long in )
{
/* find first set == 1 + count trailing zeros */
int index = 64;
if (!in) return 0;
in &= -in; /* clear all but lsb set */
/*
* for ctz remove above test and add next line
* if (in) --index;
*/
if (in & 0x00000000FFFFFFFF) index -= 32;
if (in & 0x0000FFFF0000FFFF) index -= 16;
if (in & 0x00FF00FF00FF00FF) index -= 8;
if (in & 0x0F0F0F0F0F0F0F0F) index -= 4;
if (in & 0x3333333333333333) index -= 2;
if (in & 0x5555555555555555) index -= 1;
return index;
}
--
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
prev parent reply other threads:[~2017-07-27 21:03 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-27 8:06 Sebastian Huber
2017-07-27 8:29 ` Kito Cheng
2017-07-27 8:40 ` Sebastian Huber
2017-07-27 9:01 ` Kito Cheng
2017-07-27 11:13 ` Eric Blake
2017-07-27 11:25 ` Sebastian Huber
2017-07-27 12:27 ` Corinna Vinschen
2017-07-27 12:34 ` Sebastian Huber
2017-07-27 12:49 ` Corinna Vinschen
2017-07-27 12:53 ` Sebastian Huber
2017-07-27 21:03 ` Brian Inglis [this message]
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