From: Sebastian Huber <sebastian.huber@embedded-brains.de>
To: newlib@sourceware.org
Subject: Re: [PATCH] Workaround for ffs() on LP64 targets
Date: Thu, 27 Jul 2017 12:34:00 -0000 [thread overview]
Message-ID: <fe8c56d5-c9a8-d3c5-cd65-8d9340c9e729@embedded-brains.de> (raw)
In-Reply-To: <20170727122734.GA27695@calimero.vinschen.de>
On 27/07/17 14:27, Corinna Vinschen wrote:
> On Jul 27 13:24, 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<sebastian.huber@embedded-brains.de>
>>>> ---
>>>> 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.
>> This is roughly the same implementation we had before. I do not intend to
>> optimize this.
> Still, __LP64__ is unacceptable. Cygwin would be affected by this as
> well and would have to revert to its former own ffs implementation.
>
> Reverting to a C-based implementation should only be performed on a
> case-by-case basis.
Yes, so maybe something like this
#if defined(__LP64__) && defined(__riscv)
or a target-specific ffs.c file similar to memcpy.c, etc.
--
Sebastian Huber, embedded brains GmbH
Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone : +49 89 189 47 41-16
Fax : +49 89 189 47 41-09
E-Mail : sebastian.huber@embedded-brains.de
PGP : Public key available on request.
Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
next prev parent reply other threads:[~2017-07-27 12:34 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 [this message]
2017-07-27 12:49 ` Corinna Vinschen
2017-07-27 12:53 ` Sebastian Huber
2017-07-27 21:03 ` Brian Inglis
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