From: Corinna Vinschen <vinschen@redhat.com>
To: newlib@sourceware.org
Subject: Re: [PATCH] Workaround for ffs() on LP64 targets
Date: Thu, 27 Jul 2017 12:49:00 -0000 [thread overview]
Message-ID: <20170727124844.GC27695@calimero.vinschen.de> (raw)
In-Reply-To: <fe8c56d5-c9a8-d3c5-cd65-8d9340c9e729@embedded-brains.de>
[-- Attachment #1: Type: text/plain, Size: 2130 bytes --]
On Jul 27 14:33, Sebastian Huber wrote:
> 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.
I'm inclined to favor a target-specific file. This would also allow
to implement the replacement in assembler easily.
Corinna
--
Corinna Vinschen
Cygwin Maintainer
Red Hat
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 819 bytes --]
next prev parent reply other threads:[~2017-07-27 12:49 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 [this message]
2017-07-27 12:53 ` Sebastian Huber
2017-07-27 21:03 ` Brian Inglis
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170727124844.GC27695@calimero.vinschen.de \
--to=vinschen@redhat.com \
--cc=newlib@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).