Re: [PATCH] Workaround for ffs() on LP64 targets

[Sebastian Huber <sebastian dot huber at embedded-brains dot de>]

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.

--
Sebastian Huber, embedded brains GmbH

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