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

[Sebastian Huber <sebastian.huber@embedded-brains.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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