* Is __ffs64 supposed to be zero based?
@ 2013-11-06 7:15 Ben Greear
2013-11-06 11:52 ` Clemens Ladisch
0 siblings, 1 reply; 3+ messages in thread
From: Ben Greear @ 2013-11-06 7:15 UTC (permalink / raw)
To: Linux Kernel Mailing List
Similarly named methods elsewhere seem to indicate it is supposed to be
ones-based counting (ie, bit (1<<0) would be considred 'bit 1'.
Thanks,
Ben
--
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc http://www.candelatech.com
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* Re: Is __ffs64 supposed to be zero based?
2013-11-06 7:15 Is __ffs64 supposed to be zero based? Ben Greear
@ 2013-11-06 11:52 ` Clemens Ladisch
2013-11-06 17:17 ` Ben Greear
0 siblings, 1 reply; 3+ messages in thread
From: Clemens Ladisch @ 2013-11-06 11:52 UTC (permalink / raw)
To: Ben Greear, Linux Kernel Mailing List
Ben Greear wrote:
> Similarly named methods elsewhere seem to indicate it is supposed to be
> ones-based counting (ie, bit (1<<0) would be considred 'bit 1'.
ffs() is defined to use one-based counting:
<http://pubs.opengroup.org/onlinepubs/9699919799/functions/ffs.html>
__ffs() uses zero-based counting. On gcc, it's likely to be implemented
with__builtin_ctz*(), whose documentation says:
| Returns the number of trailing 0-bits in x, starting at the least
| significant bit position.
Regards,
Clemens
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Is __ffs64 supposed to be zero based?
2013-11-06 11:52 ` Clemens Ladisch
@ 2013-11-06 17:17 ` Ben Greear
0 siblings, 0 replies; 3+ messages in thread
From: Ben Greear @ 2013-11-06 17:17 UTC (permalink / raw)
To: Clemens Ladisch; +Cc: Linux Kernel Mailing List
On 11/06/2013 03:52 AM, Clemens Ladisch wrote:
> Ben Greear wrote:
>> Similarly named methods elsewhere seem to indicate it is supposed to be
>> ones-based counting (ie, bit (1<<0) would be considred 'bit 1'.
>
> ffs() is defined to use one-based counting:
> <http://pubs.opengroup.org/onlinepubs/9699919799/functions/ffs.html>
>
> __ffs() uses zero-based counting. On gcc, it's likely to be implemented
> with__builtin_ctz*(), whose documentation says:
> | Returns the number of trailing 0-bits in x, starting at the least
> | significant bit position.
Thanks for the information!
Ben
--
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc http://www.candelatech.com
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