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SubjectRe: [patch V3] lib: GCD: add binary GCD algorithm
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On Thu, Apr 28, 2016 at 7:58 PM, Rich Felker <dalias@libc.org> wrote:
> On Thu, Apr 28, 2016 at 07:51:06PM +0200, Geert Uytterhoeven wrote:
>> On Thu, Apr 28, 2016 at 6:48 PM, George Spelvin <linux@horizon.com> wrote:
>> > Another few comments:
>> >
>> > 1. Would ARCH_HAS_FAST_FFS involve fewer changes than CPU_NO_EFFICIENT_FFS?
>>
>> No, as you want to _disable_ ARCH_HAS_FAST_FFS / _enable_
>> CPU_NO_EFFICIENT_FFS as soon as you're enabling support for a
>> CPU that doesn't support it.
>>
>> Logical OR is easier in both the Kconfig and C preprocessor languages
>> than logical NAND.
>>
>> E.g. in Kconfig, a CPU core not supporting it can just select
>> CPU_NO_EFFICIENT_FFS.
>
> How does a CPU lack an efficient ffs/ctz anyway? There are all sorts
> of ways to implement it without a native insn, some of which are
> almost or just as fast as the native insn on cpus that have the
> latter. On anything with a fast multiply, the de Bruijn sequence
> approach is near-optimal, and otherwise one of the binary-search type
> approaches (possibly branchless) can be used. If the compiler doesn't
> generate an appropriate one for __builtin_ctz, that's arguably a
> compiler bug.

m68k-linux-gcc 4.6.3 generates:

jsr __ctzsi2

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

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