[patch V3] lib: GCD: add binary GCD algorithm

On Thu, Apr 28, 2016 at 7:58 PM, Rich Felker [off-list ref] wrote:

On Thu, Apr 28, 2016 at 07:51:06PM +0200, Geert Uytterhoeven wrote:

quoted

On Thu, Apr 28, 2016 at 6:48 PM, George Spelvin [off-list ref] wrote:

quoted

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 at 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