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

[Rich Felker]

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@xxxxxxxxxxx> 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.

Rich
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