linux-kernel - Re: [patch V2] lib: GCD: add binary GCD algorithm

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Message-ID: <20160428072118.GR3430@twins.programming.kicks-ass.net>
Date:	Thu, 28 Apr 2016 09:21:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	George Spelvin <linux@...izon.com>
Cc:	akpm@...ux-foundation.org, zengzhaoxiu@....com,
	linux-kernel@...r.kernel.org, zhaoxiu.zeng@...il.com
Subject: Re: [patch V2] lib: GCD: add binary GCD algorithm

On Wed, Apr 27, 2016 at 12:20:33PM -0400, George Spelvin wrote:
> The frequent "if (USE_FFS)" conditions seem to clutter the code.
> Would it be easier to read if the two variants were just separated?
> The function is short enough that the duplication isn't too severe.

Yes please, this is _MUCH_ more readable.

> #if USE_FFS
> /* If __ffs is available, the even/odd algorithm benchmarks slower. */
> unsigned long gcd(unsigned long a, unsigned long b)
> {
> 	unsigned long r = a | b;
> 
> 	if (!a || !b)
> 		return r;
> 
> 	b >>= __ffs(b);
> 
> 	for (;;) {
> 		a >>= __ffs(a);
> 		if (a == b)
> 			return a << __ffs(r);
> 		if (a < b)
> 			swap(a, b);
> 		a -= b;
> 	}
> }
> 
> #else /* !USE_FFS */
> 
> /* If normalization is done by loops, the even/odd algorithm is a win. */
> unsigned long gcd(unsigned long a, unsigned long b)
> {
> 	unsigned long r = a | b;
> 
> 	if (!a || !b)
> 		return r;
> 
> 	r &= -r;	/* Isolate lsbit of r */
> 
> 	while (!(b & r))
> 		b >>= 1;
> 
> 	for (;;) {
> 		while (!(a & r))
> 			a >>= 1;
> 		if (a == b)
> 			return a;
> 		if (a < b)
> 			swap(a, b);
> 		a -= b;
> 		a >>= 1;
> 		if (a & r)
> 			a += b;
> 		a >>= 1;
> 	}
> }
> #endif