linux-kernel - Re: [PATCH,RFC] random: make fast

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Message-ID: <20130922030553.GA21422@thunk.org>
Date:	Sat, 21 Sep 2013 23:05:53 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jörn Engel <joern@...fs.org>,
	John Stultz <john.stultz@...aro.org>,
	Stephan Mueller <smueller@...onox.de>,
	LKML <linux-kernel@...r.kernel.org>, dave.taht@...ferbloat.net,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH,RFC] random: make fast_mix() honor its name

The following fast_mix function, with the loop unrolling, is about 70%
slower than your proposed version, but it's still four times faster
than the original byte-based fast_mix function.  This is what I'm
considering using as a compromise.

Any comments or objections?

     					- Ted

static void fast_mix(struct fast_pool *f, __u32 input[4])
{
	__u32		w;
	int i;
	unsigned	input_rotate = f->rotate;

#if 0
	for (i = 0; i < 4; i++) {
		w = rol32(input[i], input_rotate) ^ f->pool[i] ^
			f->pool[(i + 3) & 3];
		f->pool[i] = (w >> 3) ^ twist_table[w & 7];
		input_rotate = (input_rotate + (i ? 7 : 14)) & 31;
	}
#else   /* loop unrolled for speed */
	w = rol32(input[0], input_rotate) ^ f->pool[0] ^ f->pool[3];
	f->pool[0] = (w >> 3) ^ twist_table[w & 7];
	input_rotate = (input_rotate + 14) & 31;
	w = rol32(input[1], input_rotate) ^ f->pool[1] ^ f->pool[0];
	f->pool[1] = (w >> 3) ^ twist_table[w & 7];
	input_rotate = (input_rotate + 7) & 31;
	w = rol32(input[2], input_rotate) ^ f->pool[2] ^ f->pool[1];
	f->pool[2] = (w >> 3) ^ twist_table[w & 7];
	input_rotate = (input_rotate + 7) & 31;
	w = rol32(input[3], input_rotate) ^ f->pool[3] ^ f->pool[2];
	f->pool[3] = (w >> 3) ^ twist_table[w & 7];
	input_rotate = (input_rotate + 7) & 31;
#endif
	f->count += 16;
	f->rotate = input_rotate;
}
--
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