linux-kernel - Re: [PATCH 3/4] lib: vsprintf: Optimize put_dec

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Date:	Mon, 24 Sep 2012 14:29:56 +0200
From:	Michal Nazarewicz <mpn@...gle.com>
To:	George Spelvin <linux@...izon.com>, linux@...izon.com,
	vda.linux@...glemail.com
Cc:	hughd@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] lib: vsprintf: Optimize put_dec_trunc8

>>> @@ -174,20 +174,12 @@ char *put_dec_trunc8(char *buf, unsigned r)
>>>  	unsigned q;
>>>  	/* Copy of previous function's body with added early returns */
>>> -	q      = (r * (uint64_t)0x1999999a) >> 32;
>>> -	*buf++ = (r - 10 * q) + '0'; /* 2 */
>>> -	if (q == 0)
>>> -		return buf;
>>> -	r      = (q * (uint64_t)0x1999999a) >> 32;
>>> -	*buf++ = (q - 10 * r) + '0'; /* 3 */
>>> -	if (r == 0)
>>> -		return buf;
>>> -	q      = (r * (uint64_t)0x1999999a) >> 32;
>>> -	*buf++ = (r - 10 * q) + '0'; /* 4 */
>>> -	if (q == 0)
>>> -		return buf;
>>> -	r      = (q * (uint64_t)0x1999999a) >> 32;
>>> -	*buf++ = (q - 10 * r) + '0'; /* 5 */
>>> +	while (r >= 10000) {
>>> +		q = r + '0';
>>> +		r  = (r * (uint64_t)0x1999999a) >> 32;
>>> +		*buf++ = q - 10*r;
>>> +	}

All right, I now see what the loop is doing (I couldn't grasp it
yesterday) and expect for r=0 it looks legit.

On Mon, Sep 24 2012, George Spelvin wrote:
> Truthfully, it would have made *more* sense to swap q and r globally,
> so the loop had a more sensible q=quotient/r=remainder assignment,
> but I wanted to show that the unmodified tail was in fact unmodified.

The original has it a bit awkwardly because it just copies code from
put_dec_full9() with the first iteration skipped.

> The big saving from using a loop is that it avoids unnecessary
> 32x32->64-bit multiplies, falling through to the 16x16->32-bit
> code as early as possible.  Given that most numbers are small,
> this seemed like a significant win.

Ah, makes sense.

I guess the following should work, even though it's not so pretty:

static noinline_for_stack
char *put_dec_trunc8(char *buf, unsigned r) {
	unsigned q;

	if (r > 10000) {
		do {
			q = r + '0';
			r = (r * (uint64_t)0x1999999a) >> 32;
			*buf++ = q - 10 * r;
		} while (r >= 10000);
		if (r == 0)
			return buf;
	}

	q      = (r * 0x199a) >> 16;
	*buf++ = (r - 10 * q)  + '0'; /* 6 */
	if (q == 0)
		return buf;
	r      = (q * 0xcd) >> 11;
	*buf++ = (q - 10 * r)  + '0'; /* 7 */
	if (r == 0)
		return buf;
	q      = (r * 0xcd) >> 11;
	*buf++ = (r - 10 * q) + '0'; /* 8 */
	if (q == 0)
		return buf;
	*buf++ = q + '0'; /* 9 */
	return buf;
}

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +----<email/xmpp: mpn@...gle.com>--------------ooO--(_)--Ooo--

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