Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 2359172a authored by George Spelvin's avatar George Spelvin Committed by Linus Torvalds
Browse files

lib: vsprintf: optimize division by 10000 



The same multiply-by-inverse technique can be used to convert division by
10000 to a 32x32->64-bit multiply.

Signed-off-by: default avatarGeorge Spelvin <linux@horizon.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e49317d4

lib/vsprintf.c

+33 −27
Original line number Diff line number Diff line
@@ -245,17 +245,32 @@ char *put_dec(char *buf, unsigned long long n) 


/* See comment in put_dec_full9 for choice of constants */

static noinline_for_stack

char *put_dec_full4(char *buf, unsigned q)

void put_dec_full4(char *buf, unsigned q)

{

	unsigned r;

	r      = (q * 0xccd) >> 15;

	*buf++ = (q - 10 * r) + '0';

	buf[0] = (q - 10 * r) + '0';

	q      = (r * 0xcd) >> 11;

	*buf++ = (r - 10 * q)  + '0';

	buf[1] = (r - 10 * q)  + '0';

	r      = (q * 0xcd) >> 11;

	*buf++ = (q - 10 * r)  + '0';

	*buf++ = r + '0';

	return buf;

	buf[2] = (q - 10 * r)  + '0';

	buf[3] = r + '0';

}



/*

 * Call put_dec_full4 on x % 10000, return x / 10000.

 * The approximation x/10000 == (x * 0x346DC5D7) >> 43

 * holds for all x < 1,128,869,999.  The largest value this

 * helper will ever be asked to convert is 1,125,520,955.

 * (d1 in the put_dec code, assuming n is all-ones).

 */

static

unsigned put_dec_helper4(char *buf, unsigned x)

{

        uint32_t q = (x * (uint64_t)0x346DC5D7) >> 43;



        put_dec_full4(buf, x - q * 10000);

        return q;

}



/* Based on code by Douglas W. Jones found at
@@ -277,28 +292,19 @@ char *put_dec(char *buf, unsigned long long n) 
	d3  = (h >> 16); /* implicit "& 0xffff" */



	q   = 656 * d3 + 7296 * d2 + 5536 * d1 + ((uint32_t)n & 0xffff);

	q = put_dec_helper4(buf, q);



	q += 7671 * d3 + 9496 * d2 + 6 * d1;

	q = put_dec_helper4(buf+4, q);



	q += 4749 * d3 + 42 * d2;

	q = put_dec_helper4(buf+8, q);



	buf = put_dec_full4(buf, q % 10000);

	q   = q / 10000;



	d1  = q + 7671 * d3 + 9496 * d2 + 6 * d1;

	buf = put_dec_full4(buf, d1 % 10000);

	q   = d1 / 10000;



	d2  = q + 4749 * d3 + 42 * d2;

	buf = put_dec_full4(buf, d2 % 10000);

	q   = d2 / 10000;



	d3  = q + 281 * d3;

	if (!d3)

		goto done;

	buf = put_dec_full4(buf, d3 % 10000);

	q   = d3 / 10000;

	if (!q)

		goto done;

	buf = put_dec_full4(buf, q);

 done:

	while (buf[-1] == '0')

	q += 281 * d3;

	buf += 12;

	if (q)

		buf = put_dec_trunc8(buf, q);

	else while (buf[-1] == '0')

		--buf;



	return buf;