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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: George Spelvin <linux@horizon.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
George Spelvin 2012-10-04 17:12:29 -07:00 committed by Linus Torvalds
parent e49317d415
commit 2359172a75
1 changed files with 31 additions and 25 deletions
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56
lib/vsprintf.c
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@ -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);
buf = put_dec_full4(buf, q % 10000);
q = q / 10000;
q += 7671 * d3 + 9496 * d2 + 6 * d1;
q = put_dec_helper4(buf+4, q);
d1 = q + 7671 * d3 + 9496 * d2 + 6 * d1;
buf = put_dec_full4(buf, d1 % 10000);
q = d1 / 10000;
q += 4749 * d3 + 42 * d2;
q = put_dec_helper4(buf+8, q);
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;