>>> @@ -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: m...@google.com>--------------ooO--(_)--Ooo--
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