Messages in this thread | | | From | Michal Nazarewicz <> | Subject | Re: [PATCH 3/4] lib: vsprintf: Optimize put_dec_trunc8 | Date | Mon, 24 Sep 2012 14:29:56 +0200 |
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>>> @@ -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@google.com>--------------ooO--(_)--Ooo--[unhandled content-type:application/pgp-signature] | |