lib: vsprintf: optimised put_dec_trunc() and put_dec_full()
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From: Denys Vlasenko on 10 Aug 2010 12:10 2010/8/10 Michał Nazarewicz <m.nazarewicz(a)samsung.com>: >> On Sunday 08 August 2010 21:29, Michal Nazarewicz wrote: >>> >>> + /* >>> + * '(x * 0xcccd) >> 19' is an approximation of 'x / 10' that >>> + * gives correct results for all x < 81920. However, because >>> + * intermediate result can be at most 32-bit we limit x to be >>> + * 16-bit. >>> + * >>> + * Because of those, we check if we are dealing with a "big" >>> + * number and if so, we make it smaller remembering to add to >>> + * the most significant digit. >>> + */ >>> + if (q >= 50000) { >>> + a = '5'; >>> + q -= 50000; >> >> ... >>> >>> + /* >>> + * We need to check if q is < 65536 so we might as well check > > On Tue, 10 Aug 2010 05:17:48 +0200, Denys Vlasenko > <vda.linux(a)googlemail.com> wrote: >> >> You meant "need to check if q is < 81920"? > > No. 81920 is a 17 bit number and when we multiply it by 0xcccd we lose > the most significant bit. > Therefore we cannot use the '(x * 0xcccd) >> > 19' approximation for numbers which are higher then 65535. No. All x up to (exclusive) 81920 can be multiplied by 0xcccd and result still fits into 32 bits. Proof: # printf "%x\n" $((81919 * 0xcccd)) ffff7333 -- vda -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |