Re: [PATCHv2 1/3] lib: vsprintf: optimised put_dec_trunc() and put_dec_full()

From: Denys Vlasenko
Date: Mon Aug 09 2010 - 23:18:02 EST


On Sunday 08 August 2010 21:29, Michal Nazarewicz wrote:
> -- Speed --------------------------------------------------------
> orig_put_dec_full : 1.054570 1.356214 1.732636 1.725760 Original
> mod1_put_dec_full : 1.000000 1.017216 1.255518 1.116559
> mod3_put_dec_full : 1.018222 1.000000 1.000000 1.000000 Proposed
>
> orig_put_dec_trunc : 1.137903 1.216017 1.850478 1.662370 Original
> mod1_put_dec_trunc : 1.000000 1.078154 1.355635 1.400637
> mod3_put_dec_trunc : 1.025989 1.000000 1.000000 1.000000 Proposed
> -- Size ---------------------------------------------------------
> orig_put_dec_full : 1.212766 1.310345 1.355372 1.355372 Original
> mod1_put_dec_full : 1.021277 1.000000 1.000000 1.000000
> mod3_put_dec_full : 1.000000 1.172414 1.049587 1.049587 Proposed
>
> orig_put_dec_trunc : 1.363636 1.317365 1.784000 1.784000 Original
> mod1_put_dec_trunc : 1.181818 1.275449 1.400000 1.400000
> mod3_put_dec_trunc : 1.000000 1.000000 1.000000 1.000000 Proposed

In my testing on Phenom II the speed gain is smaller,
but it is indeed faster. And smaller!


> + /*
> + * '(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

You meant "need to check if q is < 81920"?

> + * if we can just call the _full version of this function.
> + */
> + if (q > 9999)
> + return put_dec_full(buf, q);

--
vda
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