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SubjectRe: [PATCH 3/4] lib: vsprintf: Optimize put_dec_trunc8
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On Fri, Aug 03 2012, George Spelvin wrote:
> If you're going to have a conditional branch after
> each 32x32->64-bit multiply, might as well shrink the code
> and make it a loop.
>
> This also avoids using the long multiply for small integers.
>
> (This leaves the comments in a confusing state, but that's a separate
> patch to make review easier.)
>
> Signed-off-by: George Spelvin <linux@horizon.com>

NAK.

> ---
> lib/vsprintf.c | 20 ++++++--------------
> 1 file changed, 6 insertions(+), 14 deletions(-)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index a8e7392..3ca77b8 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -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;
> + }

This loop looks nothing like the original code. Why are you adding '0'
at the beginning? Also, the original code switches the role of q and r,
the loop does not.

> if (r == 0)
> return buf;
> q = (r * 0x199a) >> 16;

--
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]
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