Messages in this thread | | | From | Denys Vlasenko <> | Date | Mon, 24 Sep 2012 11:06:33 +0200 | Subject | Re: [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers. |
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On Fri, Aug 3, 2012 at 7:21 AM, George Spelvin <linux@horizon.com> wrote: > Shrink the reciprocal approximations used in put_dec_full4 > based on the comments in put_dec_full9. > > Signed-off-by: George Spelvin <linux@horizon.com> > Cc: Denys Vlasenko <vda.linux@googlemail.com> > Cc: Michal Nazarewicz <mina86@mina86.com> > --- > lib/vsprintf.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > I was looking over the code and noticed that the constants could be smaller. > > diff --git a/lib/vsprintf.c b/lib/vsprintf.c > index c3f36d41..2f32fe8 100644 > --- a/lib/vsprintf.c > +++ b/lib/vsprintf.c > @@ -243,13 +243,14 @@ char *put_dec(char *buf, unsigned long long n) > > /* Second algorithm: valid only for 64-bit long longs */ > > +/* See comment in put_dec_full9 for choice of constants */ > static noinline_for_stack > char *put_dec_full4(char *buf, unsigned q) > { > unsigned r; > - r = (q * 0xcccd) >> 19; > + r = (q * 0xccd) >> 15; > *buf++ = (q - 10 * r) + '0'; > - q = (r * 0x199a) >> 16; > + q = (r * 0xcd) >> 11;
I would use 16-bit shifts instead of smaller ones. There may be CPUs on which "get upper half of 32-bit reg" operation is cheaper or smaller than a shift.
-- vda
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