[FFmpeg-devel] [PATCH] WMA Voice decoder
Fri Jan 22 17:26:57 CET 2010
"Ronald S. Bultje" <rsbultje at gmail.com> writes:
> On Fri, Jan 22, 2010 at 11:15 AM, Uoti Urpala <uoti.urpala at pp1.inet.fi> wrote:
>> x*49995 / 41 = x*(1219*41 + 16) / 41 = 1219*x + x * 16 / 41
>> In the last form x*16 is at most 1048544.
> In fastdiv form, this'd be 3 muls, an add and a shift. Is that still
> faster than 1 mul + 1 div?
Don't forget the table lookup.
Multiplication takes typically 3-5 cycles. Division takes 15-40
cycles if the CPU has a hardware divider, 50-100 cycles if not. The
fastdiv should be faster, even when including the table lookup. If
this is called at all frequently, the table should be in L2 cache,
which costs typically 10-20 cycles, still faster than a division.
mans at mansr.com
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