using mpn_preinv_divrem_1
Torbjorn Granlund
tege at swox.com
Thu Sep 1 13:52:52 CEST 2005
Ashod Nakashian <saghmos at xter.net> writes:
I understand that. What I was asking was why mpn_divrem_1 isn't
precomputing the inverse on-the-fly?
It sure does, look at the code, in mpn/generic/divrem_1.c.
DIVREM_1_NORM_THRESHOLD and DIVREM_1_UNNORM_THRESHOLD
controls this.
Precomputing the inverse should pay off for a few limb's
long numbers and more, assuming a (mul + overhead) is faster on the
target CPU than a plain div.
That's the experience of the GMP hackers.
Now, I'm willing to experiment with this on my AMD-64 3200+ to see how
far I can get. But my next question is how do you compute the inverse of
the divisor? As I stated in my previous mail, the traditional method, as
I understand it, doesn't work. There is some assumption that I'm missing.
All details are in the paper "Division by Invariant Integers using
Multiplication", <http://swox.com/~tege/divcnst-pldi94.pdf>. I
think the source code for invert_limb might also give you a hint.
--
Torbjörn
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