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Subject: Re: [boost] [review] Multiprecision review scheduled for June 8th - 17th, 2012
From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2012-06-09 03:41:29

On Fri, 1 Jun 2012, John Maddock wrote:

> Support for fused multiply-add is on my TODO list.

Consider the case of modular arithmetic, where you use an int64_t to do
the operations, and then reduce modulo some 30 bit prime number. When you
perform many operations in a row, it is interesting to avoid computing the
modulo every time but only do it often enough that there won't be an
overflow. FMA is a special case where it will remove one modulo. But in a
sum of a dozen numbers, you would want a single modulo at the end.
Expression templates should be able to help, but I guess this is a wrong
fit for this library (which is completely fine, I just thought I'd mention

(in the special case where we compute modulo 2^30 instead of a prime
number, and use an uint32_t for the computation, a finalization function
would be enough)


I just had a look at the updated introduction, and I realize I said
something wrong about the example with 11 temporaries: the optimization to
get 0 temporary in gmp-5.1 is only for mpz and mpq, not mpf. Sorry for the
misunderstanding. I may try to add the same optimization for mpf some time
(mixed precisions give me headaches when I try to handle that code), but
that's not really a plan.

Marc Glisse

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