Re: [boost] [XInt] Some after thoughts about SIMD

Subject: Re: [boost] [XInt] Some after thoughts about SIMD
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2011-03-11 16:05:38

On Fri, Mar 11, 2011 at 3:04 PM, Simonson, Lucanus J <
lucanus.j.simonson_at_[hidden]> wrote:

> If you have the same variable in more than one place in an expression you
> can map that expression to different/fewer instructions than if they were
> all different variables in different places.  There is no way to perform
> that mapping using expression templates because it cannot be detected in
> template metaprogramming which varaibles are the same, only that they have
> the same type.  Therefor you cannot map expression to the optimal
> instructions using template metaprogramming.
>

Even if a given compiler can't do such further optimizations for you, I
wouldn't consider it the end of the road for expression templates. Not that
I would necessarily suggest the following solution for all situations, but
you could always attach a type-tag to a reused variable when forming an
expression, for instance:

some_type a, b, c;

some_tagger< a_tag > a_( a );

c = a_ * b + a_;

Since a_ here has a unique tag "a_tag" associated with it, the hypothetical
DSEL can be made to treat the two instances of a_ as the same variable and
would therefore know to use specific instructions in such a case. Again, I'm
not proposing that this is a great solution as it requires some effort on
the part of the user, but by type-tagging objects that appear multiple times
in a single expression you can do the kinds of things that you are talking
about.

-- 
-Matt Calabrese