On 11/03/2011 22:05, Matt Calabrese wrote:
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.
In practice, such tagging on SIMD registers is not necessary. On bigger quantities, such as a vector, a matrix, or something like that, you can afford to simply do a runtime test if no such tagging is present.