
- Expression templates make the code slower.
For sure - expression templates are about eliminating temporaries - if the temporaries are cheap (cpp_int containing small values and no dynamic memory allocation) then there's no benefit and probably a hit from bringing in all that machinary. That's why expression templates are disabled for the fixed size cpp_int typedefs.
I'd like to propose an alternate viewpoint: in the context of numerics, expression templates are about delaying evaluation to gather more context, so you can perform the computation more efficiently. From that point-of-view there's absolutely no reason that they should ever make code slower. Just generate the appropriate code for the types and expressions involved.
Hi, I agree, the library should be able to decide whether expression templates should be an improvement or not.
A minor change in the library, defaulting the ExpressionTemplates parameter to a trait depending on the backend could help to that. The library could default to something reasonable, while the backend developer could always provide a specialization. Vicente
I like this idea. Unfortunately, I may not be the best one to program it, and I fear John would get stuck with it---if, that is, this turns out to be the way to go. So one would need to see what John thinks. So Vicente, if I understand, you are saying that one might add more *intelligence* to the kind of object used as the template parameter for enabling/disabling the expression templates. Perhaps is it like the good idea you wrote on 31-May-2012, but takes it even one step further? You wrote this, but it was for floats:
What about replacing the second bool template parameter by an enum class expression_template {disabled, enabled}; which will be more explicit. That is
typedef mp::mp_number<mp::mpfr_float_backend<300>, false> my_float; versus typedef mp::mp_number<mp::mpfr_float_backend<300>, mp::expression_template::disabled> my_float;
But I also fear that it would take q bit of time to specify this kind of back-end parameter type. Is it something one could add later, backwards compatibility being handled vie sensible default parameter? Best regards, Chris.