Re: [boost] GTL compile time vs. run time accessors

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-05-02 19:25:19

AMDG

Simonson, Lucanus J wrote:
> I think providing both compile time and runtime
> accessors is fine.  The runtime accessors can be implemented in terms of
> the compile time ones in the cases where the type doesn't provide
> indexing (such as a tuple based point.)  People wanting better
> performance with the runtime accessor can specialize it where
> appropriate.
>   

Cool.

>> Have you measured the performance when adapting a struct like this:
>> struct Point {
>>    int x;
>>    int y;
>> };
>>     
>
> Yes, when compiled with optimization it is identical to access the
> struct members directly or through the accessors (and the extra wrapper
> class from the original design as well.)  Please note: if you forget to
> use the result of your computation, dead code removal leads to a
> misleading result because the compiler is more successful in removing
> dead code in the case where there is less to be removed, so take care
> confirming my result.
>   

Ok.  There might be extra optimizations possible when the index is known
at compile time as opposed to run time.  (I'm not talking about the 
difference
between get<X>(p) and p[X], here, but the difference between when X is
known at compile time using templates to avoid code duplication vs. runtime.
and using function arguments).  This is really a property of the 
algorithm rather
than the point class, though.

In Christ,
Steven Watanabe