On Sat, Sep 09, 2006 at 06:27:40PM +0800, Joel de Guzman wrote:
Andy Little wrote:
You must've missed this: Dan wrote me an email a while back. He says: "Interestingly using fusion::fold to do maths on boost::arrays, I'm finding that with vc8.0 fusion significantly outperforms the standard library equivalent code, presumably as it has more information available at compile time, and with inlining it effectively unrolls the entire loops."
I asked Dan to add his tests to libs/fusion/example to showcase this favorable "phenomena" :-).
I can't find that, but I am probably looking in the wrong place. Do you mean Boost CVS?
Sorry, you'll have to wait a bit more. Dan says he'll do it over the weekend.
I think the main reason for this is that inlining a fusion loop means that the compiler has obeyed an explicit programmer suggestion (inline), whereas unrolling a constant size array loop is loop unrolling, which is impossible to suggest to the compiler (and typically requires higher -O levels, etc.). A while back when I implemented a vector type with constant size for loops, it slowed down the whole application by 30%. I'm very curious to see how much better fusion can do. Andy: is your fusion code in an easily accessible place where I could grab it and stick some of it into our vector class to test it in a (probably) larger example? The main issue is whether the compile will still choose to unroll everything if it sees a bunch of vector operations inside various complicated functions. Thanks, Geoffrey