How fast is D compared to C++?

Question

I like some features of D, but would be interested if they come with a runtime penalty?

To compare, I implemented a simple program that computes scalar products of m

Answer 1

Whether C++ or D is faster is likely to be highly dependent on what you're doing. I would think that when comparing well-written C++ to well-written D code, they would generally either be of similar speed, or C++ would be faster, but what the particular compiler manages to optimize could have a big effect completely aside from the language itself.

However, there are a few cases where D stands a good chance of beating C++ for speed. The main one which comes to mind would be string processing. Thanks to D's array slicing capabalities, strings (and arrays in general) can be processed much faster than you can readily do in C++. For D1, Tango's XML processor is extremely fast, thanks primarily to D's array slicing capabilities (and hopefully D2 will have a similarly fast XML parser once the one that's currently being worked on for Phobos has been completed). So, ultimately whether D or C++ is going to be faster is going to be very dependent on what you're doing.

Now, I am suprised that you're seeing such a difference in speed in this particular case, but it is the sort of thing that I would expect to improve as dmd improves. Using gdc might yield better results and would likely be a closer comparison of the language itself (rather than the backend) given that it's gcc-based. But it wouldn't surprise me at all if there are a number of things which could be done to speed up the code that dmd generates. I don't think that there's much question that gcc is more mature than dmd at this point. And code optimizations are one of the prime fruits of code maturity.

Ultimately, what matters is how well dmd performs for your particular application, but I do agree that it would definitely be nice to know how well C++ and D compare in general. In theory, they should be pretty much the same, but it really depends on the implementation. I think that a comprehensive set of benchmarks would be required to really test how well the two presently compare however.