practicality of empirical cache optimization in D vs C++

[John Colvin via Digitalmars-d-learn]

On Monday, 10 November 2014 at 19:18:21 UTC, Kirill wrote:
> Dear D community (and specifically experts on cache 
> optimization),
>
> I'm a C++ programmer and was waiting for a while to do a 
> project in D.
>
> I'd like to build a cache-optimized decision tree forest 
> library, and I'm debating between D and C++. I'd like to make 
> it similar to atlas, spiral, or other libraries that partially 
> use static optimization with recompilation and meta-programming 
> to cache optimize the code for a specific architecture 
> (specifically the latest xeons / xeon phi). Given D's compile 
> speed and meta-programming, it should be a good fit. The 
> problem that I might encounter is that C++ has a lot more 
> information on the topic, which might be significant bottleneck 
> given I'm just learning cache optimization (from a few papers 
> and "what every programmer should know about memory").
>
> From my understanding, cache optimization mostly involves 
> breaking data and loops into segments that fit in cache, and 
> making sure that commonly used variables (for example sum in 
> sum+=i) stay in cache.

Assing there isn't more frequently accessed data around, you 
would want that to stay in a register, not cache.

> Most of this should be solved by statically defining sizes and 
> paddings of blocks to be used for caching. It's more related to 
> low level -- C, from my understanding. Are there any hidden 
> stones?
>
> The other question is how mature is the compiler in terms of 
> optimizing for cache comparing to C++? I think gnu C++ does a 
> few tricks to optimize for cache and there are ways to tweak 
> cache line alignment.
>
> My knowledge on the subject is not yet concrete and limited but 
> I hope this gave an idea of what I'm looking for and you can 
> recommend me a good direction to take.
>
> Best regards,
> --Kirill

D is a good language for this sort of thing. Using various 
metaprogramming techniques it might even be fun.

Most advice for C(++) will also apply to D w.r.t. cache.

You will probably have to learn assembly and also make use of 
tools such as cachegrind and perf unless you like trying to 
optimise blind.

A word of warning: modern CPU caches are complicated and are 
sometimes difficult to understand w.r.t. performance in specific 
cases.