Re:How to get C like performance in Java
by krakensden on Wednesday July 20, 2011 @01:53PM
(#2786689)
> > Please don't write your own object pools, this isn't 1995.
> The snark was unnecessary. It is frowned upon here.
Plus he's wrong, especially for all the people writing GUI applications who want to avoid GC pauses (for Dalvik, not the JVM, but still).
Re:How to get C like performance in Java
by davidtgoldblatt on Wednesday July 20, 2011 @04:02PM
(#2787166)
On the contrary, for almost all types of common array code the compiler is really good at optimizing away the bounds checks (see http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.86.96... ). Things only get tricky when you have, for instance, arrays of indices into other arrays, or when the next array position to examine depends on the contents of the one being examined. Even there though (and even in array-heavy code), bounds checks don't typically take up a ton of program execution time.
For sparse matrix operations that is the most common pattern of access though. For linear algebraic operations on dense matrices, BLAS and LAPACK takes care of most of the issues and is super fast. I think the times I do not use BLAS/LAPACK (directly or indirectly) are when I have to apply operations on matrices that are not the typical linear algebraic operations. This comes up fairly often too in my line of work which is machine learning.
I believe it would do so automatically only when it can prove that the bound will not be violated. That wouldnt be very often in a program written in an imperative style.
Lest it be mistaken for a swipe at imperative style, functional languages don't have it much easier either. I think the problem is that for the most part, array use patterns are not very amenable to this kind of analysis. So even in functional languages you have to explicitly ask the runtime or the compiler to avoid bound-checks.
If I remember correctly, there were a few languages designed to handle this case, i.e. use compile time type-checking to ensure that all the arrays are of the right shape and that none are accessed un-safely. Fish I think is what it was called. Dont know what happened to it or the idea. Seemed pretty handy to have. I think the first place to check now would be SAC (single assignment C). But I have digressed far from the topic of this thread !
> Please don't write your own object pools, this isn't 1995.
The snark was unnecessary. It is frowned upon here.