I need high performance. Will there be a difference if I use C or C++?

Question

I need to write a program (a project for university) that solves (approx) an NP-hard problem. It is a variation of Linear ordering problems. In general, I will have very large inputs (as Graphs) and will try to find the best solution (based on a function that will 'rate' each solution)

Will there be a difference if I write this in C-style code (one main, and functions) or build a Solver class, create an instance and invoke a 'run' method from a main (similar to Java)

Also, there will be alot of floating point math going on in each iteration.

Thanks!

Answer 1

Not inherently, no.

Answer 2

No.

The biggest performance gains/flaws will be on the algorithm you implement, and how much unneeded work you perform (Unneeded work could be everything from recalculating a previous value that could have been cached, to using too many malloc/free's vs using memory pools, passing large immutable data by value instead of reference)

Answer 3

Since both are compiled, and the compilers now are very good at how to handle C++, I think the only problem would come from how well optimized your code is. I think it would be easier to write slower code in C++, but that depends on which style your model fits into best.

When it comes down to it, I doubt there will be any real difference, assuming both are well-written, any libraries you use, how well written they are, if you are measuring on the same computer.

Answer 4

As long as you don't use any virtual functions etc. you won't note any considerable performance differences. Early C++ was compiled to C, so as long as you know the pinpoints where this creates any considerable overhead (such as with virtual functions) you can clearly calculate for the differences.

In addition I want to note that using C++ can give you a lot to gain if you use the STL and Boost Libraries. Especially the STL provides very efficient and proven implementations of the most important data structures and algorithms, so you can save a lot of development time.

Effectively it also depends on the compiler you will be using and how it will optimize the code.

Answer 5

The biggest roadblock to optimal code is no longer the language (for properly compiled languages), but rather the programmer.

Answer 6

Rule of thumb - do not optimize until you know what to optimize. So start with C++ and have some working prototype. Then profile it and rewrite bottle necks in assembly. But as others noted, chosen algorithm will have much greater impact than the language.

Answer 7

Function call vs. member function call overhead is unlikely to be the limiting factor, compared to file input and the algorithm itself. C++ iostreams are not necessarily super high speed. C has 'restrict' if you're really optimizing, in C++ it's easier to inline function calls. Overall, C++ offers more options for organizing your code clearly, but if it's not a big program, or you're just going to write it in a similar manner whether it's C or C++, then the portability of C libraries becomes more important.

Answer 8

The Solver class will be constructed once, I take it, and the run method executed once... in that kind of environment, you won't see a difference. Instead, here are things to watch out for:

• Memory management is hellishly expensive. If you need to do lots of little malloc()s, the operating system will eat your lunch. Make a determined effort to re-use whatever data structures you create if you know you'll be doing the same kind of thing again soon!

• Instantiating classes generally means... allocating memory! Again, there's practically no cost for instantiating a handful of objects and re-using them. But beware of creating objects only to tear them down and rebuild them soon after!

• Choose the right flavor of floating point for your architecture, insofar as the problem permits. It's possible that double will end up being faster than float, although it will need more memory. You should experiment to fine-tune this. Ideally, you'll use a #define or typedef to specify the type so you can change it easily in one place.

• Integer calculations are probably faster than floating point. Depending on the numeric range of your data, you may also consider doing it with integers treated as fixed-point decimals. If you need 3 decimal places, you could use ints and just consider them "milli-somethings". You'll have to remember to shift decimals after division and multiplication... but no big deal. If you use any math functions beyond the basic arithmetic, of course, that would of course kill this possibility.

Answer 9

first, writing in C++ doesn't imply using OOP, look at the STL algorithms. second, C++ can be even slightly faster at runtime (the compilation times can be terrible compared to C, but that's because modern C++ tends to rely heavily on abstractions that tax the compiler).

edit: alright, see Bjarne Stroustrup's discussion of qsort and std::sort, and the article that FAQ mentions (Learning Standard C++ as a New Language), where he shows that C++-style code can be not only shorter and more readable (because of higher abstractions), but also somewhat faster.

Answer 10

When speaking of performance, anything you can do in C can be done in C++. For example, virtual methods are known to be “slow”, but if it's really a problem, you can still resort to C idioms.

C++ also brings templates, which lead to better performance than using void* for generic programming.

Answer 11

Another aspect:

C++ templates can be an excellent tool to generate type-specific / optimized code variations.

For example, C qsort requires a function call to the comparator, whereas std::sort can inline the functor passed. This can make a significant difference when compare and swap themselves are cheap.

Note that you could generate "custom qsorts" optimized for various types with a barrage of defines or a code generator, or by hand - you could do these optimizations in C, too, but at much higher cost.

(It's not a general weapon, templates help only in sepcific scenarios - usually a single algorithm applied to different data types or with differing small pieces of code injected.)

Answer 12

Good answers. I would put it like this:

1. Make the algorithm as efficient as possible in terms of its formal structure.

2. C++ will be as fast as C, except that it will tempt you to do dumb things, like constructing objects that you don't have to, so don't take the bait. Things like STL container classes and iterators may look like the latest-and-greatest thing, but they will kill you in a hotspot.

3. Even so, single-step it at the disassembly level. You should see it very directly working on your problem. If it is spending lots of cycles getting in and out of routines, try some in-lining (or macros). If it is wandering off into memory allocation and freeing, for much of the time, put a stop to that. If it's got inner loops where the loop overhead is a large percentage, try unrolling the loop.

That's how you can make it about as fast as possible.

Answer 13

hi

I would go with C++ definitely. If you are careful about your design and avoid creating heavy objects inside hotspots you should not see any performance difference but the code is going to be much simpler to understand, maintain, and expand.

Use templates and classes judiciously. avoid unnecessary object creation by passing objects by reference. Avoid excessive memory allocation, if needed, allocate memory in advance of hotspots. Use restrict keyword on memory pointers to tell compiler whenever pointers overlap or not.

As far as optimization, pay careful attention to memory alignment. Assuming you are working on Intel processor, you can make use of vector instructions, provided you tell the compiler through pragma's about your memory alignment and aliased pointers. you can also use vector instructions directly via intrinsics.

you can also automatically create hotspot code using templates and let compiler optimize it out if you have things like short loops of different sizes. To find out performance and to drill down to your bottlenecks, Intel vtune or oprofile are extremely helpful.

hope that helps

Answer 14

I do some DSP coding, where it still pays off to go to assembly language sometimes. I'd say use C or C++, either one, and be prepared to go to assembly language when you need to, especially to exploit SIMD instructions.