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Blitz++: Fast, Accurate Numerical Computing in C++January 26, 2007 The C++ programming language offers many useful features for tackling complex scientific computing problems. Inheritance, polymorphism, generic programming, and operator overloading are of primary importance. Unfortunately, most of these advanced features carry a hefty performance cost. Until recently, C++ performance lagged behind Fortran's performance by anywhere from 20 percent to orders of magnitude. As a result, the adoption of C++ for scientific computing has been slower than it should be, given the platform's popularity. So, how can you turbo charge C++ so that you can have your advanced language features but lose the poor performance? This is precisely the goal of the Blitz++ project: to develop techniques that will enable C++ to rival and even surpass the speed of Fortran for numerical computing, while preserving an object-oriented interface. The Blitz++ Numerical Library is being constructed as the solution under dual licenses—GNU GPL and Blitz++ Artistic License. Blitz++ provides a single array class called Array<T_numtype, N_rank>. This class provides a dynamically allocated N-dimensional array with reference counting, arbitrary storage ordering, subarrays and slicing, flexible expression handling, and many other useful features. Can C++ Compete with Fortran?Recent benchmarks show C++ encroaching steadily on Fortran's high-performance monopoly. For some benchmarks, C++ is even faster than Fortran! No, this is not a result of better optimizing compilers, preprocessors, or even language extensions. It's achieved simply through the use of template techniques. Through the magic of template metaprogramming, optimizations such as loop fusion, unrolling, tiling, and algorithm specialization can be performed automatically at compile time. Another successful package that uses this technique is, of course, the Boost C++ Libraries. Another goal of Blitz++ is to extend the conventional dense array model (in other words, not sparse) with new and useful features. Some examples of such extensions are flexible storage formats, tensor notation, and index placeholders. Blitz++ is more than just cool arrays; you also get matrix math, random number generation, and a variety of supporting functionality. Blitz++ Platform SupportAs is true of any template metaprogramming tool, Blitz++ is only as good as the ISO/ANSI C++ compliance of the compiler you choose. Specifically, your compiler of choice must have rock-solid member templates, partial specialization, and partial ordering of templates. The good news is that Visual Studio .NET 2003 and Visual Studio 2005 are supported directly by way of project files. Additionally, a whole raft of compilers are supported, including Intel C++ for Windows, gcc, IBM XL C/C++ for AIX, and Sun Studio 10 for Solaris. Older versions of Borland C++ and Visual Studio C++ (6.0) definitely will not work. Of course, Blitz++ comes with a complete self-test suite so you can be confident that your OS and compiler are 100 percent compliant. Getting StartedTo use the Array class, you must include the header <blitz/array.h> and use the namespace blitz: #include <blitz/array.h> using namespace blitz; The array class, called Array<T_numtype, N_rank>, has two templated parameters:
Array<int,1> x; // A one-dimensional array of int Array<double,2> y; // A two-dimensional array of double Array<complex<float>, 12> z; // A twelve-dimensional array of // complex<float> When no constructor arguments are provided, the array is empty and no memory is allocated. To create an array that contains some data, simply provide the size of the array as constructor arguments: Array<double,2> y(4,4); // A 4x4 array of double The contents of a newly created array are garbage. To initialize the array to all zeroes, you can write the following: y = 0; Putting it all together, you can write a "hello world"-type program to add two 2x2 matrices together: #include <blitz/array.h> using namespace blitz; int main() { Array<float,2> A(2,2), B(2,2), C(2,2); A = 1, 0, 2, 2; B = 0, 0, 7, 0; C = A + B; cout << "C = " << C << endl; return 0; } and the output: C = 2 x 2 [ 1 0] 9 ] The Array constructor can be manipulated in a lot of ways to achieve a variety of results: computation during initialization, setting the extent (length), setting the base of an array to be something other than zero, and building an array as the reference of another array. Here's a cool example of creating an array "B" that contains the square roots of elements in "A": Array<float,2> A(N,N); Array<float,2> B(sqrt(A)); A Complete Example: Integration TechniquesNow, take a look at three different integration methods: naïve summation, two-point trapezoid, and a three-point Simpson's rule. The following is the expression for the example: 1 #include <blitz/vector.h> 2 using namespace blitz; 3 4 int main() 5 { 6 // Integrate the expression 7 8 // to estimate the error function erf(x) on the interval [0,1]. 9 // 10 // Three methods are compared: 11 // 1. Naive summation 12 // 2. Two-point trapezoid 13 // 3. Three-point Simpson's 14 15 const int numSamples = 1024; // Number of samples 16 double dt = 1.0 / numSamples; // Distance between samples 17 Range R(0, numSamples - 1); // Index set for sample // vectors 18 Range displayRange(0, 900, 100); // Indices to output to // screen 19 20 // For comparison purposes, use the built-in erf(x) function. 21 Vector<double> z = erf(R * dt); 22 cout << " Built-in: " << z(displayRange+1) << endl; 23 24 25 // Naive summation 26 Vector<double> z1 = accumulate(M_2_SQRTPI * exp(-sqr(R * dt)) * dt); 27 cout << " Naive: " << z1(displayRange+1) << endl; 28 29 // Calculate the rms error 30 double error1 = sqrt(mean(sqr(z1 - z))); 31 cout << "RMS Error: " << error1 << endl; 32 33 34 // For the following rules, it's easier to work with a 35 // sampled integrand. 36 Vector<double> a = M_2_SQRTPI * exp(-sqr(R * dt)); 37 38 39 // Two-point trapezoid 40 Range J(1, numSamples-1); 41 Vector<double> z2 = accumulate(dt / 2.0 * (a(J) + a(J-1))); 42 43 cout << " Trapezoid: " << z2(displayRange) << endl; 44 cout << "RMS Error: " << sqrt(mean(sqr(z2-z(J)))) << endl; 45 46 47 // Three-point Simpson's (parabolic) 48 Range I(1, numSamples-2); 49 Vector<double> z3 = accumulate(dt / 6.0 * (a(I-1) + 4 * a(I) + a(I+1))); 50 51 cout << " Parabolic: " << z3(displayRange) << endl; 52 cout << " RMS Error: " << sqrt(mean(sqr(z3 - z(I)))) << endl; 53 54 55 return 0; 56 } 57 Lines 26, 41, and 49 use the accumulate functor. A functor describes an action that should be taken on each member of a list. The action usually will vary according to the list member's type. Parallel ComputationWith multi-core CPUs on the horizon, everyone is thinking about how to exploit the extra cores by adding computation threads. Although Blitz++ can be used for parallel computing, it was not designed primarily for that. For this reason, you may want to investigate some other available libraries such as POOMA before choosing to implement a parallel code using Blitz++. Using threads on Blitz++ may require a library recompile, depending on platform. In threadsafe mode, Blitz++ array reference counts are safeguarded by a mutex. By default, pthread mutexes are used. Blitz++ does not do locking for every array element access because it would result in an unacceptable performance hit. Responsibility for appropriate synchronization is squarely on your shoulders. Tough Numeric Problems? No ProblemYou've just barely scratched the surface of what Blitz++ can do for you. Some of the topics this article couldn't cover include sophisticated array expressions, stencils, slicing, and indirection. Give it a try because Blitz++ is ready, willing, and able to handle your toughest numeric problems. Book Recommendation
About the AuthorVictor Volkman has been writing for C/C++ Users Journal and other programming journals since the late 1980s. He is a graduate of Michigan Tech and a faculty advisor board member for Washtenaw Community College CIS department. Volkman is the editor of numerous books, including C/C++ Treasure Chest and is the owner of Loving Healing Press. He can help you in your quest for open source tools and libraries, just drop an e-mail to sysop@HAL9K.com. |