在 C Mex-File 中使用 BLAS 库

Question

我正在尝试为我的 C-Mex-Code 使用 BLAS 库来分析使用和不使用 BLAS 的性能差异。但是，我无法弄清楚如何正确使用 BLAS 而不会出现编译错误/警告（最终导致 Matlab 崩溃）。

下面是一个示例代码，我想使用 Blas 的 ddot() 函数 (https://software.intel.com/de-de/node/468398#D4E53C70-D8FA-4095-A800-4203CAFE64FE) 计算向量积：

  #include <mex.h>
  #include <math.h>
  #include <blas.h>

  void TestBlas(double *L, double *R, mwSize n)
  {

      int one = 1;
      double sum = ddot(&n,L,&one,L,&one);

      //Output Matrix R not defined, just for test purposes...

  } 

  void mexFunction( int nlhs, mxArray *plhs[],
                    int nrhs, const mxArray *prhs[])
  {
      double *inMatrix;               /* 1xN input matrix */
      size_t ncols;                   /* size of matrix */
      long *ncolsPr;                  /* output matrix */
      double *outMatrix;              /* output matrix */

      inMatrix = mxGetPr(prhs[0]);
      ncols = mxGetN(prhs[0]);

      /* create the output matrix */
      plhs[0] = mxCreateDoubleMatrix(1,(mwSize)ncols,mxREAL);
      outMatrix = mxGetPr(plhs[0]);

      TestBlas(inMatrix,outMatrix,(mwSize)ncols);
  }

我收到的编译器警告是：

 >> mex -largeArrayDims TestBlas.c -lmwblas
  Building with 'Xcode with Clang'.
  /Users/jhess/Dropbox/Uni/E-Technik Master/Forschungspraxis Machine Learning/Matlab/TestBlas.c:9:23: warning: passing 'mwSize *' (aka 'unsigned long *') to parameter of type 'const ptrdiff_t *' (aka 'const long *') converts between pointers to integer types with different sign [-Wpointer-sign]
      double sum = ddot(&n,L,&one,L,&one);
                        ^~
  /Applications/MATLAB_R2015a.app/extern/include/blas.h:559:22: note: passing argument to parameter 'n' here
      const ptrdiff_t *n,
                       ^
  /Users/jhess/Dropbox/Uni/E-Technik Master/Forschungspraxis Machine Learning/Matlab/TestBlas.c:9:28: warning: incompatible pointer types passing 'int *' to parameter of type 'const ptrdiff_t *' (aka 'const long *') [-Wincompatible-pointer-types]
      double sum = ddot(&n,L,&one,L,&one);
                             ^~~~
  /Applications/MATLAB_R2015a.app/extern/include/blas.h:561:22: note: passing argument to parameter 'incx' here
      const ptrdiff_t *incx,
                       ^
  /Users/jhess/Dropbox/Uni/E-Technik Master/Forschungspraxis Machine Learning/Matlab/TestBlas.c:9:35: warning: incompatible pointer types passing 'int *' to parameter of type 'const ptrdiff_t *' (aka 'const long *') [-Wincompatible-pointer-types]
      double sum = ddot(&n,L,&one,L,&one);
                                    ^~~~
  /Applications/MATLAB_R2015a.app/extern/include/blas.h:563:22: note: passing argument to parameter 'incy' here
      const ptrdiff_t *incy
                       ^
  3 warnings generated.

  MEX completed successfully.

这似乎是我将错误的变量类型（或指针类型？）传递给 BLAS 函数，但我不知道如何解决它。有人可以看看吗？ 非常感谢！

Answer 1

如果我的怀疑是正确的，您只需定义代码中涉及的整数的正确类型：

  #include <mex.h>
  #include <math.h>
  #include <blas.h>

  void TestBlas(double *L, double *R, mwSignedIndex n) //changed
  {

      mwSignedIndex one = 1; //changed
      double sum = ddot(&n,L,&one,L,&one);

      //Output Matrix R not defined, just for test purposes...

  } 

  void mexFunction( int nlhs, mxArray *plhs[],
                    int nrhs, const mxArray *prhs[])
  {
      double *inMatrix;               /* 1xN input matrix */
      size_t ncols;                   /* size of matrix */
      long *ncolsPr;                  /* output matrix */
      double *outMatrix;              /* output matrix */

      inMatrix = mxGetPr(prhs[0]);
      ncols = mxGetN(prhs[0]);

      /* create the output matrix */
      plhs[0] = mxCreateDoubleMatrix((mwSize)1,(mwSize)ncols,mxREAL); //changed
      outMatrix = mxGetPr(plhs[0]);

      TestBlas(inMatrix,outMatrix,(mwSignedIndex)ncols); //changed
  }

根据我的经验，您必须特别注意函数调用中出现的文字常量，因此请注意 1 到 (mwSize) 的转换，这是 mxCreateDoubleMatrix 所期望的。