Developer Reference for Intel® oneAPI Math Kernel Library for C

ID 766684
Date 3/22/2024
Public

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Matrix Layout for LAPACK Routines

There are two general methods of storing a two dimensional matrix in linear (one dimensional) memory: column-wise (column major order) or row-wise (row major order). Consider an M-by-N matrix A:

Column Major Layout

In column major layout the first index, i, of matrix elements ai,j changes faster than the second index when accessing sequential memory locations. In other words, for 1 i < M, if the element ai,j is stored in a specific location in memory, the element ai+1,j is stored in the next location, and, for 1 j < N, the element aM,j is stored in the location previous to element a1,j+1. So the matrix elements are located in memory according to this sequence:

{a1,1a2,1 ... aM,1a1,2a2,2 ... aM,2 ... ... a1,Na2,N ... aM,N}

Row Major Layout

In row major layout the second index, j, of matrix elements ai,j changes faster than the first index when accessing sequential memory locations. In other words, for 1 j < N, if the element ai,j is stored in a specific location in memory, the element ai,j+1 is stored in the next location, and, for 1 i < M, the element ai,N is stored in the location previous to element ai+1,1. So the matrix elements are located in memory according to this sequence:

{a1,1a1,2 ... a1,Na2,1a2,2 ... a2,N ... ... aN,1aN,2 ... aM,N}

Leading Dimension Parameter

A leading dimension parameter allows use of LAPACK routines on a submatrix of a larger matrix. For example, the submatrix B can be extracted from the original matrix A defined previously:

B is formed from rows with indices i0 + 1 to i0 + K and columns j0 + 1 to j0 + L of matrix A. To specify matrix B, LAPACK routines require four parameters:

  • the number of rows K;

  • the number of columns L;

  • a pointer to the start of the array containing elements of B;

  • the leading dimension of the array containing elements of B.

The leading dimension depends on the layout of the matrix:

  • Column major layout

    Leading dimension ldb=M, the number of rows of matrix A.

    Starting address: offset by i0 + j0*ldb from a1,1.

  • Row major layout

    Leading dimension ldb=N, the number of columns of matrix A.

    Starting address: offset by i0*ldb + j0 from a1,1.