Specify Array Layout in a Function

For an example of a specialized function, consideraddMatrixRM:

function[S] = addMatrixRM(A,B)%#codegenS = zeros(size(A)); coder.rowMajor;% specify row-major codeforrow = 1:size(A,1)forcol = 1:size(A,2) S(row,col) = A(row,col) + B(row,col);endend

ForMATLAB Coder, you can generate code foraddMatrixRMby using thecodegencommand.

codegenaddMatrixRM-args{ones(20,10),ones(20,10)}-config:lib-launchreport

Because of thecoder.rowMajorcall, the code generator produces code that uses data stored in row-major layout.

Other functions called from a row-major function or column-major function inherit the same array layout. If a called function has its own distinctcoder.rowMajororcoder.columnMajorcall, the local call takes precedence.

You can mix column-major and row-major functions in the same code. The code generator inserts transpose or conversion operations when passing data between row-major and column-major functions. These conversion operations ensure that array elements are stored as required by functions with different array layout specifications. For example, the inputs to a column-major function, called from a row-major function, are converted to column-major layout before being passed to the column-major function.

Query Array Layout of a Function

To query the array layout of a function at compile time, usecoder.isRowMajororcoder.isColumnMajor. This query can be useful for specializing your generated code when it involves row-major and column-major functions. For example, consider this function:

function[S] = addMatrixRouted(A,B)ifcoder.isRowMajor%execute this code if row-majorS = addMatrixRM(A,B);elseifcoder.isColumnMajor%execute this code if column-majorS = addMatrix_OptimizedForColumnMajor(A,B);end

This function behaves differently depending on whether it is row-major or column-major. WhenaddMatrixRoutedis row-major, it calls theaddMatrixRMfunction, which has efficient memory access for row-major data. When the function is column-major, it calls a version of theaddMatrixRMfunction optimized for column-major data.

For example, consider this function definition. The algorithm iterates through the columns in the outer loop and the rows in the inner loop, in contrast to theaddMatrixRMfunction.

function[S] = addMatrix_OptimizedForColumnMajor(A,B)%#codegenS = zeros(size(A));forcol = 1:size(A,2)forrow = 1:size(A,1) S(row,col) = A(row,col) + B(row,col);endend

Code generation for this function yields:

... /* column-major layout */ for (col = 0; col < 10; col++) { for (row = 0; row < 20; row++) { S[row + 20 * col] = A[row + 20 * col] + B[row + 20 * col]; } } ...

The generated code has a stride length of only one element. Due to the specializing queries, the generated code foraddMatrixRoutedprovides efficient memory access for either choice of array layout.

Specify Array Layout in a Class

You can specify array layout for a class so that object property variables are stored with a specific array layout. To specify the array layout, place acoder.rowMajororcoder.columnMajorcall in the class constructor. If you assign an object with a specified array layout to the property of another object, the array layout of the assigned object takes precedence.

Consider the row-major classrowMatsas an example. This class contains matrix properties and a method that consists of an element-wise addition algorithm. The algorithm in the method performs more efficiently for data stored in row-major layout. By specifyingcoder.rowMajorin the class constructor, the generated code uses row-major layout for the property data.

classdefrowMatsproperties(Access = public) A; B; C;endmethodsfunctionobj = rowMats(A,B) coder.rowMajor;ifnargin == 0 obj.A = 0; obj.B = 0; obj.C = 0;elseobj.A = A; obj.B = B; obj.C = zeros(size(A));endendfunctionobj = add(obj)forrow = 1:size(obj.A,1)forcol = 1:size(obj.A,2) obj.C(row,col) = obj.A(row,col) + obj.B(row,col);endendendendend

Use the class in a simple functiondoMath. The inputs and outputs of the entry-point function must all use the same array layout.

function[out] = doMath(in1,in2)%#codegenout = zeros(size(in1)); myMats = rowMats(in1,in2); myMats = myMats.add; out = myMats.C;end

ForMATLAB Coder, you can generate code by entering:

A = rand(20,10); B = rand(20,10); cfg = coder.config('lib'); codegen-configcfgdoMath-args{A,B}-launchreport

With default settings, the code generator assumes that the entry-point function inputs and outputs use column-major layout, because you do not specify row-major layout for the functiondoMath. Therefore, before calling the class constructor, the generated code convertsin1andin2to row-major layout. Similarly, it converts thedoMathfunction output back to column-major layout.

When designing a class for a specific array layout, consider: