Documentation

Unrolling for-loops eliminates the loop logic by creating a separate copy of the loop body in the generated code for each iteration. Within each iteration, the loop index variable becomes a constant.

You can also force loop unrolling for individual functions by wrapping the loop header in a coder.unroll function. For more information, see coder.unroll.

Limit Copying the for-loop Body in Generated Code

To limit the number of times that you copy the body of a for-loop in generated code:

1. Write a MATLAB^® function getrand(n) that uses a for-loop to generate a vector of length n and assign random numbers to specific elements. Add a test function test_unroll. This function calls getrand(n) with n equal to values both less than and greater than the threshold for copying the for-loop in generated code.

   function [y1, y2] = test_unroll() %#codegen
   % The directive %#codegen indicates that the function
   % is intended for code generation
     % Calling getrand 8 times triggers unroll
     y1 = getrand(8);
     % Calling getrand 50 times does not trigger unroll
     y2 = getrand(50);
    
   function y = getrand(n)
     % Turn off inlining to make 
     % generated code easier to read
     coder.inline('never');
   
     % Set flag variable dounroll to repeat loop body
     % only for fewer than 10 iterations
     dounroll = n < 10;
     % Declare size, class, and complexity
     % of variable y by assignment
     y = zeros(n, 1);
     % Loop body begins
     for i = coder.unroll(1:2:n, dounroll)
         if (i > 2) && (i < n-2) 
             y(i) = rand();
         end;
     end;
     % Loop body ends

2. Disable support for

   In the default output folder, codegen/lib/test_unroll, generate C static library code for test_unroll:

   codegen -config:lib test_unroll

   In test_unroll.c, the generated C code for getrand(8) repeats the body of the for-loop (unrolls the loop) because the number of iterations is less than 10:

   static void getrand(double y[8])
   {
     /*  Turn off inlining to make  */
     /*  generated code easier to read */
     /*  Set flag variable dounroll to repeat loop body */
     /*  only for fewer than 10 iterations */
     /*  Declare size, class, and complexity */
     /*  of variable y by assignment */
     memset(&y[0], 0, sizeof(double) << 3);
   
     /*  Loop body begins */
     y[2] = b_rand();
     y[4] = b_rand();
   
     /*  Loop body ends */
   }
   

   The generated C code for getrand(50) does not unroll the for-loop because the number of iterations is greater than 10:

   static void b_getrand(double y[50])
   {
     int i;
     int b_i;
   
     /*  Turn off inlining to make  */
     /*  generated code easier to read */
     /*  Set flag variable dounroll to repeat loop body */
     /*  only for fewer than 10 iterations */
     /*  Declare size, class, and complexity */
     /*  of variable y by assignment */
     memset(&y[0], 0, 50U * sizeof(double));
   
     /*  Loop body begins */
     for (i = 0; i < 25; i++) {
       b_i = (i << 1) + 1;
       if ((b_i > 2) && (b_i < 48)) {
         y[b_i - 1] = b_rand();
       }
     }