Documentation

## Unroll `for`-Loops

When the code generator unrolls a `for`-loop, instead of producing a `for`-loop in the generated code, it produces a copy of the loop body for each iteration. For small, tight loops, unrolling can improve performance. However, for large loops, unrolling can significantly increase code generation time and generate inefficient code.

### Force Loop Unrolling by Using `coder.unroll`

The code generator uses heuristics to determine when to unroll a `for`-loop. To force loop unrolling, use `coder.unroll`. This affects only the `for` loop that is immediately after `coder.unroll`. For example:

```function z = call_myloop() %#codegen z = myloop(5); end function b = myloop(n) b = zeros(1,n); coder.unroll(); for i = 1:n b(i)=i+n; end end```

Here is the generated code for the for-loop:

``` z = 6.0; z = 7.0; z = 8.0; z = 9.0; z = 10.0;```

To control when a `for`-loop is unrolled, use the `coder.unroll` `flag` argument. For example, unroll the loop only when the number of iterations is less than 10.

```function z = call_myloop() %#codegen z = myloop(5); end function b = myloop(n) unroll_flag = n < 10; b = zeros(1,n); coder.unroll(unroll_flag); for i = 1:n b(i)=i+n; end end```

To unroll a `for`-loop, the code generator must be able to determine the bounds of the `for`-loop. For example, code generation fails for the following code because the value of `n` is not known at code generation time.

```function b = myloop(n) b = zeros(1,n); coder.unroll(); for i = 1:n b(i)=i+n; end end```