Skip to main content


You can add the @unroll decorator on any loop (such as for and while) to make the Mojo compiler unroll the loop, either fully or with a given unroll factor.

For example, the compiler will unroll all 10 iterations of the following loop into 10 consecutive calls to print() (removing the for loop entirely):

for i in range(10):

The decorator also accepts an "unroll factor" argument, which specifies how many iterations to unroll at once. For example, the unroll above is equivalent to @unroll(10) because it unrolls all iterations of the loop. So if you pass a number smaller than the loop bounds, the compiler creates multiple unrolls. For example:

# Unroll every 2 iterations, leaving a loop with 5 iterations.
for i in range (10):

The result is equivalent to this:

for i in range(0, 10, 2):

However, the compiler can unroll a loop only when the following statements are true:

  • The loop's lower bound, upper bound, and induction step size are compile-time constants (they do not vary at runtime). For example, in the above code range(0, 10, 2), 0 is the lower bound, 10 is the upper bound, and 2 is the induction step size—these could instead be defined with variable names, but the values cannot vary at runtime.

  • Likewise, there are no early exits in the loop that make the loop count variable at runtime.

Compared to unroll()

The Mojo standard library also includes a function called unroll() that unrolls a given function that you want to call repeatedly, but has some important differences when compared to the @unroll decorator:

  • The @unroll decorator operates on loop expressions only, not on functions like the unroll() function does.

  • The @unroll decorator determines how to unroll the loop based on the induction variable (i), the value of which is not known when compilation begins. Whereas, the unroll() function calls upon your looping function (func) with the Int loop index parameter that is known at compile time.

    This means two things:

    • Within a loop using the @unroll decorator, the i induction variable is still a runtime variable, so you cannot use it as a parameter value (such as for SIMD[Int8, i]). Whereas, within the func callback used with the unroll() function, the Int loop index is known at compile time, so you can use it as a parameter value.

    • The unroll() function unrolls at the beginning of compilation, which might explode the program size that still needs to be compiled, depending on the amount of code that's unrolled. Whereas, the @unroll decorator performs unrolling later in the compilation, after the compiler is able to evaluate the induction variable (i), which avoids early explosion of the program size that still needs compilation.