Loop Peeling

Loop Peeling

Loop peeling is a type of loop unrolling that involves unraveling only the first few iterations of a long loop. This is also similar to “loop splitting” with two sections, where the first section is over the early range, and the second range is the main section of all remaining iterations.

Loop peeling is beneficial to the overall loop efficiency if there is code in the loop body that is only required for one or two early iterations, which can then be removed from the main loop body. Similarly, there can be benefit in unraveling the last few iterations of a loop, which is a similar technique.

One common case of loop peeling is when the first iteration is different from the rest, so peeling off a single iteration is valuable.

    for (int i = 0; i < n; i++) {
        arr[i] = (i == 0) ? 0.0f : 1.0f;
    }

In this case, we can peel off the first “i==0” iteration into a single unrolled instruction, and change the main loop to start at 1. This is also a trivial form of “loop distribution,” where we are hoisting an “if” conditional test out of the loop. The new code becomes:

    arr[0] = 0.0f;  // Peeled
    for (int i = 1 /*not 0*/ ; i < n; i++) {
        arr[i] = 1.0f;
    }

This peeled version is faster in terms of both sequential or parallel execution. The loop body has less computation and is also more amenable to vectorization.

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