# Loop splitting

Loop splitting is a compiler optimization technique. It attempts to simplify a loop or eliminate dependencies by breaking it into multiple loops which have the same bodies but iterate over different contiguous portions of the index range.

## Loop peeling

Loop peeling is a special case of loop splitting which splits any problematic first (or last) few iterations from the loop and performs them outside of the loop body.

Suppose a loop was written like this:

``` int p = 10;
for (int i=0; i<10; ++i)
{
y[i] = x[i] + x[p];
p = i;
}
```

Notice that `p = 10` only for the first iteration, and for all other iterations, `p = i - 1`. A compiler can take advantage of this by unwinding (or "peeling") the first iteration from the loop.

After peeling the first iteration, the code would look like this:

``` y[0] = x[0] + x[10];
for (int i=1; i<10; ++i)
{
y[i] = x[i] + x[i-1];
}
```

This equivalent form eliminates the need for the variable `p` inside the loop body.

Loop peeling was introduced in gcc in version 3.4. More generalised loop splitting was added in GCC 7.[1]

## Brief history of the term

Apparently the term was for the first time used by Cannings, Thompson and Skolnick[2] in their 1976 paper on computational models for (human) inheritance. There the term was used to denote a method for collapsing phenotypic information onto parents. From there the term was used again in their papers, including their seminal paper on probability functions on complex pedigrees.[3]

In compiler technology, the term first turned up in late 1980s papers on VLIW and superscalar compilation, including [4] and.[5]

## References

1. ^ GCC 7 Release Series — Changes, New Features, and Fixes - GNU Project
2. ^ Cannings, C.; Thompson, E. A.; Skolnick, H. H. (1976). "The recursive derivation of likelihoods on complex pedigrees". Advances in Applied Probability. 8 (4): 622–625. doi:10.2307/1425918. JSTOR 1425918.
3. ^ Cannings, C.; Thompson, E. A.; Skolnick, H. H. (1978). "Probability functions on complex pedigrees". Advances in Applied Probability. 10 (1): 26–61. doi:10.2307/1426718. JSTOR 1426718.
4. ^ Callahan, D.; Kennedy, Ken (1988). "Compiling Programs for Distributed-memory Multiprocessors". The Journal of Supercomputing. 2 (2): 151–169. doi:10.1007/BF00128175. S2CID 10214341.
5. ^ Mahlke, S. A.; Lin, D. C.; Chen, W. Y.; Hank, R. E.; Bringman, R. A. (1992). Effective compiler support for predicated execution using the hyperblock. 25th Annual International Symposium on Microarchitecture. pp. 45–54.