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:
<syntaxhighlight lang="c">
int p = 10;
for (int i = 0; i < 10; ++i) {
y[i] = x[i] + x[p];
p = i;
}
</syntaxhighlight>
Notice that <code>p = 10</code> only for the first iteration, and for all other iterations, <code>p = i - 1</code>. 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:
<syntaxhighlight lang="c">
y[0] = x[0] + x[10];
for (int i = 1; i < 10; ++i) {
y[i] = x[i] + x[i - 1];
}
</syntaxhighlight>
This equivalent form eliminates the need for the variable <code>p</code> inside the loop body.
Loop peeling was introduced in gcc in version 3.4. More generalised loop splitting was added in GCC 7.