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 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:
intp=10;for(inti=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(inti=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]
Apparently the term "peeling" 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. [4] [5]