At optimization level /optimize:3 or above, Visual Fortran attempts to unroll certain innermost loops, minimizing the number of branches and grouping more instructions together to allow efficient overlapped instruction execution (instruction pipelining). The best candidates for loop unrolling are innermost loops with limited control flow.
As more loops are unrolled, the average size of basic blocks increases. Loop unrolling generates multiple copies of the code for the loop body (loop code iterations) in a manner that allows efficient instruction pipelining.
The loop body is replicated some number of times, substituting index expressions. An initialization loop might be created to align the first reference with the main series of loops. A remainder loop might be created for leftover work.
The number of times a loop is unrolled can be determined either by the optimizer or by using the /unroll option, which can specify the limit for loop unrolling. Unless the user specifies a value, the optimizer optimizer will choose an unroll amount that minimizes the overhead of prefetching while also limiting code size expansion.
Array operations are often represented as a nested series of loops when expanded into instructions. The innermost loop for the array operation is the best candidate for loop unrolling (like DO loops). For example, the following array operation (once optimized) is represented by nested loops, where the innermost loop is a candidate for loop unrolling:
A(1:100,2:30) = B(1:100,1:29) * 2.0