Loops are the heart of nearly all high performance programs. The two boxes in [Figure 4] illustrate how the first few references to A and B look superimposed upon one another in the blocked and unblocked cases. In the code below, we have unrolled the middle (j) loop twice: We left the k loop untouched; however, we could unroll that one, too. Second, when the calling routine and the subroutine are compiled separately, its impossible for the compiler to intermix instructions. For example, in this same example, if it is required to clear the rest of each array entry to nulls immediately after the 100 byte field copied, an additional clear instruction, XCxx*256+100(156,R1),xx*256+100(R2), can be added immediately after every MVC in the sequence (where xx matches the value in the MVC above it). Change the unroll factor by 2, 4, and 8. Thats bad news, but good information. For many loops, you often find the performance of the loops dominated by memory references, as we have seen in the last three examples. 6.5. Loop Unrolling (unroll Pragma) - Intel Unfortunately, life is rarely this simple. Again, operation counting is a simple way to estimate how well the requirements of a loop will map onto the capabilities of the machine. The most basic form of loop optimization is loop unrolling. Other optimizations may have to be triggered using explicit compile-time options. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Address arithmetic is often embedded in the instructions that reference memory. Can we interchange the loops below? The manual amendments required also become somewhat more complicated if the test conditions are variables. This article is contributed by Harsh Agarwal. The tricks will be familiar; they are mostly loop optimizations from [Section 2.3], used here for different reasons. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, Fastest way to determine if an integer's square root is an integer. The inner loop tests the value of B(J,I): Each iteration is independent of every other, so unrolling it wont be a problem. This usually occurs naturally as a side effect of partitioning, say, a matrix factorization into groups of columns. Unrolling the outer loop results in 4 times more ports, and you will have 16 memory accesses competing with each other to acquire the memory bus, resulting in extremely poor memory performance. It is easily applied to sequential array processing loops where the number of iterations is known prior to execution of the loop.
Kos Pembedahan Polip Hidung, Articles L