And if the subroutine being called is fat, it makes the loop that calls it fat as well. If we could somehow rearrange the loop so that it consumed the arrays in small rectangles, rather than strips, we could conserve some of the cache entries that are being discarded. Lets look at a few loops and see what we can learn about the instruction mix: This loop contains one floating-point addition and three memory references (two loads and a store). First, once you are familiar with loop unrolling, you might recognize code that was unrolled by a programmer (not you) some time ago and simplify the code. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, please remove the line numbers and just add comments on lines that you want to talk about, @AkiSuihkonen: Or you need to include an extra. Because the computations in one iteration do not depend on the computations in other iterations, calculations from different iterations can be executed together. If the compiler is good enough to recognize that the multiply-add is appropriate, this loop may also be limited by memory references; each iteration would be compiled into two multiplications and two multiply-adds. This is in contrast to dynamic unrolling which is accomplished by the compiler. Lets revisit our FORTRAN loop with non-unit stride. For example, if it is a pointer-chasing loop, that is a major inhibiting factor. Significant gains can be realized if the reduction in executed instructions compensates for any performance reduction caused by any increase in the size of the program. Small loops are expanded such that an iteration of the loop is replicated a certain number of times in the loop body. However, I am really lost on how this would be done. Loop unrolling is a technique for attempting to minimize the cost of loop overhead, such as branching on the termination condition and updating counter variables. It has a single statement wrapped in a do-loop: You can unroll the loop, as we have below, giving you the same operations in fewer iterations with less loop overhead. This occurs by manually adding the necessary code for the loop to occur multiple times within the loop body and then updating the conditions and counters accordingly. Inner loop unrolling doesnt make sense in this case because there wont be enough iterations to justify the cost of the preconditioning loop. AWS Graviton3 delivers leading AES-GCM encryption performance You have many global memory accesses as it is, and each access requires its own port to memory. Before you begin to rewrite a loop body or reorganize the order of the loops, you must have some idea of what the body of the loop does for each iteration. You can imagine how this would help on any computer. Loop tiling splits a loop into a nest of loops, with each inner loop working on a small block of data. Others perform better with them interchanged. This code shows another method that limits the size of the inner loop and visits it repeatedly: Where the inner I loop used to execute N iterations at a time, the new K loop executes only 16 iterations. A programmer who has just finished reading a linear algebra textbook would probably write matrix multiply as it appears in the example below: The problem with this loop is that the A(I,K) will be non-unit stride.
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