Loop unrolling software pipelining

When compiling with mw, the information also contains a copy of the single scheduled iteration. Vliw, software pipelining, and limits to ilp professor david a. Scheduling for loops loop unrolling software pipelining interaction with register allocation hardware vs. Consider a loop containing instructions with a high latency. Alternatively, loop unrolling can be avoided thanks to software register renaming. As a hazard, though, unrolling increases program size, both in its ir form and in its. Software pipelining symbolic loop unrolling software. Algorithms to achieve software pipelining generally fall into two basic categories. It does not unroll the loop enough to eliminate the redundancies completely because the extra unrolling, like too much loop fusion, can have a negative effect on software pipelining. So, in prefetching, as in all its other optimizations, the lno tries to achieve a delicate balance. D55710 add pragmas to control software pipelining optimisation. Software pipelining is applied to a restricted set of loops, namely those containing a single fortran statement. Loop unrolling, also known as loop unwinding, is a loop transformation technique that attempts to optimize a programs execution speed at the expense of its binary size, which is an approach known as spacetime tradeoff. Tomasulo prevents register as bottleneck avoids war, waw hazards of scoreboard allows loop unrolling in hw not limited to basic blocks provided branch.

Schedule the code efficiently try to use the least number of cycles, but do not use software pipelining or loop unrolling. This is achieved through the insertion of move operations, but this may increase the initiation interval ii which nullifies the benefits of software pipelining. For example a loop might issue a load not for use in the present iteration but the next, thereby allowing extra cycles for the data to arrive from memory. Unrollbased copy elimination for enhanced pipeline scheduling. Ajit pal,department of computer science and engineering,iit kharagpur. Loop unrolling and sw pipelining consider a basic inorder pipeline with bypassing one instruction in each pipeline stage in any cycle.

Although this technique greatly increases performance by exposing ilp within loops, it is. Please do the following four questions based on th. If you consider that unrolling times the vector register widths of up to 16 for 32bit data, the total effective unrolling rivals what was needed for. Software pipelining is often used in combination with loop unrolling, and this combination of techniques is often a far better optimization than. Both units are pipelined initiate one op each cycle. We provide a formal problem definition for each scenario, and we propose and study a dedicated algorithm for each problem. In the general case, loop unrolling may not be the best way to implement software pipelining. Unlike unrolling, software pipelining can give optimal result.

Lastly, weiss and smith compared the results of using loop unrolling and software pipelining to generate scalar code for the cray1s architecture 31. To avoid stalls, a dependent instruction must be separated from the source instruction by a distance in clock cycles equal to the pipeline latency of that source instruction. Basic instruction scheduling and software pipelining. Unrolling loops several times and performing global. Start with minimum ii, schedule the body of the loop using. In the example above, we could write the code as follows assume for the moment that bignumber is divisible by 3. Software pipelining overlaps successive basic blocks from successive iterations of an innennost loop. Can fill arbitrarily long pipelines with infinitely many iterations 15745. Compiler techniques for parallelism loop unrolling. The problem of minimal loop unrolling arises either before or after software pipelining, either with a single or with multiple register types classes. The transformation can be undertaken manually by the programmer or by an optimizing compiler. Citeseerx on the effectiveness of register moves to. Software pipelining for i1, i pipelining all decode ia32 instructions into microoperations mips like instructions since it makes pipelining easier. Take one instruction from each of several iterations of the loop software overlapping of loop iterations today will show hardware.

Pipeline complications cs510 computer architectures lecture 8 3 and instruction level parallelism advanced pipelining advanced pipelining and instruction level parallelism and instruction level parallelism loop unrolling control stalls basic pipeline scheduling raw stalls dynamic scheduling with scoreboarding raw stalls dynamic scheduling with register renaming war and waw stalls dynamic. Possibly moving instructions from one iteration to the previous or the next iteration. Computer science 146 computer architecture lecture outline. Notice that, compared to unrolling, loop pipelining does not significantly increase the resource consumption of our design, indeed, with pipelining we are simply making a better use underutilized hardware resources. Multithreading which ones require a great number of. It should consider not only threadlevel parallelism, but the memory hierarchy not only unrolling, but cache tiling and register tiling, software pipelining and so forth. Usually hardware techniques require compiler support.

Every loop unrolling iteration requires pipeline to fill and. Software pipelining of nested loops for realtime dsp applications jian wang speech recognition software nortel montrcal lab. Software pipelining is often used in combination with loop unrolling, and this combination of techniques is often a far better optimization than loop unrolling alone. Pdf improving software pipelining with unrollandjam. While several articles have addressed software pipelining of single nonnested loops, little work has been done in the area of applying it to nested loops. Software pipelining approaches considering loop unrolling explore the points shown in the arrow represented on the line koptk.

Approaches to software pipelining iterative modulo scheduling similar to list scheduling, computes priorities and uses. Unlike unrolling, software pipelining can give optimal result with small code size blowup locally compacted code may not be globally optimal doall. Loop unrolling and software pipelining 1 combined with loop jamming was researched by carr et al. Compiler induction variable recognition loop invariant code motion 5. For readers who have other favorite loop optimizations, like unrolling or fusion, we note that pipelining is a complement to those other tools, not a. It works for loops with arbitrary control flow that eps must deal with, as well as for. It is for this reason that many optimizers only perform software pipelining for loops with constant bounds.

Lecture 19 software pipelining carnegie mellon university. Software pipelining of nested loops for realtime dsp. Very little or no overlap across iterations of the loop y. Symbolic loop unrolling software pipelining is a technique for reorganizing loops such that each itera tion in the softwarepipelined code is made from instructions chosen from dif ferent iterations of the original loop. Complex instructions requiring many cycles are executed by standard microprogrammed control. See the article on loop unrolling for more on solutions to this problem, but note that software pipelining prevents the use of duffs device. Additionally, the compiler may automatically perform loop unrolling. With compilers oriented to instructionlevel parallel. Loop unrolling software pipelining modulo scheduling trace scheduling predication ece 463521, profs. With software pipelining a reorganized loop contains instructions from different iterations of the original loop. It also increases code size in the new loop body, which may increase pressure on register allocation, cause register spilling. The basic concepts of loop pipelining and loop unrolling and example codes to apply these techniques are shown and the limiting factors to achieve optimal performance using these techniques are discussed.

Often, a test must be performed beforehand which jumps to an alternative, non software pipelined version of the loop in these cases. Loop unrolling benefits cpi part 1 georgia tech hpca. Loop unrolling and distributed pipelining moves the output registers at the design level to reduce the amount of combinatorial logic, thus increasing clock speed. Ppt loop unrolling powerpoint presentation free to. This article aims at tightly controling the postpass loop unrolling necessary to generate code. With vivado hls we can use the hls pipeline pragma within the loop that we wish to pipeline. Software pipelining is an is technique that reorders the instructions in a loop. Loop unrolling exposes instruction level parallelism for instruction scheduling and software pipelining and thus can improve a programs performance. Optk is the optimal unrolling degree ud of the loop. It should be noted, however, that software pipelining prevents the use of duffs device, a widely known and efficient solution to this problem. Look how complex the atlas project is, which provides an optimized version of a blas library based primarily on matrix multiplication.

Vliw, software pipelining, and limits to ilp eecs at uc. Software pipelining loop unrolling 100 iterations 25 loops with 4 unrolled iterations each. Symbolic loop unrolling to get most from pipeline with little code expansion, little overhead. Optimal exploration of the unrolling degree for software. Inst int op branch mem op fpint op fp add fp mul 1 flw f1, 0x1 2 add x7, x5, x7 lw x9, 0x2 3 addi x1, x1, 4. Software pipelining consists of scheduling instructions around the branch point in a loop. Ebcioglu proposed a software pipelining algorithm to generate code for a hypothetical machine with infinitely many hardware resouruzs 7. Dynamic branch prediction, superscalar, vliw, and software pipelining professor randy h. Loop unrolling increase instructions between loop branch by replicating loop body multiple times software pipelining reorganize loop code such that each iteration contains code chosen from different iterations trace scheduling increase parallelism by selecting more code candidates.

In computer science, software pipelining is a technique used to optimize loops, in a manner that. Both loop pipelining and loop unrolling improve the hardware functions performance by exploiting the parallelism between loop iterations. Consider a basic inorder pipeline with bypassing one instruction in each pipeline stage in any cycle. Software pipelining allows iterations to be overlapped. Their approach performs unrollandjam to increase the instructionlevel parallelism of. Distributed pipelining for clock speed optimization. This paper solves the problem of finding the minimum iteration initiation interval in the absence of resource constraints for each level of a nested loop. In other words, at most one interiteration data dependency relationship can be present in the flow graph.

Ramakrishna rau loops hewlettpackard laboratories, 1501 page mill road, bldg. The softwarepipelined loop information includes the source lines from which the loop originates, a description of the resource and latency requirements for the loop, and whether the loop was unrolled among other information. Two techniques for extracting ilp are software pipelining 6, 7 and superblock scheduling 8, 9, 10. Also, as was touched upon above, its typically useful to add some software loop unrolling, e. A method for applying loop unrolling and software pipelining to. Enhanced loop flattening for software pipelining of. Loop unrolling increases the programs speed by eliminating loop control instruction and loop test instructions. To keep a pipeline full, parallelism among instructions must be exploited by finding sequences of unrelated instructions that can be overlapped in the pipeline. The article solves the problem of finding the minimum iteration initiation interval in the absence of resource constraints for each level of a nested loop. While several papers have addressed software pipelining of inner loops, little work has been done in the area of extending it to nested loops. Loop unrolling is a loop transformation technique that helps to optimize the execution time of a program.