Vliw compilation techniques we will explore several compiler techniques that are critical for achieving highperformance on vliw processors note that some of these techniques can help improve performance on traditional processors too. The basic way it operates is that if we have for the sake of a simple example a loop that assigns 16 array elements thus. Software pipelining is often used in combination with loop unrolling, and this. 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. Introduction to software pipelining in the ia64 architecture. This paper solves an open problem regarding loop unrolling after periodic register allocation. Ece 4750 computer architecture, fall 2016 t15 advanced. Often, a test must be performed beforehand which jumps to an alternative, non software pipelined version of the loop in these cases. The transformation can be undertaken manually by the programmer or by an optimizing compiler. Consider a loop containing instructions with a high latency. Both loop pipelining and loop unrolling improve the hardware functions performance by exploiting the parallelism between loop iterations. Loop unrolling software pipelining interaction with register allocation hardware vs. A method for applying loop unrolling and software pipelining to. This combination has been shown to be remarkably effective in practice.
Additionally, the compiler may automatically perform loop unrolling. Lecture 19 software pipelining carnegie mellon university. Software pipelining, which really has nothing to do with hardware pipelining, is a loop optimization technique to make. Compiler induction variable recognition loop invariant code motion 5. I wouldnt care to pontificate on the reasons, but multithreading support has been relatively weak in conjunction with software pipelining. Loop unrolling increases the programs speed by eliminating loop control instruction and loop test instructions. The action of loop stream detection and microop caching also helps further in keeping the pipeline full across iterations of the unrolled loop. In other words, at most one interiteration data dependency relationship can be present in the flow graph. In the example above, we could write the code as follows assume for the moment that bignumber is divisible by 3. Loop unrolling and distributed pipelining moves the output registers at the design level to reduce the amount of combinatorial logic, thus increasing clock speed. An effective scheduling technique for vliw machines monica s. Software pipelining enables some loop iterations to run at top speed by eliminating raw hazards that create latencies within iteration. The algorithm also suggests the optimal unroll factor to be used, but the main difference between our ap. Software pipelining, which really has nothing to do with hardware pipelining, is a loop optimization technique to make statements within an iteration independent of each other.
Quickly get up to peak throughput, one epilogueprologue per loop software pipelining does not reduce loop overhead. Graduate computer architecture chapter 4 explore instruction. Possibly moving instructions from one iteration to the previous or the next iteration. Enhanced loop flattening for software pipelining of. Us5950007a method for compiling loops containing prefetch. Software pipelining software pipelining is an is technique that reorders the instructions in a loop. Software pipelining is a family of compiler methods for scheduling and resource allocation that exploit the fact that while the number of parallel operations available in a single iteration of a loop is often limited, operations from later iterations can be executed before earlier iterations have. Alternatively, loop unrolling can be avoided thanks to software register renaming. Tiling matrixmatrix multiply, code tuning david bindel 1 feb 2010. Vliw, software pipelining, and limits to ilp people. On the e ectiveness of register moves to minimise post. Pdf graph traverse software pipelining researchgate. For amortizing the cost of the loop counter and jump funrollloops works fine with gcc. Each task in the pipeline is similar to an iteration in a loop.
On the c6000 variants c62x, c67x, and c64x, software pipelining is completely disabled when code size flags ms2 and ms3 see c6000 compiler. The main difference is that tasks in their pipelines are. Computer science 146 computer architecture lecture outline. Unlike unrolling, software pipelining can give optimal result. Loop unrolling can then be used to amortize the cost of the scalar addition to the counter i and the conditional jump.
The idea is to increase performance by grouping loop statements so that there are less number of loop control instruction and loop test instructions. Assume that the predicatesetting compares have single cycle latency i. See the article on loop unrolling for more on solutions to this problem, but note that software pipelining prevents the use of duffs device. Loop unrolling is a technique that replicates the body of the loop. Very large improvements in running time are possible. Feb 08, 2015 loop unrolling is a program transformation that trades code size for execution speed. Loop unrolling software pipelining is symbolic loop unrolling consumes less code space. In this case, unlike methods 1 and 2, there is not any dependent relationship between the virtual prefetch instruction and memory reference instruction. Subsequently, the software pipelined instruction schedule 106 is obtained by applying the software pipelining to the loop in the software pipelining section 112. The goal of loop unwinding is to increase a programs speed by reducing or eliminating instructions that control the loop, such as pointer arithmetic and end of loop tests on.
Ece 4750 computer architecture, fall 2015 t16 advanced. Advanced computer architecture 06cs81 unit 8 loop level parallelism detection and enhancement static exploitation of ilp use compiler support for increasing parallelism supported by hardware techniques for eliminating some types of dependences applied at compile time no run time support finding parallelism reducing control and data dependencies using speculation. Loop unrolling to keep a pipeline full, parallelism among instructions must be exploited by finding sequences of unrelated instructions that can be overlapped in the pipeline. Lastly, weiss and smith compared the results of using loop unrolling and software pipelining to generate scalar code for the cray1s architecture 31.
As a hazard, though, unrolling increases program size, both in its ir form and in its. Loop unrolling exposes instruction level parallelism for instruction scheduling and software pipelining and thus can improve a programs performance. To keep a pipeline full, parallelism among instructions must be exploited by finding sequences of unrelated instructions that can be overlapped in the pipeline. Vliw, software pipelining, and limits to ilp professor david a. Time taken by loop unrolling vs normal loop geeksforgeeks. So the pipeline able cpu for example risc can execute the unrolled code in parallel. 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. Unlike unrolling, software pipelining can give optimal result with small code size blowup locally compacted code may not be globally optimal doall. Cs 152 computer architecture and engineering cs252.
Look how complex the atlas project is, which provides an optimized version of a blas library based primarily on matrix multiplication. This is achieved through the insertion of moveoperations, but this may increase the initiation interval ii which nulli. Vliw very long instruction word processors instructions are scheduled by the compiler a fixed number of operations are formatted as one big instruction called a bundle usually liw 3 operations today change in the instruction set architecture, i. Both loop pipelining and loop unrolling exploit the parallelism between loop iterations. Loop unrolling time performance time performance loop unrolled software pipelined startup overhead winddown overhead loop iteration loop iteration costs only once per loop, not once per iteration joel emer november 28, 2005 software pipelining. Loop unrolling in vliw unrolled 7 times to avoid delays 7 results in 9 clocks, or 1. 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.
This approach is most easily understood by looking at the scheduled code for the unrolled loop, which appeared in the example on page 78. With compilers oriented to instructionlevel parallel. Every loop unrolling iteration requires pipeline to fill and drain. The goal of loop unwinding is to increase a programs speed by reducing or eliminating instructions that control the loop. Loop unrolling software pipelining is symbolic loop unrolling consumes less code space actually they are targeting different things both. Both flows are supported, and the same optimization methodology is. Also, as was touched upon above, its typically useful to add some software loop unrolling, e. Basic instruction scheduling and software pipelining. Ebcioglu proposed a software pipelining algorithm to generate code for a hypothetical machine with infinitely many hardware resouruzs 7. Loop unrolling is a loop transformation technique that helps to optimize the execution time of a program. 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. Sw pipelining example symbolic loop unrolling less code space overhead paid only once vs.
Software pipelining symbolic loop unrolling software. Loop unrolling time performance time performance loop unrolled software pipelined startup overhead winddown overhead loop iteration loop iteration costs only once per loop, not once per iteration joel emer november 28, 2005 software pipelining pays startupwinddown. What is the average number of cycles per element for this new loop. Compiler id inducti ion variabbll e r recogniti ion loop invariant code motion saman amarasinghe 26 6. Loop unrolling eliminates loop maintenance overhead exposing parallelism between iterations. Scheduling loops loop bodies are small but, lot of time is spend in loops due to large number of iterations. This article aims at tightly controling the postpass loop unrolling necessary to generate code. The following is the same as above, but with loop unrolling implemented at a factor of 4. Try to optimize your code but do not use software pipelining or loop unrolling. Vliw introduction problems with classic vliw objectcode compatibility have to recompile all code for every machine, even for two machines in same generation object code size instruction padding wastes instruction memorycache loop unrollingsoftware pipelining replicates code. Outline scheduling for loops loop unrolling software pipelining interaction with register allocation hardware vs. Software pipelining vs loop unrolling produces more compact code uses less registers can better handle irregularly sized input arrays quickly get up to peak throughput, one epilogueprologue per loop software pipelining does not reduce loop overhead.
Try to optimize your code but do not use software pipelining. Distributed pipelining for clock speed optimization. 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. Complex instructions requiring many cycles are executed by standard microprogrammed control. On the e ectiveness of register moves to minimise postpass. It also increases code size in the new loop body, which may increase pressure on register allocation, cause register spilling. Optimal loop unrolling and shifting for reconfigurable.
Postpass periodic register allocation to minimise loop. The goal is to remove dependencies so that seemingly sequential instructions may be executed in parallel. In computer science, software pipelining is a technique used to optimize loops, in a manner that. Loop unrolling plus software pipelining float s0 signal0, s1 signal1, s2 signal2. However, parallelism between loop iterations is limited by two main factors. Instruction level parallelism dynamic scheduling, multiple issue, and speculation. Sw pipelining symbolic loop unrolling to get most from pipeline with little code expansion, little overhead. As mentioned above, software pipelining is almost always applied in concert with loop unrolling, further increasing the potential for exploiting parallelism. Loop unrolling is a program transformation that trades code size for execution speed. Dynamic branch prediction, superscalar, vliw, and software pipelining professor randy h. Although software pipelining is a powerful technique to extract finegrain parallelism, it generates. 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. Enhanced loop flattening for software pipelining of arbitrary.
Loop unrolling time performance time performance loop unrolled software pipelined startup overhead winddown overhead loop iteration loop iteration software pipelining pays startupwinddown costs only once per loop, not once per iteration. 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. Algorithms to achieve software pipelining generally fall into two basic categories. Symbolic loop unrolling software pipelining is a technique for reorganizing loops such that each iteration in the software pipelined code is made from instructions chosen from different iterations of the original loop. The loop code is separated into four pipeline stages. Note again that the size of one element of the arrays a double is 8 bytes. Can fill arbitrarily long pipelines with infinitely many iterations 15745. It is for this reason that many optimizers only perform software pipelining for loops with constant bounds. A technique called software pipelining contributes the biggest boost to improving looped code performance. Ajit pal,department of computer science and engineering,iit kharagpur. Time taken by loop unrolling vs normal loop we have discussed loop unrolling. Scheduling for loops loop unrolling software pipelining interaction with register allocation hardware vs.
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