During the startup phase of the program execution, where a static branch prediction might be effective, the history information is gathered and dynamic branch prediction gets effective. Static branch prediction in general is a prediction that uses information that was gathered before the execution of the program. However, the first time and most often schemes have important limitations. This paper presents a new method for branch prediction. At the execute stage it is known whether or not the branch is taken.
In the 5stage pipeline, a branch completes in two cycles if the branch went the wrong way, one incorrect instr is fetched one stall cycle per incorrect branch. By using a 2bit counter, the predictor can tolerate a branch going an unusual direction one time and keep predicting the usual branch direction. The actions for a branch are fixed for each branch during the entire execution. An intuitive illustration of the scbp transformation. The pa8500 relies on dedicated static branch prediction bits in the branch instruction itself which are populated based upon previous executions of the program. They are static because the prediction is already known before the program is executed. Prol eassisted compiler support for dynamic predication. Pdf dynamic branch prediction and control speculation. Cmsc 411 computer systems architecture lecture 9 instruction level parallelism 2 branch prediction 12% 22% 18% 11% 12% 4% 6% 9% 10% 15% 0% 5% 10% 15% 20% 25% o m p s s t o c c i c a r o 2 d l j p s o r misprediction rate cmsc 411 8 from patterson static branch prediction previously scheduled code around delayed branch to reorder. The left side of the diagram shows a pair of conditional statements and.
Static branch prediction is a simple prediction technique. So how does branch prediction even work, if either way you are still doing the same conditional check. While the simple prediction techniques provide fast lookup and power efficiency they suffer from high misprediction rate. Mcfarling, combining branch predictors, dec wrl technical. Dynamic prediction the downside to static prediction is that it has a large variance across different programs, and it requires the compiler to optimize for branches being taken. Dynamic predication has been proposed to reduce the branch misprediction penalty due to hardtopredict branch instructions. Recently executed branches correlated with next branch branch target buffer.
A recently proposed dynamic predication architecture, the divergemerge. Pdf combining static and dynamic branch prediction to. Branch hazards and static branch prediction techniques. Static methods are usually carried out by the compiler. Branch prediction increases the number of instructions available for the scheduler to issue. Static prediction strategies strategy 1 always predict that a branch is taken and its converse always predict that a branch is not taken are two examples of static prediction strategies. Abstractmodern branch predictors predict the vast majority of conditional branch. Accurate static branch prediction by value range propagation jason r. Static scheduling of instructions in instruction pipeline aca duration. Prol eassisted compiler support for dynamic predication in divergemerge processors. What branch prediction accuracy must p2 have in order to perform as well as p1. Branch prediction branch prediction is one of the ancient performance improving techniques which still finds relevance into modern architectures.
Static branch prediction good static branch predictions are invaluable information for compiler optimisation or performance estimation. Pdf branch prediction schemes have become an integral part of todays superscalar processors. How to handle control dependences critical to keep the pipeline full with correct sequence of dynamic instructions. Questions tagged branchprediction ask question in computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch e. Improved branch prediction through intuitive execution performance will begin at an estimated 40 specint95 and 60 specfp95 and will reach more than 100 specint95 and 150 specfp95, and operate at more than mhz by the year 2000. Dynamic branch prediction branch history table duration. The most often scheme requires a static branch prediction bit in the instruction format. Static branch prediction good static branch predictions are invaluable information for compiler optimisation or performance. In general, dynamic branch prediction gives better results than static branch. A case for static branch prediction in realtime systems irit. Dynamic branch prediction has improved to be much more accurate than static branch prediction on average.
Dynamic predicated execution of complex controlflow graphs based on frequently executed paths, micro 2006. Branch outcomes are calculated during the decode stage stage 2. Use the direction of a branch to base the prediction on. We made a number of changes to the source code in order to perform our branch prediction methods available below. In order to explain dynamic branch prediction, one has to differentiate it from static branch prediction. Merge into perf branch btrax has many useful functions execution path call graph analyze code coverage visualize executed. This primitive mechanism yields 60% to 70% accuracy. Accurate static branch prediction by value range propagation. This makes use of the observation that a majority of branches is taken. I know that up to pentium 4, static branch prediction on a conditional forward branch was considered not taken by the processor it seems that static prediction became random on further intel processors, thus i imagine this optimization is dealing with this.
Static branch prediction uses only sourcecode knowledge or compiler analysis to predict a branch 5 whereas dynamic prediction accounts for timevarying and inputdependent execution pattern of a branch. Stall the pipeline until we know the next fetch address guess the next fetch address branch prediction employ delayed branching branch delay slot. A lowcost method to improve branch prediction accuracy. Dynamic branch prediction branch history table youtube. What i am trying to say is, isnt branch prediction exactly the same as having no branch prediction at all because you are doing the same conditional checks anyway.
Branch prediction in arm processors that have no pu, the target of a branch is not known until the end of the execute stage. The percent of branches not taken in the spec benchmark is only 34%. Method and apparatus for performing branch prediction. Jimenez and lin, dynamic branch prediction with perceptrons, hpca 2001. A survey of techniques for dynamic branch prediction article pdf available in concurrency and computation practice and experience april 2018 with 2,967 reads how we measure reads.
Branch prediction article about branch prediction by the. When branch removal is not possible, the branch can be coded to help the processor predict it correctly based on the static branch prediction rules. The tradeoff between fast branch prediction and good branch prediction is sometimes dealt with by having two branch predictors. Static branch prediction how is static branch prediction abbreviated. Increases instruction level parallelism ilp allows useful work to be completed while waiting for the branch to resolve 9.
Static branch prediction how is static branch prediction. In conclusion, we have researched a number of branch prediction methods. Static branch prediction is an idea from the 90s that doesnt really help anymore. Over the course of the cacheconscious algorithm improvements, branch prediction results are generated in order to observe the sidee. Compiler determines likely direction for each branch using profile run. Assume p1 uses static branch prediction and assumes not taken. In both cases, care must be taken not to change the. A survey of techniques for dynamic branch prediction arxiv.
This paper investigates neural static branch prediction as proposed in 1 but it goes further and links it with a dynamic neural branch prediction as stated in 5,8. Dynamic branch prediction with perceptrons request pdf. We used the simplescaler simulator to generate our branch prediction results. Encodes that direction as a hint bit in the branch instruction format. Potential solutions if the instruction is a controlflow instruction. Virtual function call example figure 1 shows a virtual function call insidethatis responsible for 24% of all indirect jump mispredictions using a commonlyimplemented branch target bufferbased indirect jump predictor in the spec 2006 fp benchmark povray. The name programbased branch prediction is given to static branch. Bimodal predictor structure repeatedly nottaken branches will be predicted to be nottaken.
Static branch prediction branch prediction schemes can be classified into static and dynamic schemes. Prediction is decided on the computation history of the program. The best performance is obtained by predicting all branches as not taken and filling the pipeline w. The decision causing the branch prediction can change during the program execution. While the simple prediction techniques provide fast lookup and power. We looked at both static and dynamic branch prediction schemes. Pdf a survey of techniques for dynamic branch prediction. The second branch predictor, which is slower, more complicated, and with bigger tables, will override a possibly wrong prediction made by the first predictor. So most modern cpu since 2000 ignore static branch prediction hints.
Static conditional branch prediction branch prediction schemes can be classified into static and dynamic schemes. The key idea is to use one of the simplest possible neural networks, the perceptron, as an alternative to the commonly used twobit counters. Resolve a branch hazard by predicting which path will be taken. For large counter tables, each branch will map to a unique counter.