There are several dynamic branch predictor in use or being researched nowadays. This paper surveys the different techniques used for branch prediction. Branch predict ion techniques used in pipeline processors. Today, all state of the art microprocessors have branch prediction of static softwar e and dynamic hardware. The prediction is the sign of the dot product of the branch history and the perceptron weights. The branch predictor may, for example, recognize that the conditional jump is taken more often than not, or that it is taken every second time. The branch prediction using machine learning is also based on the same idea. Dynamic branch prediction has a rich history in the literature. You can create a form in indesign that includes placeholders for fields such as radio buttons, check boxes, and text fields.
Cse 560 computer systems architecture branch prediction 1 this unit. Onelevel branch predictor dynamic branch prediction on the other hand uses information about taken or not taken branches gathered at runtime to predict the outcome of a branch. Uses a branch history table bht, a table of usually twobit saturating counters which. In this scheme, a pattern history table pht of twobit saturating counters is indexed by a combination of. The taken branches t in the branch history are representedas1s,andnottakenbranchesntarerepresentedas. Dynamic branch prediction dynamic branch prediction has been a well researched topic in recent decades, with modern advances focusing on the improvement, re. An intuitive illustration of the scbp transformation. Improving the accuracy of dynamic branch prediction using. The purpose of this assignment is to supplement your knowledge about the design of different dynamic branch prediction schemes, alongside intuition about their. Branches change the program counter based on runtime information. The left side of the diagram shows a pair of conditional statements and the two most likely paths through them. Static prediction is the simplest branch prediction technique because it does not rely on information about the dynamic history of code executing. Cs 152 computer architecture and engineering lecture 14.
Branch prediction is not the same as branch target prediction. At the execute stage it is known whether or not the branch is taken. 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. Intel branch predictors 386 and 486 didnt have any sort of hardware based dynamic branch prediction block. Dynamic branch prediction dynamic branch prediction schemes utilize runtime behavior of branches to make predictions.
Dynamic branch prediction with perceptrons request pdf. Pdf branch prediction schemes have become an integral part of todays superscalar processors. Among these, dynamic branch prediction is perhaps the most popular, because it yields good results and can be im plemented without changes to the instruction set architec ture or preexisting binaries. We do not include branch target prediction or the techniques for indirect or unconditional branches. Conclusions branch prediction is an important issue in increasing the instruction fetch rate and. In order to explain dynamic branch prediction, one has to differentiate it from static branch prediction. Dynamic branch prediction on the other hand uses information about taken or not taken branches gathered at runtime to predict the outcome of a branch. Autumn 2006 cse p548 dynamic branch prediction 24 real branch prediction strategy static and dynamic branch prediction work together predicting correlated branch prediction pentium 4 4k entries, 2bit pentium 3 4 history bits gshare mips r12000 2k entries, 11 bits of pc, 8 bits of history. Dynamic branch prediction in highperformance processors is a specific instance of a general time series prediction problem that occurs in many areas of science. An alternative to branch prediction is to execute both directions of a branch speculatively branch prediction takes less resources than speculative execution of both paths only half the resources engage in useful work when both directions of a branch are executed speculatively with accurate branch prediction, it is more cost effective. If the prefetched values matches with the actual values needed at a later point of time, it reduces the latency that the processor. Dynamic branch prediction arvind learning based on past behavior. The purpose of this assignment is to supplement your knowledge about the design of different dynamic branch prediction schemes, alongside intuition about their relative performance. Dynamic branch prediction learning based on past behavior temporal correlation.
In contrast, most branch prediction research focuses on twolevel adaptive branch prediction techniques, a very specific solution to the branch prediction problem. Neural methods for dynamic branch prediction ut computer. In both cases, care must be taken not to change the. Depending on the simulated configuration, the hardware corresponding to the light shaded boxes can be either duplicated or shared. The following sections first introduce those wellknown schemes of dynamic branch predictors. The actions for a branch are fixed for each branch during the entire execution. Indian institute of technology, hyderabad summary branch predictor bp is an essential component in modern processors since high bp accuracy can improve performance and reduce energy by decreasing the number. Explicit dynamicbranch prediction with active updates. R register file read e integer execute btb bht in later bht pipeline stage corrects when btb misses a predicted taken branch btbbht only updated after branch resolves in e stage 21611 cs252s11, lecture 9 4. Instead, it predicts the outcome of a branch based solely on the branch instruction. 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. How to survive alone in the wilderness for 1 week eastern woodlands duration.
Modify the tracer reader to calculate the mis prediction rate that is, the percentage of conditional branches that were mispredicted for these two simple schemes. In this scheme, a pattern history table pht of twobit saturating counters is indexed by a combination of branch address and global or perbranch history. Underlying algorithm has regularities data that is being operated on has regularities instruction sequence has redundancies that are artifacts of way that humanscompilers think about problems is dynamic branch prediction better than static branch prediction. A dynamic multithreading processor princeton university. Seems to be there are a small number of important branches in programs that have dynamic behavior 3 cmsc 411 8 from patterson dynamic branch prediction performance.
Dynamic branch prediction summary branch history table. Proceedings of the fifth international conference on architectural support for programming languages and operating systems improving the accuracy of dynamic branch prediction using branch correlation. Dynamic branch prediction using neural networks core. The key idea is to use one of the simplest possible neural networks, the. Pdf dynamic branch prediction using machine learning. Cmsc 411 computer systems architecture lecture 9 instruction. The decision causing the branch prediction can change during the program execution. Recently executed branches correlated with next branch branch target buffer. With hardwareassisted branch prediction, only the most recent history of a branch is used to predict the outcome of that branch. Intel pentium ii 333 mhz pentium ii 1998 specint95, 9 specfp95.
Dynamic branch prediction with perceptrons ut computer science. Evaluating the performance of dynamic branch prediction. Pdf dynamic branch prediction and control speculation. Pdf a survey of techniques for dynamic branch prediction. Pentium iii has a twolevel of local history based branch predictor where each entry is 2bit saturating counter also. Modify the tracer reader to calculate the misprediction rate that is, the percentage of conditional branches that were mispredicted for these two simple schemes. In a loop branch, even if a branch is almost always taken and then not taken once, the. The branch target buffer is a table, each entry of which is composed of 3 fields. Saves each indesign layer as an acrobat layer within the pdf. Usually information about outcomes of previous occurrences of branches are used to predict the outcome of the current branch. Detailed prediction accuracy of various dynamic branch prediction schemes. I studied as much documentation as i could within my very limited. The a bility to predict the direction of branches is an important issue in modern computer architecture and advanced compilers. After knowing the schemes, each branch prediction performance is then explicitly presented through the comparison.
Static branch prediction in general is a prediction that uses information that was gathered before the execution of the program. This leads to dynamic prediction strategies in which the prediction varies, based on branch history. Dynamically improving branch prediction accuracy between. Request pdf dynamic branch prediction with perceptrons this paper presents a new method for branch prediction. However, their performances always have new and interesting discoveries based on different benchmarks and architectures. Branch prediction in arm processors that have no pu, the target of a branch is not known until the end of the execute stage. Branch prediction attempts to guess whether a conditional jump will be taken or not. Cs 152 computer architecture and engineering lecture 14 branch prediction krste asanovic. Due to the fast progress of computer technology, su and zhou sz95 showed different aspects of performance analysis.
Most studies of dynamic branch prediction focus on the history of the branch under consideration 4, 7. Dynamic branch prediction and control speculation article pdf available in international journal of high performance systems architecture 11. Static branch prediction is performed at compile time and uses static code analysis to either insert branch prediction hints into the emitted opcodes, or to reorganize the emit ted code so the branch prediction implemented by the targeted machines hardware is more effective. Perceptrons have been successfully applied in 21, 10, 11 and 12 for efficient dynamic branch prediction within twolevel adaptive schemes that are. To avoid this problem, pentium uses a scheme called dynamic branch prediction. The key idea is to use one of the simplest possible neural net works, the perceptron as an alternative to the commonly used twobit counters. Although indesign does not provide tools for adding form fields, adobe acrobat does. All branches were statically predicted as not taken. R register file read e integer execute remainder of execute pipeline. Neural methods for dynamic branch prediction 371 fig. So how does branch prediction even work, if either way you are still doing the same conditional check.
Before looking at dynamic branch prediction, we are going to look at simple static branch prediction policies of always predict taken and always predict not taken. The best performance is obtained by predicting all branches as. If the prediction is true then the pipeline will not be flushed and no clock cycles will be lost. About 80% of instructions executed in branch delay slots useful.
Strategy 2 predict that a branch will be decided the same way. The taken branches t in the branch history are represented as 1s. Branch prediction accuracy profilebased 2bit counter tournament accuracy of branch prediction profile. Static branch prediction good static branch predictions are invaluable information for compiler optimisation or performance estimation.
Dynamic branch prediction why does prediction work. Dynamic branch prediction, superscalar, vliw, and software pipelining professor randy h. Is dynamic branch prediction better than static branch prediction. Modern processor architectures increasingly make use of prefetching as a way to boost instruction level parallelism. Published in romanian journal of information science and. Branch penalties limit performance of deeply pipelined processors modern branch predictors have high accuracy 95% and can reduce branch penalties significantly required hardware support. 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. A survey of techniques for dynamic branch prediction arxiv. 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. The strength of dynamic branch prediction is that it can.
Introduction in both the architecture and compiler domains, conditional branch instructions are a barrier to higher levels of performance. Branch hazards and static branch prediction techniques. The traditional twolevel adaptive branch predictors use two levels of branch history information to make the prediction. Branch predictor bp is an essential component in modern processors since high bp accuracy can improve performance and reduce energy by decreasing the. Prediction is decided on the computation history of the program. Carnegie mellon computer architecture 38,389 views 1. 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. Most recent research on dynamic branch prediction has concentrated on twolevel adaptive techniques. Pdf branch predictor bp is an essential component in modern processors since high bp accuracy can improve performance and reduce. For static branch prediction one always assume that the branch is not taken, while for dynamic branch prediction if the branch is taken before then it is more likely to be taken again. Sas 6, 4x16 indicates 4 sets of history registers, each with a length of 6 bits. The dynamic branch predictors should have a branch target bufferbtb. In this scheme, a prediction is made for the branch instruction currently in the pipeline. In contrast, most current branch prediction research focuses on twolevel adaptive branch prediction techniques, a very specific solution to the branch prediction problem.
19 575 1225 535 1215 1341 1506 114 233 1308 1385 1333 197 12 934 664 1299 466 316 753 709 1238 292 82 314 1452 582 1242 1230 224 468 574 1057