Branch target buffer btb, interrupt support, computer architecture lec 516. Introduction branch prediction continues to be an ongoing area of research and many new ideas are being proposed today. And if its a miss, branch predictor comes into the play and predict the outcome of the branch. The target of a direct branch is predicted using a branch target buffer btb 1 a cache structure indexed by. Alpha 21264 branch predictors similar to power4 alpha 21264 branch predictor is also composed of three units local predictor, global predictor, and choice predictor. Sandy bridge, ivy bridge, and skylake intel processors. Branch history table bht and branch target buffer btb support for handling interrupts. Smith, a study of branch prediction strategies, isca 1981. The report has not been accepted for any degree and is not being submitted concurrently in candidature for any degree or other award.
Autumn 2006 cse p548 dynamic branch prediction 17 2. Branch target buffer btb keep both the branch pc and target pc in the btb. In this course, you will learn to design the computer architecture of complex modern microprocessors. Branch prediction attempts to guess whether a conditional jump will be taken or not. A study for branch predictors to alleviate the aliasing problem tieling xie, robert evans, and yul chu. This paper discusses two major issues in the design of btbs with the theme of achieving maximum performance with a limited number of bits allocated to the btb design.
Branch target prediction attempts to guess the target of a taken conditional or unconditional jump before it is computed by decoding and executing the instruction itself. Using this approach, a hybrid branch predictor can be constructed such that each component branch predictor predicts those branches for which it is best suited. All the features of this course are available for free. Branch target buffer btb that includes the addresses of conditional. For a branch history table bht with 2bit saturating counters. Applying decay strategies to branch predictors for leakage energy. Pdf branch target buffer design and optimization chris perleberg. May 26, 2016 branch classification allows an individual branch instruction to be associated with the branch predictor best suited to predict its direction. Branch prediction strategies and branch target buffer design, computer, 171, jan. The twolevel adaptive training branch prediction scheme as well as the other dynamic and static branch prediction schemes were simulated on the spec benchmark suite. 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. Download pdf download citation view references email request permissions. Pdf the performance of counter and correlationbased.
A general lowcost indirect branch prediction using target. The control unit looks up the branch target buffer during the f phase. Branch prediction and instruction delivery branch target buffer, return address prediction, tournament predictor, highperformance instruction delivery 2 correlating branch predictor general form. In computer architecture, a branch target predictor is the part of a processor that predicts the target of a taken conditional branch or an unconditional branch instruction before the target of the branch instruction is computed by the execution unit of the processor branch target prediction is not the same as branch prediction which attempts to guess whether a conditional branch will be. Once theres a hit, theres no need for branch prediction, and we can go ahead and fetch the instruction at pc in the btb. Branch prediction strategies and branch target buffer. The branch target buffer design can also be simplified to record only the result of the last execution of the branch. In this project, you will 1 design a basic tournament predictor based off the alpha. Achieving high instruction issue rates depends on the ability to dynamically predict branches.
That prediction can be generated by profiling a set of benchmarks. A study for branch predictors to alleviate the aliasing. Btb prediction is latencysensitive and a prefetchbased vir. Btb size for haswell, sandy bridge, ivy bridge, and skylake. The other is using a global history, in which the history of the last few branches determines the direction. Evaluating the performance of dynamic branch prediction. A sophisticated btb can recognize patterns, like an indirect jump that alternates between two targets. One is using a local history, in which the prediction is made solely based on the history of that branch itself. Modern processors use branch target buffers btbs to pre dict the target. Instruction cache prefetching directed by branch prediction. For instruction caches of4kb and greater, instruction cache based branch prediction performance is a strong function of line size, and a weak function of instruction cache size.
The powerpc620 has a 256 entry twoway set associative branch target buffer for predicting the branch target address and a decoupled direct mapped branch prediction buffer. Instr address predicted pc btb is a cache that holds instr addr, predicted pc for every taken branch the control unit looks up the. The target of a direct branch is predicted using a branch target buffer btb 1 a cache structure indexed by a portion of the branch address. Experiment flows and microbenchmarks for reverse engineering. Assuming no conflicts between branch address bits, and assuming all entries are initially set to 0, how many conditional branches would be mispredicted. The arm cortexa8 processor, which has a cycle branch misprediction penalty, uses a 512entry, 2way btb, and a 4096entry global history buffer 2. To compare various branch prediction strategies, we will use the spec89 benchmarks spe90 shown in figure 2. In order to know the target of branch link stack predict the target of branches. A study for branch predictors to alleviate the aliasing problem. When a lookup operation is initiated, the branch address is decoded and sent to the tag array. Are there any way to determine or any resource where i can find the branch target buffer size for haswell, sandy bridge, ivy bridge, and skylake intel processors. Following is a detailed description of one of these strategies. These benchmarks include a mix of symbolic and numeric applications.
Riseman and foster, the inhibition of potential parallelism by conditional jumps, ieee transactions on computers, 1972. But if your branch predictor says that it will be a taken branch, you dont know which instruction to fetch next, since you havent decoded this instruction yet. The address prediction is usually implemented using a branch target buffer, or btb. Btb miss target pc is computed and entered into the target buffer. Reading for this module branch prediction branch target buffers. Smith, branch prediction strategies and branch target buffer design. Dynamic branch prediction continued branch target buffer. To summarize, branch predictors fall into two categories. How does branch target prediction differ from branch prediction. Pdf instruction cache prefetching directed by branch prediction. Many researchers have studied branch prediction strategies extensively. Address tag predicted pc prediction state bits address predicted pc prediction bits may be in the prediction buffer instead implemented as an associative memory may be fully associative, direct.
Decay can reduce net leakage energy in the branch target buffer btb by 90%. Many hardwarebased indirect branch predictors maintain target values in dedicated storage 1519, 21, which can account for. Branch target prediction is not the same as branch prediction which attempts to guess whether a conditional branch will be taken or nottaken i. Branch target buffers, or btbs, can be used to improve cpu performance by maintaining target and history information of previously executed branches. Address tag predicted pc prediction state bits address predicted pc prediction bits may be in the prediction buffer instead implemented as an associative memory may be fully associative, direct mapped, or set associative. If the v h bit is 0, no further operation is made, and the predicted target address is the concatenation of the higher bits of the branch address ba h with the bits that were read from ta l array. We report relative performance estimates to show both the relative merits of various. How branch predictor and branch target buffer coexist. The most wellknown example of these is the branch target buffer, or btb 14. A btb stores previous addresses where branch redirected the control flow. The powerpc604 has a 64 entry fully associative branch target buffer for predicting the branch target address and a decoupled direct mapped 512 entry pattern history table. Branches hurt perfor outperforming the lru strategy by a small margin.
Branch prediction is not the same as branch target prediction. Address of the current instruction which directions earlier instances of this branch went. The branch target buffer btb can reduce the performance penalty of branches in pipelined processors by predicting the path of the branch and caching information used by the branch. This would mean that one has to wait until the id stage. We also have run an extensive set of experiments to demonstrate the. In a branch target buffer, you actually do in parallel with both the branch prediction outcome or the branch outcome prediction and the pc plus four. Branch prediction, branch target buffer btb, interrupt. Branch target prediction in addition to predicting the branch direction, we must also predict the branch target address branch pc indexes into a predictor table.
Branch target buffer design for embedded processors. Smith, branch prediction strategies and branch target buffer design, computer 171 pp. Another dynamic scheme also proposed by lee and smith is the static training scheme. Oneforpredictedbranchtargetsandoneforthebranchpredictor.
Target address prediction branch prediction coursera. Branch classification allows an individual branch instruction to be associated with the branch predictor best suited to predict its direction. Analysis of branch prediction strategies and branch target. 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. Some designs store n prediction bits as well, implementing a combined btb and. This paper discusses two major issues in the design of btbs with the. Hennessy, reducing the cost of branches, proceedings of the th annual international symposium on computer architecture, pp. Delivering full text access to the worlds highest quality technical literature in engineering and technology. In more parallel processor designs, as the instruction cache latency grows longer and the fetch width grows wider, branch target extraction becomes a bottleneck. According to our simulations, we suggest that substantial improvements with reduced hardware can potentially be obtained when the multiassociative branch target buffer is installed in a pipelined cpu. By examining the type of branch and the past execution behavior of that branch takennot taken it is possible to predict with high accuracy whether the branch will be taken or not taken, and by remembering the previous branch target destination, to predict the current branch target.
A general lowcost indirect branch prediction using target address pointers. Power4 provides is that dynamic branch prediction can be overdriven by software, if needed. For instruction caches of4kb and greater, instruction cache based branch prediction performance is a strong function of line size, and a. Bpb branch prediction buffer btb branch target buffer cpu. We present tracedriven simulation results comparing counter based and correlationbased prediction schemes for a variety of branch target buffer sizes.
Nov 17, 2014 a general lowcost indirect branch prediction using target address pointers. 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. We have introduced a versatile and complete simulator for evaluating the performance of dynamic branch prediction schemes. Decoupling branch prediction from the branch target buffer. Graduate computer architecture lecture 9 prediction cont dependencies, load values, data values. One of the mitigation strategies weve seen proposed, particularly more recently, is.
So in order to not waste cycles waiting for the branch to resolve, you would use a branch target buffer or btb. Bitlevel perceptron prediction for indirect branches. Branch prediction strategies and branch target buffer design published in. Branch prediction and branch target prediction are often combined into the same circuitry. We compare two schemes for dynamic branch prediction. Using this approach, a hybrid branch predictor can be constructed such that each component branch predictor predicts those branches for which it. Branch prediction strategies and branch target buffer design. By using twolevel adaptive training branch prediction, the average prediction accuracy for the benchmarks reaches 97 percent, while most of the other schemes achieve under 93. Strategies for branch target buffers acm digital library.