DINO CPU Assignment 4: Branch Predictor and Benchmarking ... branch prediction will become the most limiting factor in processor performance [1]. Details about this branch predictor can be found here. Tournament predictors: The next type of predictor is a tournament predictor. 3 GEHL-like components respectively indexed using the global conditional branch history (2 tables), the global history of the backward branches (2 tables) and a 64-entry local history (2 tables). An im-portant problem in two-level predictors is aliasing [20], and many of the recently proposed branch predictors . A two-level adaptive predictor with globally shared history buffer and pattern history table is called a "gshare" predictor if it xors the global history and branch PC, and "gselect" if it concatenates them. We present a selection of many different branch prediction schemes in order of increasing complexity. An efficient branch predictor for improved accuracy of ... The idea of the neural branch prediction was originally introduced by Vintan and Iridon [1999] and Jimenez and Lin [2002] showed that the global´ perceptron could be more accurate than any other then known global branch predictor. The high bit of the counter is taken as the prediction. PDF Microbenchmarks for determining branch predictor organization An im-portant problem in two-level predictors is aliasing [20], and many of the recently proposed branch predictors . Branch PC Prediction Global Prediction Table (Counts) n bits Global History Register (GHR) m bits If we use a local and global predictor, we might get something like 96% accuracy, giving us 1.15 cycles . compute the data ahead!! Further, a hybrid branch prediction scheme is proposed, which uses both global and local branch information, providing more accuracy than the single and correlation branch prediction schemes. Global branch prediction is used in AMD microprocessors and in Intel Pentium M, Core and Core 2. The method may proceed with generating a branch prediction (block 530). [6]. o pipeline the calculation and ahead of time!! and the branch predictor is global with one history bit. 1. The static predictor will serve as your base case, against which you compare the others. 1. Each of these predictors is a 2-bit predictor for that branch. BaseBranchPredictor details. By using Two-Level Adaptive Train-ing Branch Prediction, the average prediction accuracy for the benchmarks reaches 97 percent, while most of the other schemes achieve under 93 . Branch prediction is an optimization technique which predicts the path a code will take before it is known for sure. compute the data ahead!! Once the branch outcome is known, the counter is incremented if the branch is taken, and decremented otherwise. Based on typical branch behavior ! The outcomes of the h (12 in this figure) most recent branches are stored in a global branch history register or GBHR. Because of the existing of GR, global prediction can be as accuracy as local does. In general, it is an (m,n) predictor, with global information about m branches and each of the predictors maintained as an n-bit predictor. We present a selection of many different branch prediction schemes in order of increasing complexity. To track the global branch histories, we need to add a global branch history buffer (GHR). The value in GBHR is xored with the branch address to index an Since the bimodal scheme takes advantage of the bimodal distribution of branch behavior, it does not perform well when branches have strong dynamic behavior. Correlating Branch Predictor General form: (m, n) predictor m bits for global history, n bits for local history Records correlation between m+1 branches Simple implementation: global history can be store in a shift register Example: (2,2) predictor, 2-bit global, 2-bit local Branch address (4 bits) 2-bits per branch local predictors . • The k-bit global branch history register(BH R) from first level is used to select the correct prediction entry (predictor)within a the selected table, • Thus each of the b= 2 a tables has 2 k entries and each entry (predictor) is usually Pointer A plethora of research has been done on the subject of branch prediction. Gshare Global Branch Predictor 3. For example, data that are likely to be read in . Branch PC Predictor Global Branch History Data Value History Fig. The base branch predictor instantiates a set of registers to hold the prediction table (predictionTable). pipelines are getting deeper, accurate branch prediction is critical to achieve high performance since fetched instructions after a branch have to be flushed inside pipeline when prediction is wrong. In the next section we will explain the details of branch predictor used in this assignment. Theu experiments also determine the organization of the branch target buffer and the address bits used to access it. as local branch prediction. Each of these different branch prediction strategies have distinct advantages. - good branch prediction might get the same effect 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, 2-bit) • Pentium 3 (4 history bits) • gshare Local branch history, global pattern history table, two-level adaptive (or correlating) predictor. When branch(s) is(are) fetched we use the branch history table (BHT) to get a branch prediction. A processor includes an instruction pipeline for executing instructions including a branching instruction, a counter for counting times that the branching instruction is taken, a register for storing a global branch history as a function of a value of the counter, and a branch prediction unit for predicting branching based on the global branch history. bination of branch address and global or per-branch history. Prediction through speculative branch execution was recently pursued independently by Gonzalez y Gonzalez [7], and a similar scheme was studied by Farcy et al. A choice predictor chooses between the two. Correlated branch prediction schemes include common-correlation, gselect, global and local. For example, the predictor shown below uses a 2-bit global history to choose from among four predictors for each branch address. • These mechanisms usually employ a table which is indexed by lower bits of the branch address. Pin intercepts program execution and As before, the prediction comes from a 2-bit saturating counter. Encoded prediction is incorrect Branch condition is known here 16 Dynamic Branch Prediction Strategies • Use past behavior to predict the future • Local vs. global behaviors - Branches show surprisingly good correlation with one another and their history • They are not totally random events n-1 … 0 prediction Last branch behavior, More-Realistic Branch Prediction ! 2. The obtained results suggest that the proposed global perceptron branch predictor provides an increased accuracy rate of 10.47% at 4 kb hardware budget and 8.06% at 4-bit history length than the . We have explained the concept with a C++ example of branch prediction where a condition statement runs slower in case of unsorted data compared to sorted data. This XORed value is then used to index into a 1D BHT of 2-bit predictors. Tournament Lecture 11: Branch PredictionLecturer: Prof. Onur Mutlu (http://users.ece.cmu.edu/~omutlu/)Date: February 11, 2013.Lecture 11 slides (pdf): http://www.ece. [15] confirm these results for a single gshare predictor, and Branch Prediction for Free. ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming, pages 300-313, May 1993. Jourdan et al. oretical predictor that maintains global history but provides each branch with its own table of 2-bit counters (the pattern history table, or PHT). sample reports on the course . It has already been . For example, the predictor shown below uses a 2-bit global history to choose from among four predictors for each branch address. Fig. Global branch history • For a (2,n) branch predictor, the outcome of last two branches is relevant 11 110 1100 11001 110011 1100110 11001100 • 2-bit global branch history • implemented using a 2bit shift register 1% 0% 1% 5% 6% 6% 11% 4% 6% 5% 0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 20% nasa7 matrix300 tomcatv doducd spice fpppp gcc espresso . This solution is an implementation of a Global Share Predictor. It has a local predictor, which uses a local history table to index into a table of counters, and a global predictor, which uses a global history to index into a table of counters. a branch predictor can predict branch outcomes accurately. Dynamic Branch Prediction. branch prediction performance to be similar. The Gshare predictor is characterized by XORing the global history register with the lower bits (same length as the global history) of the branch's address. more accurate due to info on path! There are three different classes of branch predictors: local, global, and the combination How? Example: loop and if-statement branches ! Correlating Branch Predictor General form: (m, n) predictor m bits for global history, n bits for local history Records correlation between m+1 branches Simple implementation: global history can be stored in a shift register Example: (2,2) predictor, 2-bit global, 2-bit local Branch address (4 bits) 2-bits per branch local predictors Prediction • For a (1,1) predictor: each branch has two different branch prediction buffers: • The content of the two branch prediction buffers are determined by the branch to which they belong • Which of the two branch prediction buffers are used is depending on the outcome of the previous branch in the application X / Y Predictor used in case the . Whenever there is a branch resolved we need to shift in the recent branch result (taken/not taken) to GHR. Using data values for branch pred-iction directly. We argue this inefficiency is a fundamental limitation of runtime branch prediction and not a coincidental artifact due to the design of TAGE. Tournament Branch Predictor that combines 1 and 2 1.1 PIN tool Pin is a dynamic binary instrumentation engine that enables the creation of dynamic analysis tools (e.g. branch, updating just that prediction 2 Bits of global history means we look at T/NT behavior of last two branches to predict THIS branch. The buffer can be implemented as a one dimensional array. Rather . Implements a tournament branch predictor, hopefully identical to the one used in the 21264. For accuracy, a novel model based on global perceptron branch predictor is developed, which uses both global and per branch information. The processor speculatively updates a global history register having fetch group history and branch history, fetches a fetch group of instructions, and assigns a global history vector to the instructions. An predictor uses the behavior of the last branches to choose from branch predictors, each of which is an -bit predictor for a single branch.. Illustrative Example - Listing 1 showcases an H2P (Line 11, H2P-1) whose global history is affected by a loop with a variable iteration count. By less noise, I mean the papi calls. This paper studies the performance of several types of branch predictors starting from local branch predictor and global branch predictor. Problem M7.1.C Branch prediction with one global history bit While many branch prediction structures have been proposed, their performance is usually demonstrated empirically through simulations that provide little insight into the In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g., an if-then-else structure) will go before this is known definitively.The purpose of the branch predictor is to improve the flow in the instruction pipeline.Branch predictors play a critical role in achieving high effective performance in many modern pipelined microprocessor . 4 KB hardware budget, our global predictor improves misprediction rates on the SPEC 2000 integer benchmarks by 26% over a gshare predictor of To further o pipeline the calculation and ahead of time!! prediction mechanism for each static branch [9]. Tournament Predictor Example -Alpha 21264 Uses 4K 2-bit counters to choose from global and local predictor Global predictor 4K entries of 2-bit predictors Indexed by history of last 12 branches Local predictor is a two-level predictor History table with 1K 10-bit entries (for that branch) Each entry gives 10 most recent branch outcomes Another technique uses the combined history of all recent branches in making a prediction. An entry is selected by … • the 4 low-order bits of the branch address (word address), and The bimodal technique works well when each branch is strongly biased in a particular . For each branch, check the corresponding BP bits (indicated by the bold entries in the examples) to make a prediction, then update the BP bits in the following entry (indicated by the italic entries in the examples). The Global Branch History can be implemented using a shift register that shifts in the branch behavior (NT or T) when the branch is executed. When I measure outside of the loop, I introduce less noise to the measurement. o choose weight vector according to the path leading up to a branch!! Abstract—The state-of-the-art branch predictor, TAGE, re-mains inefficient at identifying correlated branches deep in a noisy global branch history. Branch Prediction Strategies Predict Not Taken Pipelines do this naturally Does not affect ISA Software Prediction Extra bit in the branch instruction Bit set by compiler or user; can use profiling Static prediction, same behavior every time Prediction Based on Branch Offsets Positive offset: predict not taken The CPU has one small branch predictor, like 2-3 bit saturated counter in Intels, and one big global branch predictor. (m,n) predictor uses behavior of last m branches to choose from 2m predictors each being an n-bit predictor. The branch's counter in the prediction table is incremented if the branch was taken, decremented if the branch was not taken. Predict forward branches not taken ! On the SPEC'89 benchmarks, very large . Static Branch Prediction • Heuristic-based: Ball/Larus -Thomas Ball and James R. Larus. H2P-1 is exactly correlated to the data-dependent branch at Line 5, and both branches are biased to be taken 33% of the time when uvec's values are uniformly distributed. Each of these predictors is a 2-bit predictor for that branch. Branch prediction is an essential part of modern microarchitectures. Branch prediction. The Branch Predictor (BP) bits in the table are the bits from the BHT. The premise of these methods is that a pattern will arise that may help in future predictions. In general, most research indicate that global history Modification Neural Branch Prediction • optimize the speed by path-based! Note that since the branch prediction buffer is NOT a cache, there's no guarantee that the predictions correspond to the "correct" branch instruction. If it is perfect . Modification Neural Branch Prediction • optimize the speed by path-based! There is a solution provided as an example. use pipeline to compute the weight Figure 14.4 shows an example of a correlating predictor, where m=2. The Two-Level Adaptive Training branch prediction scheme as well as the other dynamic and static branch prediction schemes were simulated on the SPEC benchmark suite. For example, the branch prediction may be done by multiplexing between global predictions provided by counters 362, 363 of the global predictor 360 according to the local prediction 355 and further multiplexing between tile global prediction provided by above multiplexing . 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