Breaking In-Order Branch Miss Recovery

There is wide agreement that one of the most important impediments to the performance of current and future pipelined superscalar processors is the presence of conditional branches in the instruction stream. Speculative execution seems to be one solution of choice to the branch problem, but speculative work is discarded if a branch is mispredicted. Therefore, we need a very accurate branch predictor; 95% accuracy is not good enough. This paper proposes branch classification to help improve the accuracy of branch predictors. Branch classification allows an individual branch instruction to be associated with the branch predictor best suited to predict its direction. 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. This paper suggests one classification scheme, analyzes several branch predictors, and proposes a hybrid branch predictor that achieves higher prediction accuracy than any branch predictor previously reported in the literature.

Modern embedded processors use small and simple branch predictors to improve performance. Using complex and accurate branch predictors, while desirable, is not possible as such predictors impose high power and area overhead which is not affordable in an embedded processor. As a result, for some applications, misprediction rate can be high. Such mispredictions result in energy wasted down the mispredicted path. We introduce area-aware and low-complexity pipeline gating mechanisms to reduce energy lost to possible branch mispredictions in embedded processors. We show that by using a simple gating mechanism which comes with 33-bit area overhead, on average, we can reduce the number of executed instructions by 17% (max: 30%) while paying a negligible performance cost (average 1.1%).KeywordsArea OverheadEmbed ProcessorConfidence EstimationWaste ActivityBranch InstructionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

High-performance multiprocessor systems built around out-of-order processors with aggressive branch predictors execute many memory references that turn out to be on a mispredicted branch path. Previous work that focused on uniprocessors showed that these wrong-path memory references may pollute the caches by bringing in data that are not needed on the correct execution path and by evicting useful data or instructions. Additionally, they may also increase the amount of cache and memory traffic. On the positive side, however, they may have a prefetching effect for memory references on the correct path. While computer architects have thoroughly studied the impact of wrong-path effects in uniprocessor systems, there is no previous work on its effects in multiprocessor systems. In this paper, we explore the effects of wrong-path memory references on the memory system behavior of shared-memory multiprocessor (SMP) systems for both broadcast and directory-based cache coherence. Our results show that these wrong-path memory references can increase the amount of cache-to-cache transfers by 32%, invalidations by 8% and 20% for broadcast and directory-based SMPs, respectively, and the number of writebacks by up to 67% for both systems. In addition to the extra coherence traffic, wrong-path memory references also increase the number of cache line state transitions by 21% and 32 % for broadcast and directory-based SMPs, respectively. In order to reduce the performance impact of these wrong-path memory references, we introduce two simple mechanisms - filtering wrong-path blocks that are not likely-to-be-used and wrong-path aware cache replacement - that yield speedups of up to 37%.

A branch predictor is the part of the processor that determines whether a conditional branch in the instruction flow of a program is likely to be taken or not. Almost all pipelined processors have branch predictors, because they must guess the address of the next instruction to fetch before the current conditional instruction has been executed [1]. Hence, attempts have been made to design accurate branch predictors. This project attempts at modeling different branch predictors and comparing their performances for a set of benchmarks.

Designers have invested much effort in developing accurate branch predictors. To maintain accuracy, current processors update the predictor regularly and frequently. Although this aggressive approach helps to achieve high accuracy, for a large number of branches, quite often, updating the branch predictor unit is unnecessary as there is already enough information available to the predictor to predict the branch outcome accurately. Therefore, the current approach appears to be inefficient since it results in unnecessary energy consumption. The author introduces the power-aware branch predictor update (PABU). PABU uses a simple power efficient structure to identify well behaved accurately predicted branch instructions. Once such branches are identified, the predictor is no longer accessed to update the associated data. The key to the success of the proposed technique is a power efficient method that can effectively identify such branches. The author exploits branch instruction behaviour to identify such branch instructions. He shows that it is possible to reduce the number of predictor updates considerably without losing performance. The technique is evaluated by studying energy and performance tradeoffs for SPEC2000 benchmarks. It is shown that the technique can reduce branch prediction energy consumption considerably for both floating point and integer benchmarks. This comes with a negligible impact on performance.

Current processors exploit out-of-order execution and branch prediction to improve instruction level parallelism. When a branch prediction is wrong, processors flush the pipeline and squash all the speculative work. However, control-flow independent instructions compute the same results when they re-enter the pipeline down the correct path. If these instructions are not squashed, branch misprediction penalty can significantly be reduced. In this paper we present a novel mechanism that detects control-flow independent instructions, executes them before the branch is resolved, and avoids their re-execution in the case of a branch misprediction. The mechanism can detect and exploit control-flow independence even for instructions that are far away from the corresponding branch and even out of the instruction window. Performance figures show that the proposed mechanism can exploit control-flow independence for nearly 50% of the mispredicted branches, which results in a performance improvement that ranges from 14% to 17,8% for realistic configurations of forthcoming microprocessors.

Efficient architectural exploration of TAGE branch predictor for embedded processors

Conditional branch prediction remains one of the most important enabling technologies for high-performance microprocessors. A small improvement in accuracy can result in a large improvement in performance as well as a significant reduction in energy wasted on wrong-path instructions. Neural-based branch predictors have been among the most accurate in the literature. The recently proposed scaled neural analog predictor, or SNAP, builds on piecewise-linear branch prediction and relies on a mixed analog/digital implementation to mitigate latency as well as power requirements over previous neural predictors. We present an optimized version of the SNAP predictor, hybridized with two simple two-level adaptive predictors. The resulting optimized predictor, OH-SNAP, delivers very high accuracy compared with other state-of-the-art predictors.

The need for multiple branch prediction is inherent to wide instruction fetching. This paper presents a completion time multiple branch predictor called the Tree-based Multiple Branch Predictor (TMP) that builds on previous single branch prediction techniques. It employs a tree structure of branch predictors, or tree-node predictors, and achieves accurate multiple branch prediction by leveraging the high accuracies of the individual branch predictors. A highly-efficient TMP design uses the 2-bit saturating counters for the tree-node predictors. To achieve higher prediction rate, the TMP employs two-level schemes for the tree-node predictors resulting in a three-level TMP design. Placing the TMP at completion time reduces the critical latency in the front-end of the pipeline; the resultant longer update latency does not significantly impact the overall performance. In this paper the TMP is applied to a trace cache design and shown to be very effective in increasing its performance.

The Precomputed-Branch architecture: Efficient branches with compiler support

Modern wide-issue superscalar processors tend to adopt deeper pipelines in order to attain high clock rates. This trend increases the number of on-the-fly instructions in processors and a mispredicted branch can result in substantial amounts of wasted work. In order to mitigate these wasted works, an accurate branch prediction is required for the high performance processors. In order to improve the prediction accuracy, we propose the bimode++ branch predictor. It is an enhanced version of the bimode branch predictor. Throughout execution from the start to the end of a program, some branch instructions have the same result at all times. These branches are defined as extremely biased branches. The bimode++ branch predictor is unique in predicting the output of an extremely biased branch with a simple hardware structure. In addition, the bimode++ branch predictor improves the accuracy using the refined indexing and a fusion function. Our experimental results with benchmarks from SpecFP, SpecINT, multi-media and server area show that the bimode++ branch predictor can reduce the misprediction rate by 13.2% to the bimode and by 32.5% to the gshare.

As modern microprocessors employ deeper pipelines and issue multiple instructions per cycle, they are becoming increasingly dependent on accurate branch prediction. Because hardware resources for branch-predictor tables are invariably limited, it is not possible to hold all relevant branch history for all active branches at the same time, especially for large workloads consisting of multiple processes and operating-system code. The problem that results, commonly referred to as aliasing in the branch-predictor tables, is in many ways similar to the misses that occur in finite-sized hardware caches.In this paper we propose a new classification for branch aliasing based on the three-Cs model for caches, and show that conflict aliasing is a significant source of mispredictions. Unfortunately, the obvious method for removing conflicts --- adding tags and associativity to the predictor tables --- is not a cost-effective solution.To address this problem, we propose the skewed branch predictor, a multi-bank, tag-less branch predictor, designed specifically to reduce the impact of conflict aliasing. Through both analytical and simulation models, we show that the skewed branch predictor removes a substantial portion of conflict aliasing by introducing redundancy to the branch-predictor tables. Although this redundancy increases capacity aliasing compared to a standard one-bank structure of comparable size, our simulations show that the reduction in conflict aliasing overcomes this effect to yield a gain in prediction accuracy. Alternatively, we show that a skewed organization can achieve the same prediction accuracy as a standard one-bank organization but with half the storage requirements.

Modern processors usually have a deep pipeline, superscalar architecture to obtain higher performance. As pipelines are getting deeper, accurate branch prediction is critical to achieve high performance, since fetched instructions after a branch have to be flushed from inside the pipeline when the prediction is wrong. This paper studies the performance of several types of branch predictors, starting from local branch predictor and global branch predictor. Simulation results show that the global history predictor outperforms the local history predictor due to the characteristic that branches tend to be correlated. However, the global history predictor still suffers an aliasing problem that degrades performance. Four techniques are proposed to alleviate the aliasing problem. The performance is evaluated by using Simplescalar with SPEC CINT95 benchmark programs. The proposed predictors display better performance over the conventional predictors after careful configuration of each.

During the 1990s Two-level Adaptive Branch Predictors were developed to meet the requirement for accurate branch prediction in high-performance superscale processors. However, while two-level adaptive predictors achieve very high prediction rates, they tend to be very costly. In particular, the size of the second level Pattern History Table (PHT) increases exponentially as a function of history register length. Furthermore, many of the prediction counters in a PHT are never used; predictions are frequently generated from non-initialised counters and several branches may update the same counter, resulting in interference between branch predictions. In this paper, we propose a Cached Correlated Two-Level Branch Predictor in which the PHT is replaced by a Prediction Cache. Unlike a PHT, the Prediction Cache saves only relevant branch prediction information. Furthermore, predictions are never based on uninitialised entries and interference between branches is eliminated. We simulate three versions of our Cached Correlated Branch Predictors. The first predictor is based on global branch history information while the second is based on local branch history information. The third predictor exploits the ability of cached predictors to combine both global and local history information in a single predictor. We demonstrate that our predictors deliver higher accuracy than conventional predictors at a significantly lower cost.

Pipeline stalls due to conditional branches represent one of the most significant impediments to realizing the performance potential of deeply pipelined, superscalar processors. Many branch predictors have been proposed to help alleviate this problem, including the Two-Level Adaptive Branch Predictor, and more recently, two-component hybrid branch predictors.In a less idealized environment, such as a time-shared system, code of interest involves context switches. Context switches, even at fairly large intervals, can seriously degrade the performance of many of the most accurate branch prediction schemes. In this paper, we introduce a new hybrid branch predictor and show that it is more accurate (for a given cost) than any previously published scheme, especially if the branch histories are periodically flushed due to the presence of context switches.