Intel branch predictors 386 and 486 didnt have any sort of hardware based dynamic branch prediction block. In particular, discuss whether or not your results support the use of the traditional static scheme of backward. The static prediction rules are no longer used, so neither are the prefixes. The decision causing the branch prediction can change during the program execution. Cmsc 411 computer systems architecture lecture 9 instruction. Pdf combining static and dynamic branch prediction to. The best performance is obtained by predicting all branches as. About 80% of instructions executed in branch delay slots useful in computation. It comprises hardwarefixed prediction and compilerdirected prediction. 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. What branch prediction accuracy must p2 have in order to perform as well as p1.
Encodes that direction as a hint bit in the branch instruction format. For static branch prediction, we have prediction not taken, prediction taken and predict based on the opcode. Static branch prediction is used in processors where the expectation is that the branch behavior is highly predictable at compile time. 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. Pentium iii has a twolevel of local history based branch predictor where each entry is 2bit saturating counter also.
First, several compiler optimizations such as code layout, scheduling, predication, etc. In conclusion, we have researched a number of branch prediction methods. They are static because the prediction is already known before the program is executed. If the decoder spots a branch that the bpu hadnt predicted, it has to resteer the fetcher. Backward branches will be predicted to be taken, since that is the most common case. Support for speculation in general, when we reorder instructions, register renaming can ensure we do not violate register data dependences. At the execute stage it is known whether or not the branch is taken. A further refinement of strategy 1 is to make a prediction based on the type of branch, determined, for example, by.
What if the branch condition is calculated by the end of e1. The accuracy of the static predictor will depend on the type of code being. Branch prediction static software migration to nios ii gen2 processors altera recommends that all bsp packages in the nios ii software build tools projects are rebuilt after. Static branch prediction performs worse than existing dynamic techniques, but is useful for performing static compiler optimizations. We used the simplescaler simulator to generate our branch prediction results. Pdf improving static branch prediction in a compiler. Static prediction is the simplest branch prediction technique because it does not rely on information about the dynamic history of code executing.
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. Static methods are usually carried out by the compiler. Operands read either from register file real or speculative or via common data bus advantage. 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. And it is also slightly more efficient for the instruction cache. Previous attempts at static branch prediction have either used simple heuristics, which can be quite inaccurate, or put the burden onto the programmer by using execution profiling data or source code hints. This had positive effect both because of cpu prediction block more efficient usage and more. We made a number of changes to the source code in order to perform our branch prediction methods available below. Carnegie mellon computer architecture 38,389 views 1.
Modify the tracer reader to calculate the misprediction rate that is, the percentage of conditional branches that were mispredicted for these two simple schemes. Software methods usually cooperate with hardware methods. 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. A static branch scheme software techniques is very simple and easy. In this article, we investigate a new approach to programbased branch prediction that uses a body of existing programs to predict the branch behavior in a new program. With static branch prediction, we must x a prediction per branch in the program before the program runs.
Compiler determines likely direction for each branch using profile run. Static branch prediction good static branch predictions are invaluable information for compiler optimisation or performance estimation. One of the way to save some cpu resources was to analyse if branches and rewrite if condition so normal flow doesnt break linear instructions sequence. The bimode branch predictor is aimed at the elimination of destructive aliasing in global history indexed schemes. I once had significant amount of performance optimization actions on arm7,9. Static branch prediction always taken sun supersparc always not taken mips r4000, intel i486 instruction specific. Static branch prediction how is static branch prediction.
Static branch prediction predicts always the same direction for the same branch during the whole program execution. Engineering lecture 14 branch prediction krste asanovic. For the static schemes you do not have to run a simulation, just use the branch characterization results. Without duplicating code, the static prediction for the second conditional must favor the left side or the right side, penalizing one of the correlated paths. The code should be structured as for the static prediction this is not strictly necessary, but most cpus use the static prediction first time a branch is encountered. To improve the branch prediction accuracy, a combination of static and dynamic techniques is employed. Static branch prediction branch prediction schemes can be classified into static and dynamic schemes. Branch prediction doesnt come from theory but from a strictly practical observation. Static prediction static prediction is used to guide code scheduling strategies simple strategy for all branches in the code based on opcode or direction of branches profile based. Static conditional branch prediction branch prediction schemes can be classified into static and dynamic schemes. 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.
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 march 10, 2020 l109 overall probability a branch is taken is 6070% but. 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 predict ion techniques used in pipeline processors. Proceedings of the acm sigplan 95 conference on programming language design and implementation pldi, jun 1995, pages 6778. Static branch prediction can also be used to assist dynamic predictors. This paper presents several new techniques for static branch prediction and profiling. 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 code around branches need to predict branch statically during compile. The two prefixes, 0x2e, which hinted that the branch wouldnt be taken, and 0x3e, which hinted that the branch would be taken, modified the static prediction rules used by the branch predictor. Branch hazards and static branch prediction techniques. Cs 152 computer architecture and engineering lecture 14.
Branch target buffers and return address predictors. Prediction is decided on the computation history of the program. Improving static branch prediction accuracy is an important problem with various interesting applications. A good dynamic predictor can outperform profiledriven static prediction by a large margin. Isa can attach preferred direction semantics to branches, e. Branch prediction is the process of correctly predicting whether branches will be taken or not before they are actually executed.
Branch predictors need to be warmed up during sampled simulation. We looked at both static and dynamic branch prediction schemes. Instead, it predicts the outcome of a branch based solely on the branch instruction. Taken ee524 cpts561 computer architecture 10 accuracy of different schemes frequency of mispredictions 0% 2% 4% 6% 8% 10% 12% 14% 16% 18%. Static branch prediction always taken sun supersparc always not. In both cases, care must be taken not to change the. 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. They are more widely used than the static branch predictors as well. Aug 01, 2017 lecture 11 branch prediction carnegie mellon computer architecture 20 onur mutlu duration. The first technique combines multiple predictions of a branchs outcome into a prediction of the probability that the branch is taken. The compiler should not use conditional execution unless the condition is unpredictable.
Accurate static branch prediction by value range propagation. For example, some architecture have a likely bit into the instruction opcode that can be set by the compiler if a branch is determined to be likely taken. Static branch prediction overall probability a branch is taken is 6070% but. Branch prediction is required in the design to reduce the core cpi loss that arises from the longer pipeline.
Assume p1 uses static branch prediction and assumes not taken. Static branch frequency and program profile analysis. Branch outcomes are calculated during the decode stage stage 2. This scheme, shown in figure 1, splits the secondlevel. Lecture9 branch prediction cs422spring 2018 cse iit kanpur. If a branch destination is wrong or if a nonbranch was misguessed as a branch its noted there, the btb is corrected, and the fetchers are resteered to the right destination. All branches were statically predicted as not taken.
Dynamic branch prediction uses run time information when predicting. 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. 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. Download the paper pdf file and the slides pdf file. Lecture 11 branch prediction carnegie mellon computer architecture 20 onur mutlu duration. This paper presents a new approach to static branch prediction called value range propagation. Some compilerbased static branch prediction uses profile, for example, ia64 uses sptk static predict taken and spnt static predict not taken.
Instruction sequence has redundancies that are artifacts of way that humans compilers think about problems. Assume on a load the value is available at the end of m3. Branch penalties limit performance of deeply pipelined processors modern branch predictors have high accuracy 95% and can reduce branch penalties significantly required hardware support. Optional static branch prediction higher performance multiplier improved and more deterministic divider.