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AGREE PREDICTOR. (Only a sketch for the branch direction prediction in Software Creation pdf417 2d barcode in Software AGREE PREDICTOR. (Only a sketch for the branch direction prediction




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AGREE PREDICTOR. (Only a sketch for the branch direction prediction generate, create pdf417 none with software projects 2 of 5 Industrial is given; it requires Software barcode pdf417 a decoupled BTB PHT.). 4.1 Branch Prediction Fetch Stage. Access BTB:. If BTBhit then / retu rns index / begin bias BTB[Biasarray [index]]; Access PHT; agreebit higherbit[PHT[PCsubset ]] pred XNOR(bias,agreebit); end else pred false; ...

. Commit Stage. if index is valid / w e had a BTB hit / then if brcd = bias then PHT[PCsubset ] (PHT[PCsubset ],bias) / reinforce / else PHT[PCsubset ] (PHT[PCsubset ],not bias). Now, two branches that Software barcode pdf417 alias in the PHT and are both strongly biased, but possibly in different directions, will not destroy each other. Of course, the scheme does not help if one branch is strongly biased and the other one is not. A related design is to associate not only a bias bit with each BTB entry, but also a saturating counter that indicates the con dence that can be given to the bias bit.

If the counter is saturated, then the prediction is that of the bias bit, and the PHT is neither consulted nor updated. Otherwise, the PHT is consulted and updated upon knowledge of the branch outcome, as in the agree scheme. The advantage of this scheme is that biased branches will be predicted with their entries saturating counters and will not access the PHT.

The latter is used only for less predictable branches. Thus, the amount of aliasing is reduced by the lter imposed on very predictable branches, because they do not in uence the PHT contents. We can mitigate the effects of destructive aliasing by replicating the PHT.

One possible design is to have two PHTs, accessed similarly, for example in a gshare fashion, where one of the PHTs is intended for branches biased towards taken and the other for branches biased towards not taken. A third PHT, a choice PHT accessed through the branch address only, will decide which of the two direction PHTs predictions should be chosen for that particular branch. In this bi-mode predictor, updating is done only for the PHT that was chosen to give the direction.

The choice PHT is also updated unless its prediction was not the branch outcome but the direction PHT was correct. In other words, if for example the choice PHT said to consult the taken PHT and the latter correctly predicted not taken, the choice PHT is left unchanged. In this design, aliasing of branches that are mostly taken (or mostly not taken) is not destructive.

Of course, destructive aliasing still occurs in the case of one branch outcome agreeing with the choice while the other does not. A second possible design is the skewed predictor, whereby three different hashing functions index three PHTs. A majority vote indicates the predicted direction.

By use of the skewing functions, the probability of aliasing in more than one of the PHTs is made quite small. In some sense, the skewing provides associativity (the. Front-End: Branch Prediction, Instruction Fetching, and Register Renaming Choice PHT Taken Not taken Tag Hash GR T/NT hit High bit Pred. Figure 4.14. The YAGS PDF-417 2d barcode for None predictor (adapted from Eden and Mudge [EM98]).

. same reasoning has bee n applied as a proposal to decrease con ict misses in caches; see 6). If the prediction is wrong, the three PHTs are updated; if it is correct only those that are correct are updated. The rationale for the partial updating is that the PHT that had the incorrect prediction must be holding the correct prediction for another branch.

The bias bits and con dence counters added to a BTB entry represent a small additional investment for a design that is already expensive in storage. On the other hand, having three PHTs instead of one in the replicated PHT designs can be overly taxing. What might be best in designs that either have no BTB or use a decoupled BTB PHT organization is to take advantage of the bias that exists from most branches as in the bi-mode predictor and to limit the aliasing of those less predictable branches as in the skewed case.

At the same time, we do not want to expand storage unduly. A proposal along these lines is the YAGS ( yet another global scheme ), shown in Figure 4.14.

As in the bi-mode predictor, YAGS has a choice PHT that is indexed by the branch address and directs the prediction to one of two predictors. However, because the choice predictor already indicates a bias towards taken or not taken and therefore takes care of neutral aliasing, what would be useful is to discover instances where the bias is not followed consistently. So we can replace the bi-mode direction PHTs by structures that indicate exceptions to the direction predicted by the choice PHT.

More precisely, when the choice PHT indicates taken and the outcome is not taken, the branch address is entered, via a gshare-like indexing, in a not taken cache (NT cache in the gure). The T cache is lled in the same way for a not taken choice and a taken outcome. An entry in these caches consists of a (partial) tag and a 2-bit saturating counter.

Now, when a prediction is to be made, the choice PHT and the caches are accessed concurrently. If the choice PHT indicates taken (respectively, not taken ) and there is a miss in the NT cache (respectively, T cache), the prediction is taken (respectively not taken ). If there is a hit, the prediction is what is given by the 2-bit counter in the NT cache (respectively, T cache).

This is the function of the three muxes in Figure 4.14, summarized in another format in Table 4.3.

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