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Branch Prediction in Software Connect pdf417 in Software Branch Prediction




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4.1 Branch Prediction using barcode encoding for software control to generate, create pdf417 2d barcode image in software applications. Reading Data Matrix ECC200 PHTs GR (a) 1 Global regist er, several PHTs (global set). Selected bits of the PC point to a PHT; the GR points to a row of the PHTs. PC.

PHT BHT (b) Table of local history registers (BHT) and one PHT (set global). Selected bits of the PC point to a local history register, which in turn points to a saturating counter in the PHT. PC.

PHTs (c) Table of local history registers (BHT) and several PHTs (set set). Selected bits of the PC point to a local history register, which in turn points to a row in the PHTs. Other bits of the PC select the PHT.

. Figure 4.9. Alternatives in two-level predictors. scheme would be low er on most programs. Arguably, what is needed is a scheme that can predict both the correlations and the patterns. These hybrid predictors will be introduced in Section 4.

1.8. The functions to be performed in each stage of the pipeline are similar to those of the global register scheme, which itself is a two-level predictor.

Changes occur in the functions to access the BHT if it is present instead of a single global register, and in the choice of the PHT. The repair mechanism is more complex, as we see next..

4.1.6 Repair Mechan barcode pdf417 for None isms Because the repair mechanism introduces some complexity, it is important to know whether there is a bene t to having speculative updating and a repair mechanism.

. Front-End: Branch Prediction, Instruction Fetching, and Register Renaming 10010 GR Shifted GR 0010 0 1 Predictor Predicted branch result, say 1 Figure 4.10. Repair mechanism for global register (or gshare) predictor.

At prediction time, the GR is shifted left. If the prediction is taken (bit 1), a 0 is inserted at the right of the GR before inserting it into the FIFO queue. At repair time, the right new GR will be at the head of the queue.

. FIFO queue of old GRs The answer is an em phatic yes: speculative updating without repair is sometimes better and sometimes worse than nonspeculative updating, but speculative updating with repair is always a win. If the number of in- ight branches is limited, because for example of the checkpointing that has to be done for repair (see the next paragraph) and for other structures such as the register map (see Section 4.4 of this chapter), the win is not as great, but still suf ciently important to warrant the repair mechanism.

Repair in the global predictor and gshare schemes is based on checkpointing the global history register at each prediction. This is what we indicated earlier with the notation GRold GR. Since the front-end does not disturb program order, the global register up to the current bit not included can be checkpointed and entered in a FIFO queue.

At the same time, the predicted outcome is shifted in the global register. If the prediction is correct, when the branch is committed its checkpointed global register will be at the head of the queue, because commitment is also in program order. The head of the queue is simply discarded.

In the case of a misprediction with repair at commit time, the head of the queue appended with the bit complement of the prediction becomes the new global register, and all other entries are discarded. A simple optimization, shown in Figure 4.10, inserts the corrected GR in the FIFO queue.

If repair is to be done at execute time, then as soon as a misprediction has been detected, the global register can be restored similarly, but only the entries in the queue between the one corresponding to the misprediction and the tail are removed. To implement this optimization, each branch must carry a tag indicating the index of its entry in the FIFO queue. Such a scheme is used in the global predictor of the Alpha 21264.

It requires little hardware: a FIFO queue where each entry is a pair (tag, history register) and where the number of entries is the maximum number of branches that can be in ight in the back-end at any given time (or some smaller number, if a full queue is a structural hazard). Note that the saved global register, and the PC for gshare, are also needed for indexing the PHT for nonspeculative update purposes. Of course, other optimizations are possible (see the exercises).

When a local scheme is used, speculative update with repair is still bene cial, but somewhat less so than in global schemes, because many of the entries in the BHT are.
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