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An agree predictor is a two-level adaptive predictor with globally shared history buffer and pattern history table, and an additional local saturating counter. The outputs of the local and the global predictors are XORed with each other to give the final prediction.
The local predictor is a two-level table which records the history of individual branches. It consists of a 1,024-entry by 10-bit branch history table. A two-level table was used as the prediction accuracy is similar to that of a larger single-level table while requiring fewer bits of storage. It has a 1,024-entry branch prediction table.
This approach is employed in a variety of areas, including branch prediction in pipelined processors, value prediction for exploiting value locality, prefetching memory and files, and optimistic concurrency control in database systems. [1] [2] [3] Speculative multithreading is a special case of speculative execution.
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The general rule for a two-level adaptive predictor with an n-bit history is that it can predict any repetitive sequence with any period if all n-bit sub-sequences are different.[8] The advantage of the two-level adaptive predictor is that it can quickly learn to predict an arbitrary repetitive pattern.
With predication, all possible branch paths are coded inline, but some instructions execute while others do not. The basic idea is that each instruction is associated with a predicate (the word here used similarly to its usage in predicate logic) and that the instruction will only be executed if the predicate is true.
Branch target prediction is not the same as branch prediction, which guesses whether a conditional branch will be taken or not-taken in a binary manner. In more parallel processor designs, as the instruction cache latency grows longer and the fetch width grows wider, branch target extraction becomes a bottleneck. The recurrence is:
If the branch is not taken, the pipeline stays full. If the branch is taken, the instruction is flushed (marked as if it were a NOP), and one cycle's opportunity to finish an instruction is lost. Branch Likely: Always fetch the instruction after the branch from the instruction cache, but only execute it if the branch was taken.
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