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The branch predictor may, for example, recognize that the conditional jump is taken more often than not, or that it is taken every second time. Branch prediction is not the same as branch target prediction. Branch prediction attempts to guess whether a conditional jump will be taken or not.
Better branch predictors are developed and validated statistically by running them in simulation on a variety of test programs. Good predictors usually count the outcomes of previous executions of a branch. Faster, more expensive computers can then run faster by investing in better branch prediction electronics.
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.
Elimination of unnecessary branch instructions can make the execution of necessary branches, such as those that make up loops, faster by lessening the load on branch prediction mechanisms. Elimination of the cost of a branch misprediction which can be high on deeply pipelined architectures.
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:
Superscalar, branch prediction PowerPC e500: Dual 7 stage Multi-core PowerPC e600: 3-issue 7 stage Superscalar out-of-order execution, branch prediction PowerPC e5500: 2010 4-issue 7 stage Out-of-order, multi-core PowerPC e6500: 2012 Multi-core PowerPC 603: 4 5 execution units, branch prediction, no SMP PowerPC 603q: 1996 5 In-order PowerPC 604 ...
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.
Branch prediction is where the hardware makes educated guesses on whether a particular branch will be taken. In reality one side or the other of the branch will be called much more often than the other. Modern designs have rather complex statistical prediction systems, which watch the results of past branches to predict the future with greater ...