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Branch prediction attempts to guess whether a conditional jump will be taken or not. Branch target prediction attempts to guess the target of a taken conditional or unconditional jump before it is computed by decoding and executing the instruction itself. Branch prediction and branch target prediction are often combined into the same circuitry.
In computer architecture, predication is a feature that provides an alternative to conditional transfer of control, as implemented by conditional branch machine instructions. Predication works by having conditional ( predicated ) non-branch instructions associated with a predicate , a Boolean value used by the instruction to control whether the ...
An example of a simple hardware branch prediction scheme is to assume that all backward branches (i.e. to a smaller program counter) are taken (because they are part of a loop), and all forward branches (to a larger program counter) are not taken (because they leave a loop).
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 ...
If the prediction is true, the predicted execution is allowed to commit; however, if there is a misprediction, execution has to be unrolled and re-executed. Common forms of this include branch predictors and memory dependence prediction. A generalized form is sometimes referred to as value prediction. [7]
In computer architecture, a branch target predictor is the part of a processor that predicts the target, i.e., the address of the instruction that is executed next, of a taken conditional branch or unconditional branch instruction before the target of the branch instruction is computed by the execution unit of the processor.
On average, every fifth instruction executed is a branch, so without any intervention, that's a high amount of stalling. Techniques such as branch prediction and speculative execution are used to lessen these branch penalties. Branch prediction is where the hardware makes educated guesses on whether a particular branch will be taken.
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.