-Branch Prediction and Speculative Execution

1.5 Program Order and Dependencies

Branch Prediction and Speculative Execution

Although one or more instructions may begin execution during each cycle, each instruction takes several (or many) cycles to complete. Thus, when a branch instruction is decoded, its branch condition may not yet be known. However, the R10000 processor can predict whether the branch is taken, and then continue decoding and executing subsequent instructions along the predicted path.

When a branch prediction is wrong, the processor must back up to the original branch and take the other path. This technique is called speculative execution. Whenever the processor discovers a mispredicted branch, it aborts all speculatively-executed instructions and restores the processor's state to the state it held before the branch. However, the cache state is not restored (see the section titled "Side Effects of Speculative Execution").

Branch prediction can be controlled by the CP0 Diagnostic register. Branch Likely instructions are always predicted as taken, which also means the instruction in the delay slot of the Branch Likely instruction will always be speculatively executed. Since the branch predictor is neither used nor updated by branch-likely instructions, these instructions do not affect the prediction of "normal" conditional branches.

Generated with CERN WebMaker