Abstract: A mechanism is described that predicts the usefulness of a prefetching instruction during the instruction's decode cycle. Prefetching instructions that are predicted as useful (prefetch useful data) are sent to an execution unit of the processor for execution, while instructions that are predicted as not useful are discarded. The prediction regarding the usefulness of a prefetching instructions is performed utilizing a branch prediction mask contained in the branch history mechanism. This mask is compared to information contained in the prefetching instruction that records the branch path between the prefetching instruction and actual use of the data. Both instructions and data can be prefetched using this mechanism.

Abstract: A mechanism is described that prefetches instructions and data into the cache using a branch instruction as a prefetch trigger. The prefetch is initiated if the predicted execution path after the branch instruction matches the previously seen execution path. This match of the execution paths is determined using a branch history queue that records the branch outcomes (taken/not taken) of the branches in the program. For each branch in this queue, a branch history mask records the outcomes of the next N branches and serves as an encoding of the execution path following the branch instruction. The branch instruction along with the mask is associated with a prefetch address (instruction or data address) and is used for triggering prefetches in the future when the branch is executed again. A mechanism is also described to improve the timeliness of a prefetch by suitably adjusting the value of N after observing the usefulness of the prefetched instructions or data.