Search results
Results from the WOW.Com Content Network
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.
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.
Fetch the prediction for the addresses of the targets of branches in that run of instructions Select the address corresponding to the branch predicted taken As the predictor RAM can be 5–10% of the size of the instruction cache, the fetch happens much faster than the instruction cache fetch, and so this recurrence is much faster.
Historically, branch prediction took statistics, and used the result to optimize code. A programmer would compile a test version of a program, and run it with test data. The test code counted how the branches were actually taken. The statistics from the test code were then used by the compiler to optimize the branches of released code.
Control dependencies are dependencies introduced by the code or the programming algorithm itself. They control the order in which instructions occur within the execution of code. [4] One common example is an "if" statement. "if" statements create branches in a program.
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.
CHAID can be used for prediction (in a similar fashion to regression analysis, this version of CHAID being originally known as XAID) as well as classification, and for detection of interaction between variables. [4] [5] [6]
Branch prediction doesn't purport to perfectly predict the target of a conditional branch; as the article indicates, either the programmer or compiler provides hints in branch instructions (static branch prediction), or the hardware makes a prediction based on past behavior. And, as the article also indicates, those aren't 100% certain ...