What is a Gshare branch predictor?
Gshare The Gshare “global sharing” branch predictor is a (2, 2) predictor. It uses both global branch history and the location of a branch instruction to create an index into a table of 2-bit counters. The 2-bit counters are like the ones at the end of the previous lecture.
What is Tage branch predictor?
The TAGE predictor, TAgged GEometric length predictor, was introduced in [25]. TAGE relies on several predictor tables indexed through independent functions of the global branch/path history and the branch address. This allows to efficiently capture correlation on recent branch outcomes as well as on very old branches.
What is PHT in branch prediction?
A branch is predicted using the PHT if the counter is not saturated. If the counter is saturated, it means that the branch is highly biased in the direction indicated by the bias bit, and therefore the bias bit is used as a prediction.
How does a branch predictor work?
In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g., an if–then–else structure) will go before this is known definitively. The purpose of the branch predictor is to improve the flow in the instruction pipeline.
How does branch prediction help in processor performance?
It illustrates the use extra logic to generate a fast (but possibly wrong) estimate for the branch condition and target address. The processor fetches and executes the predicted instructions at full speed, but needs to revert all intermediate results when the prediction turns out to having been wrong.
What is an example of branch prediction?
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 attempts to guess whether a conditional jump will be taken or not.
How are history bits used for branch prediction?
One or more bits that reflect the recent history of the instruction (history bits) can be associated with each conditional branch instruction. These bits are referred to as a taken/not taken switch that directs the processor to make a particular decision the next time the instruction is encountered.
What is two-level predictor?
A two-level adaptive predictor remembers the history of the last n occurrences of the branch and uses one saturating counter for each of the possible 2n history patterns. This means that the last two occurrences of the branch are stored in a two-bit shift register.
What is correlating predictor?
There is a correlation between different branches. Branch Predictors that use the behavior of other branches to make a prediction are called Correlating or two-level predictors. One of these predictors will be chosen based on the behavior of the earlier branches and its prediction used.
How does a 1 bit branch predictor work?
How is 1-bit branch prediction implemented in hardware? – Each table entry contains one prediction bit for that branch, e.g., o for predict not taken (N), 1 for predict taken (T). The prediction bit is used to predict the branch outcome. It is updated after the branch’s actual outcome is known.
Where does branch prediction happen?
In order to avoid wasting cycles on frontend pipeline bubbles, each taken jump needs a BTB hit. This branch prediction must happen very early in the CPU pipeline, before instruction decode is finished. This same mechanism is needed for any taken branch, whether it’s unconditional, conditional or a function call.
How does one-level branch prediction work in a table?
•One-level (bimodal or non-correlating) branch prediction uses only one level of branch history. •These mechanisms usually employ a table which is indexed by lower N or “a” bits of the branch address. •Each table entry (or predictor) consists of n history bits, which form an n-bit automaton or saturating counters.
Is there a processor that can predict branch direction?
Some superscalar processors (MIPS R8000, Alpha 21264, and Alpha 21464 (EV8)) fetch each line of instructions with a pointer to the next line. This next-line predictor handles branch target prediction as well as branch direction prediction.
When is a decision made in dynamic branch prediction?
In static prediction, all decisions are made at compile time, before the execution of the program. Dynamic branch prediction uses information about taken or not taken branches gathered at run-time to predict the outcome of a branch.
When to use the branch history Register in adaptive predictor?
Two-level adaptive predictor. The branch history register is used for choosing which of the four saturating counters to use. If the history is 00, then the first counter is used; if the history is 11, then the last of the four counters is used.