What is Gshare branch predictor?

What is 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 bimodal branch predictor?

The most well known technique, referred to here as bimodal branch prediction, makes a prediction based on the direction the branch went the last few times it was executed. More recent work has shown that significantly more accurate predictions can be made by utilizing more branch history.

What is Tage 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. The TAGE predictor uses (partially) tagged components as the PPM-like predictor [17].

What are the different techniques for branch prediction explain?

Some dynamic branch prediction techniques are: 1-bit branch prediction technique. 2-bit branch prediction technique. Correlating branch prediction technique.

What is Gselect?

Global history with selected address bits also known as gselect n/m uses n-bits of branch PC and m-bits from global history. One could also say that bimodal predictor is gselect n/0 and global branch predictor is gselect 0/m.

What is PHT 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.

What is static and dynamic branch prediction?

Static prediction is the simplest branch prediction technique because it does not rely on information about the dynamic history of code executing. Instead, it predicts the outcome of a branch based solely on the branch instruction.

How accurate is branch prediction?

This scheme was implemented in the MIPS R10000 processor and the results showed prediction accuracy of ~90%.

What is a branch target buffer?

A branch target buffer is a buffer that is index by the branch instruction address with a tag for remaining bits. The info stored can be branch taken history and/or target address so it doesnt have to be recomputed.

What is the difference between static branch prediction and dynamic branch prediction?

Static branch prediction makes fixed prediction of a branch to either taken or not taken; Dynamic branch prediction makes prediction based on previous history situation, by looking at history table.

What are the two stages of the dynamic branch prediction?

1. Integrated branch prediction—The branch predictor becomes part of the instruction fetch unit and is constantly predicting branches, so as to drive the fetch pipeline. 2. Instruction prefetch—To deliver multiple instructions per clock, the instruction fetch unit will likely need to fetch ahead.

How good is branch prediction?

Using a random or pseudorandom bit (a pure guess) would guarantee every branch a 50% correct prediction rate, which cannot be improved (or worsened) by reordering instructions. (With the simplest static prediction of “assume take”, compilers can reorder instructions to get better than 50% correct prediction.)

What is an advantage of static branch prediction?

What is an advantage of static branch prediction? Increases hardware complexity. Increased performance. Low branch prediction accuracy (no better than chance).

What is static branch prediction?

How many cycles does branch prediction take?

On modern processors it takes between one and twenty CPU cycles. There are at least four categories of control flow instructions: unconditional branch (jmp on x86), call/return, conditional branch (e.g. je on x86) taken and conditional branch not taken.

How can branch prediction be improved?

One thing you can do in a high-level language is to eliminate branches by expressing the problem in terms of lookups or arithmetic. This helps branch prediction work better on the remaining branches, because there’s more “history” available. I’ve made huge performance improvements to bottleneck code with this approach.

What is the difference between branch target buffer and branch prediction buffer?

Branch prediction buffers contain prediction about whether the next branch will be taken (T) or not (NT), but it does not supply the target PC value. A Branch Target Buffer (BTB) does this. The control unit looks up the branch target buffer during the “F” phase.

Why do we need a branch target buffer?

The Branch Target Buffer (BTB) can reduce the performance penalty of branches in pipelined processors by predicting the path of the branch and caching information used by the branch.

What is the disadvantage of dynamic branch prediction?

Question: What is a disadvantage of a dynamic branch prediction method, like history table? (Select all that apply) Increases hardware complexity. Increased performance. Simple implementation. Low branch prediction accuracy (no better than chance).

What if branch prediction is wrong?

Once the branch is decided, if the prediction was correct nothing happens but if the prediction was wrong, the pipeline simply switches to processing the correct instruction at the next clock.