Control hazards, which arise from control-flow changes like branches and jumps in a program, disrupt the smooth flow of instructions in a pipeline. Solving these hazards is crucial for maintaining pipeline performance. Control hazards are solved by predicting which instruction should be fetched and flushing the pipeline if the prediction is later determined to be wrong or by stalling the pipeline until the decision is made.
There are two primary strategies employed by modern processors to mitigate control hazards:
1. Branch Prediction
This is the most common and effective technique. Instead of waiting for the branch instruction to complete and determine the next instruction's address, the processor attempts to predict the outcome of the branch (taken or not taken) and the target address.
Here's how it generally works:
- When a branch instruction enters the pipeline, the processor makes a prediction about whether the branch will be taken (jumping to a new location) or not taken (continuing sequentially).
- Based on the prediction, the processor starts fetching instructions from the predicted path.
- While these instructions are being fetched and processed speculatively, the branch instruction completes its execution and the actual outcome is determined.
- If the prediction was correct, the speculatively fetched instructions are allowed to complete, and the pipeline continues without interruption.
- If the prediction was wrong, the instructions that were fetched down the incorrect path are flushed from the pipeline, and the pipeline is reset to fetch instructions from the correct path. This process incurs a performance penalty known as a misprediction penalty.
Techniques used in branch prediction range from simple static methods (always predict taken or not taken) to complex dynamic methods that use hardware structures to learn past behavior of branches:
- Static Prediction: Prediction is fixed at compile time or based on instruction type (e.g., loops predict taken, forward branches predict not taken).
- Dynamic Prediction: Hardware predicts based on the history of the branch's execution. This often involves:
- Branch History Tables (BHT): Store a few bits indicating the recent outcomes of branches.
- Pattern History Tables (PHT): Use global history combined with local history.
- Branch Target Buffers (BTB): Store predicted target addresses to speed up fetching the first instruction of the target path.
Effective branch prediction significantly reduces the stalls associated with control hazards by minimizing mispredictions.
2. Pipeline Stalling (Inserting Bubbles)
A simpler, but less performant, method is to stall the pipeline.
- When a branch instruction is detected, the processor simply halts or stalls the instruction fetch unit.
- No new instructions are fetched into the pipeline.
- This stalling continues until the branch instruction completes execution and the correct next instruction address is known.
- Once the decision is made and the target address is calculated, the pipeline resumes fetching instructions from the determined path.
This creates "bubbles" or empty stages in the pipeline, wasting cycles. While simple to implement, stalling drastically reduces the overall throughput of the pipeline, especially in programs with frequent branches.
Comparison of Techniques
| Feature | Branch Prediction | Pipeline Stalling |
|---|---|---|
| Performance | High (if prediction accuracy is high) | Low (introduces stalls) |
| Complexity | High (requires dedicated prediction hardware) | Low (simple control logic) |
| Overhead | Misprediction penalty (pipeline flush) | Always incurs stall penalty for every branch |
| Hardware | Requires prediction tables (BHT, BTB, etc.) | Minimal additional control logic |
In practice, modern high-performance processors heavily rely on sophisticated dynamic branch prediction mechanisms to hide the latency of control dependencies. Stalling is generally avoided as much as possible for performance-critical paths.
By implementing prediction techniques or, in simpler designs, stalling the pipeline, control hazards can be effectively managed, allowing instruction pipelines to operate more efficiently.
