Indirect Control Transfer Bias Prediction to Cut Mispredictions
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Solution Overview
Problem
Existing processors face inefficiencies and performance degradation due to mispredictions of biased control transfer instructions, which require re-fetching instructions from correct target addresses, leading to additional delays and reduced efficiency.
Innovation Solution
Implementing a bias prediction circuit to predict whether control transfer instructions are biased true or biased false, allowing these instructions to bypass the more complex instruction prediction circuit, thereby reducing the workload and improving efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a control transfer prediction circuit is implemented to predict execution paths, then instruction processing efficiency is improved, but prediction accuracy for biased control transfer instructions deteriorates leading to more mispredictions
Solution Approach 1:
The prediction circuit is segmented into two distinct circuits: a bias prediction circuit that handles biased control transfer instructions and an instruction prediction circuit that handles non-biased instructions. This segmentation allows each circuit to be optimized for its specific function, improving overall prediction accuracy while maintaining high processing efficiency.
Solution Approach 2:
The bias prediction circuit acts as an intermediary that pre-determines the bias status of control transfer instructions before they reach the main instruction prediction circuit. This intermediary function filters and categorizes instructions, allowing the main prediction circuit to focus only on non-biased instructions, thereby improving overall accuracy.
2Speed
If speculative instruction fetching is performed based on predictions, then processing speed is improved, but performance deteriorates when mispredictions occur due to discarded work and re-fetching
Solution Approach 1:
The bias prediction circuit performs preliminary action by determining the bias status of control transfer instructions before speculative fetching occurs. This pre-determination allows the system to make more accurate predictions about instruction execution paths, reducing mispredictions and the associated performance penalties from discarded work and re-fetching.
3Productivity
If multiple biased control transfer instructions are processed in parallel, then productivity is improved, but complexity of managing and resolving mispredictions increases
Solution Approach 1:
By segmenting the prediction circuit into separate bias prediction and instruction prediction circuits, the system can independently manage and resolve mispredictions for different instruction types. This segmentation simplifies the overall management complexity even when processing multiple instructions in parallel, as each circuit can handle its specific subset of instructions independently.
Data Source
AI summary
A processor may include an indirect control transfer prediction circuit. During fetch of an indirect control transfer instruction from memory to an instruction cache of the processor, the indirect control transfer prediction circuit may predict whether the indirect control transfer instruction is biased. Responsive to a prediction that the indirect control transfer instruction is biased, the indirect control transfer prediction circuit may cause the indirect control transfer instruction to be executed as an unconditional direct control transfer instruction according to the predicted bias.