Virtual Run-Ahead Resteering for Hard-to-Predict Branches
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Solution Overview
Problem
Branch mispredictions, particularly for hard-to-predict branches, significantly contribute to stalled cycles in modern computing designs, especially in wider and deeper cores, reducing instruction-level parallelism and degrading performance.
Innovation Solution
A virtual run-ahead mechanism in the core is employed to replay a subset of the backslice of hard-to-predict branches from the out-of-order engine directly, allowing early resolution of branch mispredictions by overriding branch predictions later in the pipeline, thus reducing the need for full pipeline flushes and improving performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional branch prediction is used for hard-to-predict branches, then the branch predictor structure is simple, but branch misprediction rate increases causing pipeline stalls
Solution Approach 1:
The patent performs preliminary branch evaluation by replaying a subset of the backslice (instructions leading to the branch) from the out-of-order engine to determine the actual branch outcome before the branch is executed in the main pipeline. This early determination allows the system to override incorrect branch predictions made by traditional predictors, preventing pipeline stalls caused by mispredictions of hard-to-predict branches
2Reliability
If full pipeline flush is performed on branch misprediction, then correct branch path is ensured, but instruction-level parallelism is lost and performance degrades
Solution Approach 1:
The patent extracts only the necessary subset of instructions (backslice) leading to the branch for replay and evaluation, rather than flushing the entire pipeline. By isolating and processing only the relevant instructions needed to determine branch outcome, the system maintains correctness of branch execution while preserving instruction-level parallelism in the rest of the pipeline that does not depend on the mispredicted branch
3Loss of time
If virtual run-ahead mechanism is implemented, then branch misprediction resolution is accelerated, but device complexity increases
Solution Approach 1:
The patent implements a virtual run-ahead mechanism that reuses existing out-of-order engine resources and backslice infrastructure for branch evaluation purposes. By making these existing components serve multiple functions (both their original purposes and branch outcome determination), the system accelerates misprediction resolution without proportionally increasing device complexity, as the same hardware structures are leveraged for dual purposes
Data Source
AI summary
Techniques for early resteering of a branch misprediction are described. Examples detailed herein use a virtual run-ahead mechanism in the core. On a branch misprediction for an hard-to-predict (H2P) branch, a subset of the backslice of the H2P branch are replayed from the out-of-order engine directly (while the main thread is flushing and restarting execution from the front-end).