BTAC Management via BHT Feedback to Reduce Pipeline Flushes

Solution Overview

Problem

Conventional branch prediction techniques suffer from the multiple flush cycle problem, where repeated pipeline flushes occur due to conflicts between the branch target address cache (BTAC) and the branch history table (BHT), leading to performance and power consumption issues.

Innovation Solution

A branch target address cache (BTAC) and a branch predictor circuit are configured to store and manage state information, allowing the BTAC entry to be updated based on the actual branch resolution, reducing mispredictions and conflicts between the BTAC and the branch predictor circuit.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If standard cache replacement algorithms are used to manage BTAC entries, then the BTAC can be implemented with simple replacement logic, but conflicts between BTAC and BHT predictions occur frequently causing multiple pipeline flushes

Engineering Contradiction:

ImproveBTAC management logicVSAvoidbranch prediction accuracy

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent implements feedback by monitoring BHT prediction outcomes and using this information to dynamically manage BTAC entries. When a BHT misprediction is detected, the corresponding BTAC entry is proactively removed to prevent future conflicts. This feedback mechanism coordinates the two-stage predictor to reduce mispredictions while maintaining simple replacement logic through targeted entry management.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent applies preliminary action by proactively removing BTAC entries before conflicts can occur. When the BHT stage predicts a different outcome than what would be implied by BTAC contents, the system preemptively removes the conflicting BTAC entry, preventing the multiple flush cycle problem before it can manifest.

Inventive Principle:

Principle #10Preliminary action

2Device complexity

If the BTAC is updated only after pipeline completion, then the replacement logic remains simple, but the probability of misprediction conflicts between BTAC and BHT increases

Engineering Contradiction:

ImproveBTAC update mechanismVSAvoidpipeline flush cycles

Core Design Contradiction:

Device complexityVSLoss of time

Solution Approach 1:

The system performs preliminary action by proactively managing BTAC entries based on BHT prediction outcomes before pipeline flushes occur. When a BHT misprediction is detected, the corresponding BTAC entry is removed in advance, preventing future conflicts and avoiding the time loss associated with multiple pipeline flush cycles.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements feedback by using BHT prediction outcomes to trigger proactive BTAC entry management. This feedback loop allows the system to update BTAC contents based on actual prediction performance, reducing misprediction conflicts and preventing pipeline flushes before they occur.

Inventive Principle:

Principle #23Feedback

3Reliability

If proactive BTAC entry removal is implemented, then misprediction conflicts are reduced, but the BTAC management complexity increases

Engineering Contradiction:

Improvebranch prediction accuracyVSAvoidBTAC management logic

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent implements feedback by monitoring BHT prediction outcomes and using this information to dynamically manage BTAC entries. When a BHT misprediction is detected, the corresponding BTAC entry is proactively removed to prevent future conflicts. This feedback mechanism coordinates the two-stage predictor to reduce mispredictions while maintaining simple replacement logic through targeted entry management.

Inventive Principle:

Principle #23Feedback

Data Source

PatentUS8935517B2System and method for selectively managing a branch target address cache of a multiple-stage predictor

Publication Date: 2015.01.13 QUALCOMM INC

AI summary

A multiple stage branch prediction system including a branch target address cache (BTAC) and a branch predictor circuit is disclosed. The BTAC is configured to store a BTAC entry. The branch predictor circuit is configured to store state information. The branch predictor circuit utilizes the state information to predict the direction of a branch instruction and to manage the BTAC entry based on the stored state information in response to actual resolution of the branch instruction.