BTB Prefetching via Back-Annotation for Branch Prediction

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Solution Overview

Problem

Existing branch prediction systems face challenges in accurately predicting branch targets, particularly in workloads with a large number of branches, leading to increased latency and reduced performance due to the need for larger branch prediction structures.

Innovation Solution

A branch prediction system that anticipates branch prediction entries by detecting patterns during instruction execution and dynamically constructing lists of future entries, using backpropagation to efficiently prefetch branch target buffer entries, adapting to workload changes, and utilizing a hierarchy of BTBs to reduce latency.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If the storage capacity of branch prediction structures is increased to track a large working set of branches, then the ability to anticipate branches is improved, but the latency required to resolve branches increases

Engineering Contradiction:

Improveability to track and anticipate branchesVSAvoidlatency to resolve branches

Core Design Contradiction:

Adaptability or versatilityVSLoss of time

Solution Approach 1:

The branch target buffer is divided into multiple levels (L1 BTB, L2 BTB, L3 BTB) with different capacities and access speeds. Each level handles a portion of the branch prediction workload, allowing the system to track a large working set of branches while maintaining low latency for frequently accessed branches in faster upper levels.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces a hierarchical dimension to the branch prediction structure, organizing BTB entries across multiple levels rather than using a single flat structure. This dimensional organization allows simultaneous optimization for both capacity (deeper hierarchy) and speed (upper levels).

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Measurement precision

If dedicated storage structures are used to remember branch details, then prediction accuracy is improved, but the device complexity increases

Engineering Contradiction:

Improvebranch prediction accuracyVSAvoidcomplexity of branch prediction structures

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The hierarchical BTB structure serves multiple functions: it stores branch target information, tracks branch patterns across different levels, and provides both accurate prediction and capacity for large working sets. This multi-functional design reduces the need for separate specialized structures.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent implements nested BTB levels where L2 and L3 BTBs are effectively nested within the hierarchical structure, with each level containing or referencing entries from deeper levels. This nesting allows compact organization of complex prediction data.

Inventive Principle:

Principle #7Nested doll (Nesting)

Data Source

PatentUS20250110744A1BTB prefecthing via back-annotation

Publication Date: 2025.04.03 MICROSOFT TECHNOLOGY LICENSING LLC

AI summary

A branch prediction system is configured to perform a method that includes identifying processor branch instructions and building branch target buffer (BTB) branch prediction entries corresponding to the branch instructions. The BTB branch prediction entries are stored in a hierarchy of BTBs. Target branch instruction are identified that have a target BTB entry following execution of a first branch instruction having a first BTB branch prediction entry. A target BTB entry reference is added to the first BTB branch prediction entry.