Branch Target Buffer Victim Cache for Low Latency Prediction

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Solution Overview

Problem

Existing branch prediction mechanisms face challenges in achieving both high capacity and low latency in branch target buffer (BTB) operations, particularly in workloads with a large set of branches that are difficult to predict accurately, leading to increased latency and performance impacts.

Innovation Solution

Implementing a branch target buffer (BTB) victim cache that stores evicted entries from other BTBs, allowing for faster access and reduced latency by using a victim cache that is accessed in parallel with BTB lookups, and incorporating a miss queue and eviction queue to manage these entries.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Quantity of substance

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:

Improvestorage capacityVSAvoidbranch resolution latency

Core Design Contradiction:

Quantity of substanceVSLoss of time

Solution Approach 1:

The branch target buffer is divided into multiple levels (BTB0, BTB1, BTB2) with different capacities and access characteristics. Each level handles a portion of the branch working set, allowing the system to track more branches overall while maintaining low latency for frequently accessed branches in smaller, faster upper levels.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The branch target buffers are organized as a hierarchical nested structure where BTB0, BTB1, and BTB2 are arranged in levels with BTB0 being the smallest and fastest, BTB1 being medium-sized, and BTB2 being the largest. This nested organization allows the system to provide fast access for hot branches while accommodating cold branches in larger lower levels, resolving the capacity-latency tradeoff.

Inventive Principle:

Principle #7Nested doll (Nesting)

2Quantity of substance

If multiple levels of branch target buffer storage are used to increase capacity, then more branches can be tracked, but the access speed decreases compared to single-level structures

Engineering Contradiction:

Improvebranch tracking capacityVSAvoidaccess speed

Core Design Contradiction:

Quantity of substanceVSSpeed

Solution Approach 1:

The system dynamically selects which BTB level to access based on the branch being predicted. Frequently accessed branches are kept in faster upper levels (BTB0, BTB1) while less frequently accessed branches reside in slower lower levels (BTB2). This dynamic organization optimizes access speed for the active branch working set while maintaining overall high capacity.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

Different levels of the BTB hierarchy are optimized for different access patterns and capacities. BTB0 provides fastest access for hot branches, BTB1 provides medium access speed for warm branches, and BTB2 provides largest capacity for cold branches. Each level has locally optimized characteristics matching its intended usage pattern.

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS20250110743A1Branch target buffer victim cache

Publication Date: 2025.04.03 MICROSOFT TECHNOLOGY LICENSING LLC

AI summary

Improved branch target buffer (BTB) structures are provided. A device can include branch target buffers storing entries corresponding to branch instructions and corresponding targets of the branch instructions. The device can include a victim cache storing a branch target buffer entry that has been evicted from a branch target buffer of the branch target buffers. The device can include branch prediction circuitry configured to access the victim cache responsive to receiving respective miss indications from each branch target buffer of the branch target buffers.