Branch Target Buffer Prefetching for Lower Prediction Latency

Solution Overview

Problem

Existing processor pipeline designs face challenges in accurately predicting branch instructions while maintaining low latency, as increasing branch prediction structure capacity impacts performance due to slower access times in larger branch target buffers.

Innovation Solution

Implement a hierarchy of branch target buffers with a branch target buffer prefetch table that anticipates future branch predictions, using branch prediction circuitry to detect patterns and prefetch entries to faster cache levels, reducing access latency.

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 access latency increases due to slower access times in larger branch target buffers

Engineering Contradiction:

Improvebranch prediction storage capacityVSAvoidbranch prediction access latency

Core Design Contradiction:

Quantity of substanceVSLoss of time

Solution Approach 1:

The branch target buffer is divided into multiple levels (L1 BTB and L2 BTB), with each level serving a specific purpose. The L1 BTB provides fast access for frequently accessed branches, while the L2 BTB provides larger capacity for less frequently accessed branches. This segmentation resolves the contradiction by organizing storage capacity across different speed tiers.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements a branch target buffer prefetching mechanism that proactively loads branch target buffer entries into the L1 BTB before they are actually needed. The prefetching is triggered by detecting branch misses in the L1 BTB, and the system pre-loads the target addresses into the L1 BTB in advance, thereby reducing the latency impact when those branches are subsequently executed.

Inventive Principle:

Principle #10Preliminary action

2Quantity of substance

If a hierarchy of branch target buffers is implemented with multiple levels, then the overall storage capacity is improved, but the device complexity increases due to additional buffer levels and management logic

Engineering Contradiction:

Improvebranch prediction storage capacityVSAvoidbranch target buffer structure complexity

Core Design Contradiction:

Quantity of substanceVSDevice complexity

Solution Approach 1:

The branch target buffer is divided into multiple levels (L1 BTB and L2 BTB), with each level serving a specific purpose. The L1 BTB provides fast access for frequently accessed branches, while the L2 BTB provides larger capacity for less frequently accessed branches. This segmentation resolves the contradiction by organizing storage capacity across different speed tiers.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces a prefetching mechanism that acts as an intermediary between the L2 BTB and the L1 BTB. When a branch miss occurs in the L1 BTB, the prefetching logic retrieves the target address from the L2 BTB and pre-loads it into the L1 BTB, thereby mediating the interaction between the two levels and simplifying the overall management complexity.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Loss of time

If branch target buffer entries are pre-loaded into faster cache levels, then the access latency is reduced, but the use of energy increases due to additional prefetching operations

Engineering Contradiction:

Improvebranch prediction access latencyVSAvoidprefetching energy consumption

Core Design Contradiction:

Loss of timeVSUse of energy by moving object

Solution Approach 1:

The patent implements a branch target buffer prefetching mechanism that proactively loads branch target buffer entries into the L1 BTB before they are actually needed. The prefetching is triggered by detecting branch misses in the L1 BTB, and the system pre-loads the target addresses into the L1 BTB in advance, thereby reducing the latency impact when those branches are subsequently executed.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The prefetching mechanism uses feedback from branch miss detections to intelligently trigger prefetching operations. When a branch miss occurs in the L1 BTB, the system uses this feedback information to determine that prefetching is needed, and subsequently loads the required entries from the L2 BTB. This feedback-based approach ensures that prefetching energy is consumed only when actually beneficial, rather than continuously.

Inventive Principle:

Principle #23Feedback

Data Source

PatentUS20250265090A1Device for performing branch target buffer entry prefetching

Publication Date: 2025.08.21 MICROSOFT TECHNOLOGY LICENSING LLC

AI summary

A branch prediction device includes a hierarchy of successively slower to access branch target buffers that store branch target buffer entries identifying branch instructions, branch prediction circuitry configured to predict future branch instructions, and a branch target buffer prefetch table coupled to receive candidate entries corresponding to predicted future branch instruction branch target buffer misses, each entry of the candidate entries corresponding to a precursor branch instruction, and to receive predicted precursor branch instructions that trigger promotion of an entry in a branch target buffer of the branch target buffers to a faster branch target buffer of the branch target buffers.