Branch Target Address Cache Entry Replacement Logic
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Solution Overview
Problem
Conventional branch target address caches (BTACs) are limited to storing information about only two branch instructions per 16-byte cache line, leading to performance issues when three or more branches are present, as the existing design complexity and power consumption considerations hinder efficient handling of multiple branches within the same fetch quantum.
Innovation Solution
Implementing a microprocessor with a BTAC that employs a priority-based replacement scheme for branch instructions, where each entry can store branch prediction information for up to N branch instructions, with update logic determining and prioritizing branch types for replacement, and using a pseudo-random mechanism to override priority in specific cases, allowing for efficient handling of additional branch instructions within the same cache line.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If the BTAC is designed to store information for only two branch instructions per 16-byte cache line, then timing is improved and power consumption and die size are reduced, but performance deteriorates when three or more branches are present in the same fetch quantum
Solution Approach 1:
The patent implements a dynamic replacement policy where the BTAC can adaptively store up to four branch instructions per 16-byte cache line when needed, rather than being fixed at two. The update logic dynamically determines whether to replace existing entries based on branch type priorities, allowing the cache to expand its capacity from 2 to 4 entries as needed while maintaining the physical constraint of the cache structure.
2Area of stationary object
If the BTAC is designed to store information for only two branch instructions per 16-byte cache line, then die size is reduced, but performance deteriorates when three or more branches are present in the same fetch quantum
Solution Approach 1:
The patent implements a dynamic replacement policy where the BTAC can adaptively store up to four branch instructions per 16-byte cache line when needed, rather than being fixed at two. The update logic dynamically determines whether to replace existing entries based on branch type priorities, allowing the cache to expand its capacity from 2 to 4 entries as needed while maintaining the physical constraint of the cache structure.
3Productivity
If the BTAC stores branch prediction information for more than two branch instructions, then performance is improved when multiple branches are present, but device complexity increases significantly
Solution Approach 1:
The patent applies different priorities to different branch instruction types stored in the BTAC. Instead of treating all branches equally, the system assigns higher priority to certain branch types (such as indirect branches and loops) and lower priority to others (such as sequential branches). This local differentiation allows the cache to intelligently retain the most important branch predictions while replacing less critical ones, achieving better performance without proportionally increasing complexity.
Solution Approach 2:
The patent changes the parameter of branch instruction type as a key factor in the replacement decision. By categorizing branches into different types with different priorities, the system transforms the replacement policy from a simple counter-based approach to a type-aware approach. This parameter change enables the BTAC to handle multiple branches effectively while keeping the replacement logic manageable through classification rather than complex analysis.
Data Source
AI summary
A microprocessor includes a branch target address cache (BTAC), each entry thereof configured to store branch prediction information for at most N branch instructions. An execution unit executes a branch instruction previously fetched in a fetch quantum. Update logic determines whether the BTAC is already storing information for N branch instructions within the fetch quantum (N is at least two), updates the BTAC for the branch instruction if the BTAC is not already storing information for N branch instructions, determines whether a type of the branch instruction has a higher replacement priority than a type of the N branch instructions if the BTAC is already storing information for N branch instructions, and updates the BTAC for the branch instruction if the type of the branch instruction has a higher replacement priority than the type of the N branch instructions already stored in the BTAC.