Branch Target Buffer Segmentation for Unconditional Branch Prioritization
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Solution Overview
Problem
Server workloads with large instruction footprints cause cache misses and branch target buffer (BTB) issues, leading to processing delays due to limited cache capacity and BTB entries, resulting in erroneous instruction fetching and pipeline stalls.
Innovation Solution
A data processing apparatus with a branch prediction unit comprising a BTB, a prefetch buffer, and a return address stack, prioritizing storage for unconditional branch instructions and using spatial footprints to efficiently prefetch instructions, allowing for effective BTB-directed prefetching without increasing silicon footprint or storage demands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If BTB capacity is increased to capture tens of thousands of branch targets, then branch prediction accuracy improves, but silicon footprint and storage demands increase significantly
Solution Approach 1:
The patent segments the unified BTB into separate dedicated structures: an unconditional branch target buffer for unconditional branches and a conditional branch target buffer for conditional branches. This segmentation allows each buffer to be optimized independently for its specific branch type, improving prediction accuracy for both types while keeping the total silicon footprint manageable by avoiding the need for a single large unified buffer.
2Loss of time
If BTB capacity is increased to reduce BTB misses, then processing delays are reduced, but device complexity and storage demands increase
Solution Approach 1:
By segmenting the BTB into dedicated unconditional and conditional branches buffers, the patent reduces device complexity compared to a single large unified buffer. Each dedicated buffer can use simpler access logic optimized for its specific branch type, reducing overall complexity while maintaining low processing delays through specialized optimization.
Solution Approach 2:
The patent implements preliminary action by using the return address stack to pre-fetch and store return addresses before they are needed for branch prediction. This allows the unconditional branch target buffer to be pre-populated with likely targets, reducing BTB misses and processing delays without requiring a larger buffer structure.
3Device complexity
If unified BTB stores both conditional and unconditional branch instructions, then storage utilization is simplified, but prediction accuracy for specific branch types deteriorates
Solution Approach 1:
The patent applies segmentation by creating separate dedicated buffers for unconditional and conditional branches, which improves prediction accuracy for each branch type by providing specialized storage and access logic. This segmentation resolves the trade-off by showing that the increased structural complexity is justified by the significant improvement in reliability for each branch category.
Solution Approach 2:
The patent applies local quality by optimizing each dedicated buffer differently according to the specific characteristics of its branch type. The unconditional branch buffer and conditional branch buffer can have different access patterns, entry formats, and replacement policies tailored to their respective branch types, improving overall prediction accuracy while maintaining manageable complexity through localized optimization.
Data Source
AI summary
A branch target buffer, BTB, is provided to store at least one BTB entry corresponding to a respective branch in a control flow in a sequence of machine-readable instructions of a computer program. The BTB has a tag field to compare with a program counter of a fetch address generator and at least one further field to store information characteristic of the branch instruction identified by the corresponding tag field and allowing a conditional branch to be distinguished from an unconditional branch instruction. The BTB has a predetermined storage capacity and is utilized such that unconditional branch instructions are preferentially allocated storage space in the BTB relative to conditional branch instructions.