Branch Target Buffer Segmentation for Direct and Indirect Branches
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Solution Overview
Problem
Modern processors face performance penalties due to Branch Target Buffer (BTB) mispredictions and inefficiencies in BTB usage, particularly in high code-footprint workloads, where limited information at allocation time leads to front-end stalls and re-steering, impacting total execution time.
Innovation Solution
A BTB configuration with multiple independent memory structures to store branch target information based on branch instruction types, utilizing page and branch target hotness information to reduce mispredictions, featuring a hot cache for frequent direct branches and a target cache for less frequent indirect branches, optimizing cache usage by splitting direct and indirect branch information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single BTB structure is used to store all branch target information, then the BTB size is reduced, but the misprediction rate increases due to limited information available at allocation time
Solution Approach 1:
The patent divides the BTB into multiple independent memory structures: a first portion for direct branch instructions and a second portion for indirect branch instructions. This segmentation allows each portion to be optimized for its specific branch type, improving prediction accuracy by reducing conflicts between different branch instruction characteristics while maintaining manageable complexity through specialized storage regions.
2Productivity
If branch target information is stored without differentiation by branch type, then the BTB configuration is simplified, but performance penalties occur due to front-end stalls and re-steering in high code-footprint workloads
Solution Approach 1:
The patent applies local quality by configuring different portions of the BTB with specialized characteristics matched to specific branch instruction types. The first portion is optimized for direct branches while the second portion is optimized for indirect branches, allowing each region to have the appropriate structure and allocation behavior for its intended purpose, thereby improving overall prediction accuracy and reducing execution time penalties.
3Reliability
If the BTB uses a unified allocation policy for all branch instructions, then the allocation process is simplified, but misprediction penalties increase due to inefficient usage in high code-footprint workloads
Solution Approach 1:
The patent segments the BTB into specialized portions that handle different branch instruction types separately. This segmentation enables the allocation process to be optimized for each branch type's specific characteristics, improving prediction accuracy by preventing allocation conflicts between direct and indirect branches while maintaining a structured approach to memory management.
4Productivity
If the BTB stores all branch instructions uniformly, then the storage structure is simplified, but front-end stalls occur due to limited information available at allocation time
Solution Approach 1:
The patent divides the BTB storage structure into multiple independent portions, each specialized for specific branch instruction types. This segmentation provides the front-end with more accurate and relevant prediction information by preventing mixing of different branch characteristics, thereby reducing stalls and improving throughput while maintaining a structured storage organization.
Data Source
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AI summary
In one embodiment, a processor includes: a decode circuit to decode a branch instruction, the branch instruction comprising a hint field to provide spatial information regarding a distance between the branch instruction and a target instruction of the branch instruction; a branch predictor to predict whether the branch instruction is to be taken; and a branch target buffer (BTB) coupled to the branch predictor. The BTB, based at least in part on the spatial information, may allocate an entry for the branch instruction in one of a first portion of the BTB and a second portion of the BTB. Other embodiments are described and claimed.