Processor Alias Queue Store Collision Detection
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Solution Overview
Problem
Out-of-order processors face inefficiencies due to speculative execution leading to memory violations and costly load instruction replays, as they struggle to accurately detect and prevent store collisions between load and store instructions.
Innovation Solution
A processor with a register alias table and reorder buffer that uses an alias queue to store and compare instruction pointers of colliding load and store instructions, providing dependency information to prevent subsequent memory violations by accurately identifying colliding instruction pairs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If out-of-order speculative execution is used to improve processing throughput, then productivity increases, but memory violations and load instruction replays occur causing processing efficiency to deteriorate
Solution Approach 1:
The system performs preliminary detection of store collisions by comparing instruction pointers against the alias queue before load instructions are executed. When a potential collision is detected, the load instruction is prevented from executing speculatively, avoiding memory violations and replays while maintaining out-of-order execution benefits.
Solution Approach 2:
The alias queue stores historical instruction pointer pairs from previous store collisions. The detection mechanism continuously monitors current instructions against this feedback information, dynamically adjusting execution decisions based on past collision patterns to prevent future memory violations.
2Reliability
If store collision detection accuracy is improved to reduce memory violations, then reliability improves, but device complexity increases due to additional detection mechanisms
Solution Approach 1:
Instead of complex real-time analysis of instruction dependencies, the system uses instruction pointers as simplified copies/identifiers to represent instructions. These pointers are stored in the alias queue and compared against current instructions, providing accurate collision detection through simple pointer matching rather than complex dependency analysis.
Solution Approach 2:
The alias queue serves as an intermediary data structure between the store and load instruction streams. It mediates the collision detection process by storing historical instruction pointer pairs, allowing the system to detect collisions through simple comparisons rather than direct complex interaction between store and load execution paths.
3Loss of time
If load instruction replays are prevented through accurate collision detection, then loss of time is reduced, but device complexity increases due to alias queue management
Solution Approach 1:
The alias queue stores simple instruction pointer values rather than complex instruction metadata. These pointers are lightweight, easy to compare, and can be quickly cleared or updated. The queue entries are short-lived, containing only the essential information needed for collision detection without maintaining complex state information.
Data Source
AI summary
A register alias table for a processor including an alias queue, load and store comparators, and dependency logic. Each entry of the alias queue stores instruction pointers of a pair of colliding load and store instructions that caused a memory violation and a valid value. The store comparator compares the instruction pointer of a subsequent store instruction with those stored in the alias queue, and if a match occurs, indicates that a store index of the subsequent store instruction is valid. The load comparator determines whether the instruction pointer of a subsequent load instruction matches an instruction pointer stored in the alias queue. If so, dependency logic provides a store index, if valid, as dependency information for the subsequent load instruction.