Processor Load Store Scheduling to Prevent Pipeline Flushes
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Solution Overview
Problem
Superscalar processors face performance degradation due to out-of-order load/store operations, particularly when a store operation precedes a load operation to the same target address, leading to severe recovery penalties, as the execution pipeline must be flushed and instructions reissued.
Innovation Solution
A scheduling scheme that identifies problematic store operations and forces in-order execution for such operations, preventing selection of a group of instructions for execution until a preceding group is executed, thereby maintaining program order and avoiding pipeline flushes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If superscalar processors execute operations out of order to improve parallel execution, then processor performance and throughput are improved, but pipeline flushes and recovery penalties occur when load/store address violations are detected
Solution Approach 1:
The patent applies preliminary action by detecting potential load/store address violations before they cause pipeline conflicts. The system pre-identifies instructions that may violate program order semantics and prevents their out-of-order execution in advance, rather than detecting and recovering from violations after they occur. This is achieved by tracking address dependencies and predicting potential conflicts before execution, thereby avoiding the need for pipeline flushes and recovery operations.
Solution Approach 2:
The patent implements feedback mechanisms by continuously monitoring the execution state and address dependencies of instructions in the pipeline. When a potential load/store address violation is detected, the system provides feedback to the instruction scheduler to adjust execution ordering. This feedback loop enables dynamic adaptation of instruction execution order based on real-time dependency analysis, preventing violations without requiring full pipeline flushes.
2Productivity
If superscalar processors allow stores to execute ahead of loads to the same address, then execution parallelism is increased, but program order semantics are violated requiring pipeline recovery
Solution Approach 1:
The system performs preliminary detection of load/store address dependencies before execution. By analyzing address generation units and tracking potential conflicts between loads and stores targeting the same memory location, the system proactively prevents out-of-order execution that would violate program order semantics. This preliminary intervention allows the processor to maintain semantic correctness while still permitting parallel execution of non-conflicting instructions.
Solution Approach 2:
The patent applies preliminary anti-action by implementing mechanisms that preemptively counteract potential semantic violations. The system identifies instructions that would violate program order if executed out-of-order and applies counter-measures such as reordering or delaying execution of problematic instructions before they can cause violations. This prevents the need for later recovery operations while maintaining both parallelism and semantic correctness.
3Reliability
If the processor flushes the pipeline to recover from address violations, then program correctness is restored, but execution efficiency is severely degraded
Solution Approach 1:
The patent prevents pipeline flushes by detecting and resolving potential address violations before they occur. The system continuously monitors instruction dependencies and identifies conflicts between loads and stores to the same address, taking corrective action in advance by adjusting execution order or blocking problematic instructions. This eliminates the need for pipeline flushing while maintaining program correctness, thereby preserving execution efficiency.
Solution Approach 2:
The system converts the potential harm of out-of-order execution (address violations) into a benefit by using the violation detection mechanism to improve overall execution efficiency. Instead of allowing violations to occur and then recovering through costly pipeline flushes, the detection capability is used proactively to guide instruction scheduling decisions. This transforms what would be a harmful event into a useful mechanism for optimizing execution order and maintaining efficiency.
Data Source
AI summary
A method and apparatus are provided to control the order of execution of load and store operations. Also provided is a computer readable storage device encoded with data for adapting a manufacturing facility to create the apparatus. One embodiment of the method includes determining whether a first group, comprising at least one or more instructions, is to be selected from a scheduling queue of a processor for execution using either a first execution mode or a second execution mode. The method also includes, responsive to determining that the first group is to be selected for execution using the second execution mode, preventing selection of the first group until a second group, comprising at least one or more instructions, that entered the scheduling queue prior to the first group is selected for execution.