Selective Load Deferral in Speculative Execution
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Solution Overview
Problem
Modern microprocessors face inefficiencies in speculative execution due to the need to re-execute code when encountering store instructions with unknown destination addresses, leading to duplicated computational work and potential queue overflows when deferring all load instructions.
Innovation Solution
The processor selectively defers load instructions based on predicted store addresses, features, and temporal locality in the store queue, allowing execution to continue if the load instruction is unlikely to depend on a deferred store with an unknown destination address, thereby avoiding unnecessary re-execution and queue overflow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor defers all load instructions following a store with an unknown destination address, then the processor avoids read-after-write hazards, but the deferred queue may overflow and speculative execution is wasted
Solution Approach 1:
The patent applies local quality by differentiating the handling of load instructions based on their specific characteristics. Instead of uniformly deferring all loads, the system analyzes each load's address calculation dependencies and selectively defers only those that are likely to read from the deferred store's destination address. This selective approach maintains reliability for critical loads while allowing non-critical loads to execute speculatively, thus improving productivity.
Solution Approach 2:
The system changes the parameter of load instruction handling from a binary state (all loads deferred or all loads executed) to a nuanced state based on dependency analysis. By examining whether a load's address calculation depends on the deferred store's destination register, the system dynamically adjusts the deferral decision, optimizing both safety and performance.
2Reliability
If the processor transitions to scout mode upon encountering a load following a deferred store, then the processor ensures correctness by avoiding read-after-write hazards, but the processor must re-execute all speculative instructions
Solution Approach 1:
The patent extracts the problematic load instructions from the stream of speculative executions. By identifying and isolating only the loads that depend on the deferred store's destination address, the system can handle these critical loads separately through deferral, while allowing other loads to execute speculatively without triggering full re-execution.
Solution Approach 2:
The system performs preliminary analysis of load instructions to predict whether they will read from the deferred store's destination address. By making this prediction in advance before committing to speculative execution, the system can proactively defer only the necessary loads, avoiding the need to re-execute all speculative instructions later.
3Reliability
If the processor defers load instructions to resolve data dependencies, then the processor maintains correct execution order, but the deferred queue size increases and may overflow
Solution Approach 1:
The patent applies partial action by deferring only the subset of load instructions that are necessary to maintain correctness, rather than deferring all loads. This selective deferral approach reduces the burden on the deferred queue, minimizing the risk of overflow while still ensuring execution correctness for critical dependencies.
Data Source
AI summary
The described embodiments provide a system for executing instructions in a processor. While executing instructions in an execute-ahead mode, the processor encounters a store instruction for which a destination address is unknown. The processor then defers the store instruction. Upon encountering a load instruction while the store instruction with the unknown destination address is deferred, the processor determines if the load instruction is to continue executing. If not, the processor defers the load instruction. Otherwise, the processor continues executing the load instruction.