Early Issue Recovery for Conditional Load Instructions
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Solution Overview
Problem
Dynamic scheduling in processors with out-of-order execution faces challenges in reducing scheduling latency due to the need for complex circuitry to track operand availability, which is impractical for out-of-order processors.
Innovation Solution
The technique involves issuing instructions independently of operand availability, utilizing recovery circuitry to manage potential data hazards, and selecting between alternative outcomes based on operand dependency, allowing for reduced latency without additional hardware complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If instructions are issued only after all source operands become available, then data hazards are avoided, but scheduling latency increases and processing speed decreases
Solution Approach 1:
The instruction is issued in advance before the operand is fully ready. The decode stage issues the instruction speculatively, and if the operand is not ready by the execute stage, a replay is triggered. This preliminary issuance reduces scheduling latency while maintaining reliability through the replay mechanism.
Solution Approach 2:
The system uses feedback from the execute stage to the issue stage to determine whether a replay is needed. If the operand is not ready when the instruction reaches the execute stage, the feedback triggers a replay of the instruction after the operand becomes available, ensuring correct execution while allowing early issuance.
2Loss of time
If scoreboard-based scheduling is used to reduce scheduling latency, then instruction issuance can begin earlier, but hardware complexity increases significantly
Solution Approach 1:
The patent extracts the complex scoreboard tracking mechanism from the issue stage and replaces it with a simple replay mechanism. Instead of monitoring operand availability at issue time, the system only needs to detect whether the operand is ready at the execute stage and replay if necessary, significantly reducing hardware complexity.
Solution Approach 2:
The system discards the complex scoreboard state tracking and recovers correctness through simple replay of the instruction. Rather than maintaining detailed state information about operand availability, the system allows the instruction to replay after the operand becomes ready, achieving the same goal with much simpler hardware.
3Productivity
If instructions are issued independently of operand availability, then processing speed increases, but data hazards may occur requiring recovery mechanisms
Solution Approach 1:
The instruction is issued preliminarily before the operand is ready, enabling early execution and improving processing speed. The replay mechanism then ensures reliability by re-executing the instruction after the operand becomes available if needed, resolving the tension between speed and reliability.
Solution Approach 2:
The replay mechanism acts as a cushion against data hazards. By having the capability to replay the instruction, the system prepares in advance for the possibility that the operand may not be ready, allowing early issuance without compromising reliability. The cushion absorbs the potential harm of issuing too early.
Data Source
AI summary
At least one instruction of a sequence of program instructions has a plurality of alternative outcomes including at least a first outcome that is independent of at least one operand and a second outcome that is dependent on the at least one operand. The at least one operand is a value generated by a preceding instruction in the sequence. The at least one instruction is issued for execution independently of when the at least one operand is generated by the preceding instruction. Recovery circuitry is provided to perform a recovery operation in the event that the second outcome is to be executed for the at least one instruction and the at least one operand has not been generated by the preceding instruction when the at least one instruction is to be executed by said instruction execution circuitry.