Processor Register Renaming With Retire-Phase Partial-Write Merging
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Solution Overview
Problem
The x86 architecture's PRF renaming style faces sizing issues due to complex operand sizes, leading to inefficiencies when handling partial write operations, which existing solutions like multiple source dependency and pre-merge operations introduce additional inefficiencies and timing issues.
Innovation Solution
A processor system with a physical register file, rename table, mapping logic, size tracking logic, and merge logic that allocates physical registers for write instructions, tracks write sizes, and merges partial write results during the retire phase, ensuring correct register contents are maintained without requiring double read ports or complex pre-merge operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If partial write operations are handled with separate physical registers, then write operations can be executed independently, but read operations cannot retrieve complete register contents when the source size exceeds the write size
Solution Approach 1:
The patent performs merge operations during the retire phase before read operations are executed. The merge logic proactively combines partial write results with the original larger write result in advance, ensuring that when a read operation occurs, the complete register contents are already available in the physical register file without requiring additional complex detection mechanisms at read time.
2Reliability
If multiple source dependency is used to detect partial write operations, then correct register contents can be maintained, but additional read ports are required increasing device complexity
Solution Approach 1:
The patent extracts the merge operation from the read path and places it in the retire path. By moving the merge logic to the retire phase, the system eliminates the need for additional read ports that would be required if merge operations were performed at read time. The extraction separates the merge function from the critical read path, maintaining simplicity in the read ports while ensuring correctness.
3Reliability
If pre-merge operations are performed to combine partial write results, then complete register contents are available for reads, but instruction level parallelism decreases due to cycle detection requirements
Solution Approach 1:
The patent implements dynamic merge operations that are conditionally executed based on the retirement order of instructions. The merge logic dynamically determines whether a merge is needed by checking if a partial write instruction retires before its corresponding larger write instruction. This dynamic approach allows the system to maintain high instruction level parallelism by only performing merges when necessary, rather than forcing sequential execution of all write operations.
4Reliability
If partial write results are merged before read instructions, then correct data is provided to reads, but critical timing issues and bottlenecks are introduced
Solution Approach 1:
The patent introduces the physical register file as an intermediary structure that holds both the original larger write result and the partial write results separately until retirement. The merge logic acts as a mediator that combines these results only when needed during the retire phase. This intermediary approach avoids introducing timing bottlenecks in the critical read path while ensuring data correctness is maintained through the retirement-phase merge operation.
Data Source
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AI summary
A processor including a physical register file, a rename table, mapping logic, size tracking logic, and merge logic. The rename table maps an architectural register with a larger index and a smaller index. The mapping logic detects a partial write instruction that specifies an architectural register that is already identified by an entry of the rename table mapped to a second physical register allocated for a larger write operation, and includes an index for the allocated register for the partial write instruction into the smaller index location of the entry. The size tracking logic provides a merge indication for the partial write instruction if the write size of the previous write instruction is larger. The merge logic merges the result of the partial write instruction with the second physical register during retirement of the partial write instruction.