Physical Register Release Timing for Fewer Processor Stalls
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Solution Overview
Problem
Register conflicts in modern processors lead to stalls and decreased performance due to concurrent use of the same register by multiple instructions, which traditional register renaming techniques fail to address efficiently.
Innovation Solution
Implementing early physical register release by detecting conditions where a subsequent instruction consumes and overwrites a register, allowing the physical register associated with a previous instruction to be released back to the free list at least one clock cycle before the subsequent instruction retires, thereby enabling more efficient use of physical registers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional register renaming is used to resolve register conflicts, then instruction-level parallelism is improved, but physical registers are held until retirement causing stalls and decreased performance
Solution Approach 1:
The patent applies preliminary action by detecting when a subsequent instruction is about to consume and overwrite a register's data, and proactively releasing the physical register one clock cycle before the subsequent instruction retires. This advance release prevents stalls that would otherwise occur while waiting for the register to become available, thereby improving instruction-level parallelism without sacrificing data correctness.
2Productivity
If physical registers are released earlier to improve performance, then processor throughput is enhanced, but register correctness may be compromised
Solution Approach 1:
The patent implements feedback by continuously monitoring the retirement status of subsequent instructions that consume register data. The mapper circuit receives feedback about when a subsequent instruction has retired and overwritten the destination register, and uses this feedback to determine the appropriate time to release the physical register. This ensures the register is released at the precise moment when it is safe to do so, maintaining both throughput and correctness.
3Adaptability or versatility
If more physical registers are allocated to eliminate conflicts, then register availability is improved, but the size of the register file increases
Solution Approach 1:
The patent changes the temporal parameter of register availability by introducing early release mechanisms. Instead of increasing the number of physical registers, the system changes when registers are released back to the free list - specifically, one clock cycle before the subsequent instruction that would consume the data. This parameter change in release timing effectively increases register availability without adding more physical registers, avoiding increased device complexity.
Data Source
AI summary
A processor with early physical register release is disclosed. The processor includes a pipeline and is configured to operate according to an instruction set architecture defining a number of architectural registers. The processor includes a register file having a number of physical registers, and a mapper configured to map physical registers to architectural registers. In response to detecting a first condition in which first and second instructions share the same destination architectural register, with the second instruction also being a consumer of the destination architectural register, the mapper designates a first physical register (mapped as the architectural register for the first instruction) to be available after execution, and at least one clock cycle prior to retirement, of the second instruction.