Register Rename Table Encoding for Fast Register Clearing
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Solution Overview
Problem
Existing register renaming mechanisms in out-of-order processors incur significant performance and power consumption costs when handling register clearing events, as they often require updating physical registers, which occupies write bandwidth and delays other operations.
Innovation Solution
Implement a register rename circuitry that uses cleared-register encodings in rename table entries to handle register clearing events, allowing operations to proceed with predetermined values without updating physical registers, thus reducing the need for write bandwidth and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If physical registers are updated to handle register clearing events, then register clearing is achieved, but write bandwidth is occupied and other operations are delayed
Solution Approach 1:
A cleared register indicator (intermediary flag) is introduced between the architectural register file and the execution units. Instead of physically clearing register data, the indicator signals to execution units that the register contains a cleared value, allowing other operations to proceed without waiting for the actual register update.
Solution Approach 2:
The clearing operation is extracted from the physical register update process. The cleared register indicator separates the logical clearing function from the physical register write operation, allowing the register data to remain unchanged while still achieving the effect of clearing for subsequent operations.
2Reliability
If physical registers are updated to handle register clearing events, then register clearing is achieved, but power consumption increases
Solution Approach 1:
The cleared register indicator serves as an intermediary that achieves register clearing functionality without requiring energy-intensive physical register writes. The indicator can be set with minimal power consumption while avoiding the energy cost of actually writing data to physical registers.
Solution Approach 2:
Instead of physically copying or writing the cleared value to the register, a lightweight indicator copy is used. This indicator copy requires significantly less energy than a full register write operation while achieving the same logical effect.
3Productivity
If rename entries are set to cleared-register encoding, then operations can proceed with predetermined values, but rename table complexity increases
Solution Approach 1:
The cleared-register encoding is applied locally to individual rename entries rather than requiring a global restructuring of the rename table. Each rename entry can independently indicate whether its corresponding architectural register is cleared, maintaining simplicity while enabling high throughput.
Data Source
AI summary
An apparatus comprises at least one register rename table structure comprising rename entries for indicating register mappings between architectural registers and corresponding physical registers, and register rename circuitry to update the register mappings indicated by the rename entries. Rename entries corresponding to at least one set of architectural registers support a cleared-register encoding. In response to an operation specifying a source architectural register for which a corresponding rename entry is set to the cleared-register encoding, the register rename circuitry controls the processing circuitry to process that operation with a source operand corresponding to the source architectural register being treated as having a predetermined value. In response to detection of a register clearing event indicating that at least one set of architectural registers is to be treated as having been cleared to the predetermined value, the register rename circuitry sets rename entries corresponding to the at least one set of architectural registers to the cleared-register encoding.