Pre-decode Error Handling via Branch Correction in Pipelined Processors
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Solution Overview
Problem
Pre-decode errors in processor pipelines lead to performance and power management degradation due to pipeline flushing and re-fetching of instructions, which is inefficient and costly in terms of both time and energy.
Innovation Solution
A method is introduced where an incorrectly pre-decoded instruction is corrected by forcing a branch correction procedure, treating the instruction as a mispredicted branch with its target address, allowing it to be re-fetched and re-decoded without flushing the pipeline, thereby eliminating the need for a dedicated path to the next fetch address calculator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pre-decode information is stored in cache memory to assist in fully decoding instructions, then decoding efficiency is improved, but pre-decode errors occur leading to pipeline flushing and performance degradation
Solution Approach 1:
The pre-decoder performs preliminary decoding of instructions and stores pre-decode information in the cache memory before full decoding is needed. This preliminary action speeds up the decoding process by having decoding information ready in advance, while the error detection and correction mechanisms ensure reliability is maintained.
Solution Approach 2:
The system implements feedback mechanisms where pre-decode errors are detected during decode stages, triggering exception handling that flushes the pipeline and re-fetches instructions. This feedback loop identifies and corrects errors, ensuring that the productivity gains from pre-decoding do not compromise overall system reliability.
2Ease of operation
If pipeline flushing is performed when pre-decode errors are detected, then error handling is simplified, but performance and power consumption are significantly degraded
Solution Approach 1:
The error detection and correction functionality is extracted as a separate module (pre-decode error detector and corrector) that operates independently from the main pipeline. This allows errors to be handled without necessarily flushing the entire pipeline, maintaining productivity while preserving ease of error handling through dedicated error management logic.
Solution Approach 2:
The system changes the state parameter of the pipeline by forcing a branch correction procedure with a target address when errors are detected. This parameter change allows the pipeline to continue execution with corrected branch information rather than flushing, thereby maintaining throughput while still handling errors effectively.
3Reliability
If a dedicated path to the next fetch address calculator is added for error correction, then pre-decode error handling is improved, but device complexity increases
Solution Approach 1:
The branch correction procedure serves multiple functions: it handles pre-decode errors, manages mispredicted branches, and maintains fetch address calculation. By making the branch correction logic multi-functional, the system improves error correction capability without adding dedicated separate paths, thereby avoiding increased device complexity.
Solution Approach 2:
The error correction path is merged with the existing branch prediction and fetch address calculation logic. Instead of creating a separate dedicated path for error correction, the system combines error handling with the universal branch correction mechanism already present in the pipeline, reducing overall device complexity while maintaining reliability.
Data Source
AI summary
In a pipelined processor where instructions are pre-decoded prior to being stored in a cache, an incorrectly pre-decoded instruction is detected during execution in the pipeline. The corresponding instruction is invalidated in the cache, and the instruction is forced to evaluate as a branch instruction. In particular, the branch instruction is evaluated as “mispredicted not taken” with a branch target address of the incorrectly pre-decoded instruction's address. This, with the invalidated cache line, causes the incorrectly pre-decoded instruction to be re-fetched from memory with a precise address. The re-fetched instruction is then correctly pre-decoded, written to the cache, and executed.