Semiconductor Pipeline Branch Prediction Error Handling
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Solution Overview
Problem
In semiconductor devices performing pipeline processing, branch prediction errors lead to operation delays and redundant operations, particularly when a conditional branch instruction is involved, causing inefficiencies and delays due to the need to abandon and re-read instructions.
Innovation Solution
A semiconductor device design that includes a flip-flop for holding instructions and a memory for transmitting and receiving instructions, with control signals from the arithmetic portion, allowing the device to manage instruction flow and prevent redundant operations by holding the existing instruction even if branch prediction is incorrect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If branch prediction is used to allow the reading portion to keep operating, then circuit operation speed is improved, but operation delay increases when prediction is wrong
Solution Approach 1:
The patent applies preliminary action by pre-fetching instructions into the reading portion using branch prediction before they are actually needed. When a branch instruction is encountered, the prediction unit speculatively fetches instructions from the predicted target address, allowing the pipeline to continue operating without waiting for the actual branch outcome. This preliminary fetching action resolves the contradiction by maintaining high-speed operation while having a mechanism to handle prediction errors.
Solution Approach 2:
The patent implements feedback through the branch outcome detection mechanism that monitors actual branch execution results and compares them with predictions. When a prediction error is detected, the feedback signal triggers the abandonment of incorrectly fetched instructions and initiates refetching from the correct address. This feedback loop resolves the contradiction by enabling rapid correction of prediction errors, minimizing operation delay while maintaining overall high-speed operation.
2Reliability
If the reading portion abandons existing instructions when branch prediction is wrong, then correct instructions can be read, but redundant operations occur and operation delay increases
Solution Approach 1:
The patent applies discarding and recovering by abandoning (discarding) incorrectly predicted instructions in the reading portion when a branch prediction error is detected, then recovering by refetching the correct instructions from the actual branch target. This mechanism ensures instruction correctness while minimizing productivity loss through rapid recovery, resolving the contradiction between reliability and productivity.
3Reliability
If the same instruction is re-read after being abandoned, then the arithmetic portion can process correct instructions, but redundant operations increase processing time
Solution Approach 1:
The patent applies skipping by rapidly flushing abandoned instructions from the reading portion pipeline when a prediction error is detected, and immediately refetching correct instructions. This rushing through of the error correction process minimizes the time loss from reprocessing, resolving the contradiction between ensuring instruction accuracy and minimizing reprocessing time.
Data Source
AI summary
In a semiconductor device performing pipeline processing with the use of a reading portion reading an instruction and an arithmetic portion performing an operation in accordance with the instruction, the instruction held in the reading portion is transmitted from the flip-flop to the memory when branch prediction turns out to be wrong. Note that the arithmetic portion controls transmission and reception of the instruction between the flip-flop and the memory which are included in the reading portion. This enables elimination of redundant operations in the reading portion in the case where an instruction read by the reading portion after the branch prediction turns out to be wrong is a subroutine, or the like. That is, the instruction held in the memory is transmitted back to the flip-flop without rereading of the same instruction by the reading portion, whereby the instruction can be output to the arithmetic portion.