Speculative Vector Load Management for Branch Prediction Accuracy
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Solution Overview
Problem
In vector processing devices, speculative execution of vector load instructions after branch instructions leads to potential data misuse when branch target prediction fails, resulting in performance degradation due to memory access latency, as subsequent vector instructions may execute with incorrect data.
Innovation Solution
A processor with an instruction issue unit, data acquisition unit, determination unit, and vector load management unit that issues vector load instructions based on branch target prediction, acquires vector data only when prediction is successful, and discards data when prediction fails, ensuring accurate processing and masking memory access latency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If speculative execution of vector load instruction is performed after branch instruction, then processing latency is reduced, but data accuracy deteriorates when branch target prediction fails
Solution Approach 1:
The vector load instruction is executed speculatively in advance based on branch target prediction before the actual branch target is determined. The execution unit performs the load operation preliminarily, and the validity determination unit later verifies whether the speculation was correct. If valid, the result is used; if invalid, it is discarded. This preliminary action reduces latency while maintaining accuracy through subsequent validation.
Solution Approach 2:
The system implements a feedback mechanism where the validity determination unit compares the speculatively executed vector load instruction with the actual branch target outcome. When the branch target is determined, the system feedbacks the validity result to control whether the speculatively loaded data is committed or discarded. This feedback loop ensures data accuracy is maintained despite speculative execution.
2Speed
If vector load instruction is executed speculatively, then memory access latency is masked, but subsequent vector instructions may execute with incorrect data
Solution Approach 1:
The validity determination unit acts as an intermediary between the speculative execution unit and the subsequent vector instruction execution. It mediates by validating whether the speculatively loaded data is correct before allowing it to be used by subsequent instructions. This intermediary layer masks memory access latency while preventing incorrect data from propagating through the pipeline.
3Productivity
If branch target prediction is used to issue vector load instruction, then pipeline efficiency is improved, but performance degrades when prediction fails
Solution Approach 1:
The system performs preliminary speculative execution of the vector load instruction based on branch target prediction without waiting for certain prediction outcomes. This preliminary action keeps the pipeline full and efficient, while the validity determination unit subsequently ensures that only correct results are committed, preventing performance degradation from prediction failures.
Solution Approach 2:
When branch target prediction fails, the system discards the incorrectly loaded vector data and recovers by loading the correct data from the actual branch target. This discard and recover mechanism allows the pipeline to maintain high efficiency through speculation while ensuring correctness by eliminating wrong results and reloading valid data.
Data Source
AI summary
Provided is a processor including an instruction issue unit that issues a vector load instruction read from a main memory based on branch target prediction of a branch target in a branch instruction, a data acquisition unit that starts issue of a plurality of acquisition requests for acquiring a plurality of vector data based on the issued vector load instruction from the main memory, a determination unit that determines a success or a failure of the branch target prediction after the branch target is determined, and a vector load management unit that, when the branch target prediction is determined to be a success, acquires all vector data based on the plurality of acquisition requests and then transfers all the vector data to a vector register, and, when the branch target prediction is determined to be a failure, discards the vector data acquired by the issued acquisition requests.