Processor Macro-Op Fusion with Destructive Instructions
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Solution Overview
Problem
Existing macro-op fusion techniques in RISC-V architectures fail to capitalize on numerous opportunities to enhance efficiency due to limited application of destructive instructions, leading to performance penalties from additional instruction encoding and pipeline complexities.
Innovation Solution
Implementing fusion with destructive instructions by allowing ordinary standalone instructions to augment the argument list of a destructive instruction, enabling fusion even with intervening instructions, and utilizing a decoupled vector unit to separate and process vector instructions independently, thus avoiding performance penalties and pipeline bottlenecks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If macro-op fusion is applied to destructive instructions, then processor efficiency is improved and instruction count is reduced, but instruction encoding complexity and pipeline complexity increase
Solution Approach 1:
The patent merges a first instruction that writes to a destination register with a second destructive instruction that reads from and writes to the same register, creating a fused macro-operation. This combining eliminates the need for separate encoding of move and destructive operations, reducing overall instruction encoding complexity while improving processor efficiency through reduced instruction count.
Solution Approach 2:
The fused macro-op structure is designed to handle multiple instruction types universally - it can fuse move instructions with destructive instructions, and can accommodate different types of destructive operations. This multi-functional approach allows the same fusion mechanism to handle various instruction combinations without requiring separate encoding schemes for each case.
2Reliability
If additional instructions are added to preserve input arguments, then correctness is maintained, but performance deteriorates due to increased instruction count
Solution Approach 1:
Instead of adding separate preserve instructions, the patent combines the preservation function into the fused macro-op itself. The first instruction in the fusion sequence writes the value that would need to be preserved, and this same value is then used as an input to the second destructive instruction within the same fused operation, eliminating the need for additional preserve instructions and maintaining performance.
Solution Approach 2:
The first instruction in the fusion sequence performs the necessary data preparation in advance, writing values to registers that will be used by the subsequent destructive instruction. This preliminary action ensures that data is available when needed without requiring separate preserve operations, thus maintaining both correctness and performance.
3Device complexity
If destructive instructions are used to save encoding space, then instruction encoding efficiency is improved, but instruction count increases leading to performance penalties
Solution Approach 1:
The patent merges multiple instructions including destructive operations into fused macro-operations, reducing the total instruction count that needs to be decoded and executed. By combining a move instruction with a destructive instruction into a single macro-op, the encoder can represent complex operations more compactly while the processor executes fewer instructions, improving both encoding efficiency and performance.
Data Source
AI summary
Systems and methods are disclosed for fusion with destructive instructions. For example, an integrated circuit (e.g., a processor) for executing instructions includes a fusion circuitry that is configured to detect a sequence of macro-ops stored in a processor pipeline of the processor core, the sequence of macro-ops including a first macro-op identifying a first register as a destination register followed by a second macro-op identifying the first register as both a source register and as a destination register, wherein one or more intervening macro-ops occur between the first macro-op and the second macro-op in the program order; determine a micro-op that is equivalent to the first macro-op followed by the second macro-op; and forward the micro-op to at least one of the one or more execution resource circuitries for execution. For example, the sequence of macro-ops may be detected in a vector dispatch stage of a processor pipeline.