Instruction Predication via Address Pattern Matching
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Solution Overview
Problem
Instruction pipelining in computers faces inefficiencies due to branch misprediction, which leads to the flushing of instructions in the pipeline, wasting resources and reducing throughput.
Innovation Solution
Implementing instruction predication based on address pattern matching using a set of instruction address mask, match, and count registers, allowing dynamic predication without dedicated predicate register bits, thus enabling instructions to be treated as no-ops when their addresses fall within specified ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If predicate registers are used for instruction predication, then instruction execution can be conditionally enabled or disabled, but additional opcode and register address space is consumed
Solution Approach 1:
The patent introduces a mediator mechanism (predication logic unit) that intercepts and evaluates branch outcomes independently of the traditional predicate register system. This mediator unit receives branch condition codes and generates predication signals that control instruction execution without requiring modifications to the instruction format or consuming additional opcode space.
Solution Approach 2:
The patent extracts the predication control logic from the instruction format itself and places it in a separate hardware unit (predication logic). This separation allows the instruction format to remain unchanged while still enabling conditional execution, effectively removing the burden of additional opcode requirements from the instruction set architecture.
2Productivity
If branch prediction is used to maintain pipeline flow, then instruction throughput is improved, but misprediction causes pipeline flushing and performance penalty
Solution Approach 1:
The patent implements preliminary action by pre-computing branch outcomes and preparing predication signals before instructions reach the execution stage. The predication logic unit evaluates branch conditions in advance and sets up control signals that will determine whether subsequent instructions execute, allowing the pipeline to continue flowing without stalls even when predictions are made.
Solution Approach 2:
The patent converts the harmful effect of branch misprediction (pipeline flushing) into a beneficial mechanism by using the mispredicted branch outcome itself as the predication signal. Instead of discarding mispredicted instructions, the system treats them as conditionally executed instructions with a false predicate, effectively transforming the penalty into a controlled no-op that maintains pipeline efficiency.
3Reliability
If instructions are flushed on branch misprediction, then correct instruction execution is ensured, but pipeline resources are wasted and throughput decreases
Solution Approach 1:
The patent converts the harmful waste of flushing pipeline resources into a beneficial reuse by redirecting mispredicted instructions through the predication mechanism. Instead of discarding these instructions, the system executes them with a predication signal that prevents their side effects, thereby recovering the invested pipeline resources and eliminating the energy waste associated with flushing.
Solution Approach 2:
The patent implements a selective discarding and recovering mechanism where instructions are not completely discarded on misprediction but are instead recovered and re-executed with modified predication controls. This allows the system to recover the computational investment in mispredicted instructions while still ensuring correctness by preventing their harmful effects through the predication logic.
Data Source
AI summary
A particular method includes receiving, at a processor, an instruction and an address of the instruction. The method also includes preventing execution of the instruction based at least in part on determining that the address is within a range of addresses.