DFA Fall-Through Transitions for Cache-Efficient Regex Acceleration

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Solution Overview

Problem

The exponential growth of deterministic finite automaton (DFA) graphs due to limited transitions between states leads to inefficiencies in regular expression processing, particularly in hardware accelerators, which can slow down search operations.

Innovation Solution

Implementing 'fall-through' transitions in DFA graphs, where transitions to combined states are made without consuming payload bytes, reducing the graph size and improving cache hit rates.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If traditional DFA transitions are used where each transition consumes a payload byte, then the DFA graph can be implemented with standard transition rules, but the graph size grows exponentially and cache hit rates decrease

Engineering Contradiction:

Improvecache hit rateVSAvoidDFA graph size

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The DFA graph is segmented into multiple levels: a compressed DFA graph stored in cache memory and a full DFA graph stored in external memory. The compressed graph contains only frequently accessed states and transitions, while less frequently accessed states are in the full graph. This segmentation allows the critical path to be cached while maintaining complete pattern matching capability.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements a nested structure where the compressed DFA graph is embedded within the full DFA graph. When a state in the compressed graph is encountered, it is processed directly. When a state not in the compressed graph is encountered, the system transitions to the full graph (analogous to opening a nested doll). This nested arrangement optimizes cache utilization while preserving complete DFA functionality.

Inventive Principle:

Principle #7Nested doll (Nesting)

2Productivity

If the DFA graph size is reduced to fit in cache, then cache hit rates improve, but the number of transitions and states must be significantly reduced

Engineering Contradiction:

Improvesearch performanceVSAvoidDFA graph completeness

Core Design Contradiction:

ProductivityVSDevice complexity

Solution Approach 1:

The system performs preliminary analysis of the DFA graph to identify frequently accessed states and transitions before generating the compressed graph. This preliminary action allows the compressed graph to be optimized for the most common matching paths, ensuring that the majority of pattern matching operations can be completed using only the cached portion of the graph.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent changes the parameter of transition execution by introducing fall-through transitions that do not consume payload bytes. This parameter change allows the DFA to transition between states in the compressed graph without advancing through the payload, enabling more states to be included in the cached graph while maintaining correct matching behavior.

Inventive Principle:

Principle #35Parameter changes

3Volume of stationary object

If fall-through transitions are implemented to reduce graph size, then the DFA graph fits better in cache, but the transition logic becomes more complex

Engineering Contradiction:

ImproveDFA graph memory footprintVSAvoidtransition execution logic

Core Design Contradiction:

Volume of stationary objectVSDevice complexity

Solution Approach 1:

The DFA engine automatically detects and executes fall-through transitions based on simple flags stored in the transition table. Each transition entry includes a flag indicating whether it is a fall-through transition. The engine self-manages the complex logic by checking this flag and conditionally consuming payload bytes, eliminating the need for complex external control logic while enabling compressed graph representation.

Inventive Principle:

Principle #25Self-service

4Device complexity

If standard DFA processing is used where each transition consumes a payload byte, then the processing logic is simple, but the DFA graph becomes too large for cache memory

Engineering Contradiction:

Improveprocessing logic simplicityVSAvoidDFA graph size

Core Design Contradiction:

Device complexityVSVolume of stationary object

Solution Approach 1:

The patent modifies the transition execution parameter by introducing a fall-through flag in the transition table. This flag changes the behavior of transition execution: when set, the transition occurs without consuming a payload byte; when not set, the transition consumes a payload byte as in traditional DFA. This simple parameter change enables the compressed graph representation while keeping the processing logic relatively simple.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS20250355970A1Fall-through slots for deterministic finite automatons in a regular expression accelerator

Publication Date: 2025.11.20 MICROSOFT TECHNOLOGY LICENSING LLC

AI summary

Systems and methods for implementing fall-through slots for deterministic finite automatons (DFAs) in a regular expression (regex) accelerator are provided. A method includes compiling a set of regular expression patterns to generate an output file, wherein the output file comprises information related to a deterministic finite automaton (DFA) graph, including fall-through information indicative of whether a transition associated with any nodes of the DFA graph comprises a fall-through transition. The method further includes during processing of a payload, executing transitions associated with the DFA graph, including any fall-through transitions.