Trace Indexing Unit for Speculative Execution Monitoring

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Solution Overview

Problem

Monitoring the activities of data processing units, particularly in speculative execution and out-of-order processors, is challenging due to space constraints and the difficulty in identifying and managing trace data, especially when instructions are mis-speculated or take long to complete, leading to inefficiencies in trace data generation and bandwidth usage.

Innovation Solution

A trace unit with a trace indexing unit that associates index values with trace data, allowing for efficient tagging and later reference of specific trace data items, reducing the need for explicit tracing and saving space by generating index values in a predetermined sequence, enabling effective cancellation and commitment of speculative instructions and data transfers.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If trace data is generated for all instructions including speculative ones, then complete monitoring is achieved, but bandwidth and storage requirements increase significantly

Engineering Contradiction:

Improvemonitoring completenessVSAvoidtrace data volume

Core Design Contradiction:

ReliabilityVSQuantity of substance

Solution Approach 1:

The patent extracts only the necessary trace data by using selective indexing. Instead of capturing all instructions, the system indexes only specific instructions that need to be tracked, thereby reducing trace data volume while maintaining monitoring effectiveness for critical operations.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent changes the parameter of trace data generation from comprehensive to selective. By introducing an index mechanism that identifies specific instructions, the system transforms the trace data generation process to capture only essential information, reducing bandwidth and storage requirements.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If speculative instructions are traced without cancellation mechanism, then all execution paths are monitored, but false trace data increases and reliability decreases

Engineering Contradiction:

Improvetrace data accuracyVSAvoidtrace management complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent implements a feedback mechanism where the trace unit receives indication signals from the data processing unit about speculative instruction outcomes. This feedback allows the system to identify and cancel false trace data, improving trace accuracy while managing complexity through structured communication protocols.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent discards false trace data generated from mis-speculated instructions by using cancellation mechanisms. The system recovers computational resources by not processing invalid trace paths, thereby improving reliability while managing complexity through efficient trace data management.

Inventive Principle:

Principle #34Discarding and recovering

3Reliability

If trace data is buffered until speculation is resolved, then trace accuracy is improved, but buffer space requirements become undesirably large

Engineering Contradiction:

Improvetrace data definitivenessVSAvoidbuffer space

Core Design Contradiction:

ReliabilityVSVolume of stationary object

Solution Approach 1:

The patent applies preliminary action by pre-indexing instructions before speculative execution. The trace unit pre-calculates and stores index values for instructions that will be executed speculatively, allowing immediate trace generation without requiring large buffers to hold unresolved speculation data.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent uses copying by creating index references instead of buffering complete trace data. Instead of storing all speculative trace data in buffers, the system creates index copies that point to relevant trace information, reducing buffer space requirements while maintaining trace accuracy.

Inventive Principle:

Principle #26Copying

4Quantity of substance

If only selected instructions are traced, then bandwidth is reduced, but identifying corresponding trace data for cancellation becomes difficult

Engineering Contradiction:

Improvetrace data volumeVSAvoidtrace data identification difficulty

Core Design Contradiction:

Quantity of substanceVSDifficulty of detecting and measuring

Solution Approach 1:

The patent introduces an intermediary index mechanism that bridges the gap between selected instructions and their corresponding trace data. The index serves as a mediator that enables easy identification and matching of trace data items with their parent instructions, simplifying the cancellation process while maintaining reduced bandwidth benefits.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentEP2583179B1System and method for tracing the activity of a data processing unit supporting speculative instruction execution and out-of-order data transfers

Publication Date: 2017.03.22 ARM LTD

AI summary

A trace unit is provided which is configured to generate items of trace data indicative of processing activities of a data processing unit The trace unit comprises a trace indexing unit configured to associate an index value with at least a subset of the items of trace data generated by the trace unit. The trace indexing unit is configured to generate each index value as one of a predetermined sequence of index values, wherein an n+1th index value in the predetermined sequence can be determined from only an nth index value in the predetermined sequence.