Hardware Transactional Memory Out-of-Order Execution
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Solution Overview
Problem
Traditional Hardware Transactional Memory (HTM) processors are limited in their ability to utilize out-of-order execution optimizations within speculative regions, leading to reduced execution performance due to the need for atomicity and isolation, which restricts instruction-level parallelism.
Innovation Solution
Implementing a hardware transactional memory system that supports out-of-order processing and branch prediction within speculative regions, allowing instructions to be executed out of program order while maintaining atomicity and isolation, and employing a multi-level speculative buffer to manage speculative data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional HTM processors limit the feature set to maintain atomicity and isolation, then reliability is improved, but productivity deteriorates due to inability to use out-of-order execution
Solution Approach 1:
The processor segments the instruction stream into speculative regions marked by special instructions, allowing out-of-order execution within these segments while maintaining atomicity boundaries. The speculative region acts as an isolated execution context where out-of-order instructions can be processed without compromising the atomicity guarantee of the overall transaction.
Solution Approach 2:
The processor dynamically changes the execution state parameter by detecting abort conditions and transitioning from out-of-order execution mode to ordered commitment mode. When an abort condition is detected, the processor flushes the reorder buffer and retries the speculative region, effectively changing the execution parameters to maintain reliability while maximizing productivity during successful executions.
2Productivity
If out-of-order execution is enabled within speculative regions, then productivity is improved, but device complexity increases due to additional hardware facilities
Solution Approach 1:
The processor employs universal hardware facilities such as the reorder buffer and branch prediction unit that serve multiple functions: they support both out-of-order execution and atomic transaction management. The reorder buffer, for example, is used to hold speculative results and also to detect abort conditions, thereby reducing the need for separate dedicated hardware and lowering overall device complexity.
Solution Approach 2:
The processor introduces an intermediary abort detection mechanism that mediates between the out-of-order execution engine and the atomicity commitment logic. This intermediary monitors execution for abort conditions and coordinates the transition between speculative and committed states, simplifying the interaction between complex hardware components while maintaining reliability.
3Reliability
If abort conditions are detected immediately, then reliability is improved, but loss of time increases due to premature aborts from mispredicted paths
Solution Approach 1:
The processor performs preliminary actions by pre-fetching and speculatively executing instructions along predicted branch paths before the actual branch outcome is known. The abort detection mechanism waits until it is certain that an abort condition exists (i.e., the instruction is not on a mispredicted path) before triggering an abort, thereby avoiding premature aborts and reducing retry overhead while maintaining transaction correctness.
Data Source
AI summary
A processing core of a plurality of processing cores is configured to execute a speculative region of code a single atomic memory transaction with respect one or more others of the plurality of processing cores. In response to determining an abort condition for issued one of the plurality of program instructions and in response to determining that the issued program instruction is not part of a mispredicted execution path, the processing core is configured to abort an attempt to execute the speculative region of code.