Lecture Notes: Out-of-Order Execution

Introduction

• Transition to an exciting topic: Out-of-Order (OoO) Execution in computing.
• OoO Execution allows instructions to be executed out of their original sequence.
• Despite initial resistance, it has become prevalent in modern computing.
• Complexity is managed by hardware to optimize instruction processing.

Background and Recap

• Previous classes discussed pipelining, precise exceptions, and register renaming.
• Today’s focus: completing OoO execution and introducing load/store handling.
• Reference readings provided; emphasis on a state-of-the-art processor paper from 1999.

Mechanisms of In-Order Dispatch

• Pipeline structure with reorder buffer for instruction reordering and precise exception handling.
• Instructions dispatch in order if source registers are ready.
• Completion may occur out of order; results first written to reorder buffer.

Fundamentals of Out-of-Order Execution

• In-Order vs. Out-of-Order Execution:
  • In-Order: Instructions dispatch as per program order, potentially causing stalls.
  • Out-of-Order: Instructions dispatch as soon as inputs are ready, improving efficiency.
• Instruction Scheduling:
  • Static (compiler-based) vs. Dynamic (hardware-based) scheduling.
• Handling Instruction Dependencies:
  • Register renaming to remove false dependencies.
  • Focus on flow dependencies to ensure correct value production and consumption.

Out-of-Order Execution Implementation

• Key Components:
  • Register Renaming for eliminating false dependencies (anti and output dependencies).
  • Reservation Stations for buffering and scheduling instructions.
  • Common Data Bus for value broadcasting and readiness detection.

Detailed Example and Simulation

• Pipeline Simulation:
  • Simulated sequences with various pipeline stages.
  • Illustrations of decode, execute, and write-back processes.
  • Handling of dependencies via reservation stations and tag broadcasting.
• Reverse Engineering Data Flow Graph:
  • Using machine state to reconstruct program data flow graph.

Modern Out-of-Order Execution

• Precise Exceptions and Instruction Retirement:
  • Use of reorder buffers for precise exceptions and in-order state updates.
  • Front-end and back-end (architectural) register files.
• Physical Register File Utilization:
  • Centralizing value storage to minimize replication and power consumption.
• Current Processor Architectures:
  • Examples of various processors utilizing OoO execution (Intel Pentium, AMD Zen 2).
  • Trends show increasing instruction windows and physical register file sizes.

Conclusion

• The adoption of Out-of-Order execution is crucial for achieving high performance in modern processors.
• Complex optimizations ensure efficient execution while managing hardware resources.
• Future lectures to cover load/store handling and branch prediction.