Out-of-order execution lets processors run instructions faster by shuffling their order. The reorder buffer and commit stage are key players in this process, keeping track of instructions and ensuring the final results match the original program order.

The reorder buffer holds completed instructions until they're ready to be committed. Meanwhile, the commit stage updates the processor's state with these results. Together, they maintain program accuracy while allowing for speedy, out-of-order execution.

Reorder Buffer Functionality

Enabling Out-of-Order Execution

• The reorder buffer is a hardware structure that enables out-of-order execution by allowing instructions to complete execution in an order different from the original program order
• Acts as a temporary storage for instructions that have finished execution but have not yet committed their results to the architectural state (registers, memory)
• Maintains the original program order of instructions and ensures that the architectural state is updated in the correct order during the commit stage
• Facilitates speculation by allowing the processor to execute instructions speculatively and discard the results if a misprediction occurs (branch prediction)

Handling Dependencies and Exceptions

• Helps in handling dependencies, such as write-after-write (WAW) and write-after-read (WAR) hazards, by providing a mechanism to detect and resolve conflicts
  • WAW hazard: When a later instruction writes to the same destination before an earlier instruction has committed its result
  • WAR hazard: When a later instruction reads a value before an earlier instruction has written to it
• Enables precise exceptions by preserving the necessary information to restore the architectural state to a consistent point when an exception or interrupt occurs
  • Precise exceptions: The ability to identify the exact instruction that caused an exception and the architectural state at that point
  • Allows for accurate exception handling and debugging

Commit Stage Significance

Updating Architectural State

• The commit stage is the final stage in the out-of-order execution pipeline where the results of executed instructions are permanently written to the architectural state
• Ensures that the architectural state is updated in the original program order, maintaining the appearance of sequential execution
• During the commit stage, the results stored in the reorder buffer are checked for correctness and any exceptions or interrupts that occurred during execution are handled
• If an instruction has completed without any exceptions or misspeculations, its results are committed to the architectural registers or memory

Maintaining Program Semantics

• Plays a crucial role in maintaining the precise state of the processor, allowing for accurate exception handling and consistent program behavior
• Helps in preserving the sequential semantics of the program, even in the presence of out-of-order execution and speculation
• Responsible for retiring instructions from the reorder buffer, freeing up resources for subsequent instructions
• Ensures that the processor's visible state is consistent with the original program order, providing a predictable and repeatable execution model

Reorder Buffer vs Commit Stage Interaction

Exception and Interrupt Handling

• When an exception or interrupt occurs during the execution of an instruction, the reorder buffer and commit stage work together to ensure precise exception handling
• The reorder buffer keeps track of the instructions in the pipeline and their respective states, including any exceptions or interrupts that have occurred
• If an exception or interrupt is detected, the commit stage stops committing instructions and begins the process of handling the exception or interrupt
• The processor uses the information stored in the reorder buffer to determine the precise architectural state at the point of the exception or interrupt

Restoring Architectural State

• Instructions that have completed execution but have not yet been committed are discarded from the reorder buffer, effectively rolling back the state to the point of the exception or interrupt
• The processor saves the necessary information, such as the program counter and processor status, to handle the exception or interrupt
• Once the exception or interrupt is handled, the processor can resume execution from the point where it left off, using the information stored in the reorder buffer to restore the architectural state
• The interaction between the reorder buffer and commit stage ensures that exceptions and interrupts are handled precisely and that the processor can recover from them without corrupting the program's state

Reorder Buffer Impact on Performance

Buffer Size Considerations

• The size of the reorder buffer affects the amount of out-of-order execution and speculation that can be performed by the processor
• A larger reorder buffer allows for more instructions to be executed out-of-order, potentially increasing the overall performance by exploiting more instruction-level parallelism
  • Instruction-level parallelism: The ability to execute multiple independent instructions simultaneously
  • Larger reorder buffer provides more opportunities for finding and exploiting parallelism
• However, a larger reorder buffer also consumes more hardware resources, such as transistors and power, which can impact the processor's area and energy efficiency
• The optimal size of the reorder buffer depends on factors such as the target application domain, available hardware resources, and power constraints

Commit Policy Trade-offs

• The commit policy determines when and how instructions are committed from the reorder buffer to the architectural state
• A common commit policy is the in-order commit, where instructions are committed in the original program order, ensuring precise exceptions and a consistent architectural state
• Alternative commit policies, such as committing multiple instructions per cycle or committing instructions out-of-order, can potentially improve performance but may require additional hardware complexity and verification efforts
  • Committing multiple instructions per cycle: Allows for faster retirement of instructions and better utilization of commit bandwidth
  • Out-of-order commit: Committing instructions as soon as their dependencies are resolved, potentially reducing the latency of long-latency instructions
• The choice of commit policy affects the trade-off between performance, complexity, and the ability to handle precise exceptions
• The combination of reorder buffer size and commit policy should be carefully considered based on the target application domain, power and area constraints, and the desired balance between performance and resource utilization