True Dependency

Definition

True dependency, also known as data dependency, occurs when one instruction relies on the result of a previous instruction to execute correctly. This concept is crucial for understanding how instructions can be executed in parallel without errors, as it highlights the constraints imposed on instruction scheduling and execution within a processor. Recognizing true dependencies is essential for optimizing performance through techniques that enhance instruction-level parallelism.

5 Must Know Facts For Your Next Test

1. True dependencies restrict instruction reordering since an instruction cannot be executed until its dependent instruction completes.
2. They are commonly represented in data flow graphs where edges connect instructions based on their data dependencies.
3. Compilers employ techniques such as loop unrolling and instruction scheduling to minimize the impact of true dependencies on performance.
4. In superscalar architectures, multiple instructions can be issued simultaneously, but true dependencies must still be respected to maintain correctness.
5. Out-of-order execution is a strategy that helps mitigate the latency caused by true dependencies by allowing other independent instructions to execute while waiting for the dependent instruction to complete.

Review Questions

• How do true dependencies affect instruction scheduling in processors?
  • True dependencies significantly impact instruction scheduling because they create constraints on how instructions can be reordered for execution. When one instruction relies on the result of another, the scheduler must ensure that the dependent instruction waits for its prerequisite to finish before executing. This restriction can limit the ability to fully utilize available parallelism and may lead to stalls or delays in execution if not managed properly.
• Discuss the relationship between true dependencies and data hazards, providing examples of how they manifest in program execution.
  • True dependencies directly lead to data hazards, specifically read-after-write hazards, which occur when an instruction attempts to read a value that has not yet been written by a preceding instruction. For example, if Instruction A computes a value and Instruction B needs that value to proceed, a true dependency exists. If these instructions are executed out of order, Instruction B could potentially use stale data, causing errors. Recognizing and managing these hazards is crucial for maintaining correct program execution and optimizing performance.
• Evaluate the effectiveness of out-of-order execution in handling true dependencies within modern processors.
  • Out-of-order execution is an effective method for handling true dependencies as it allows processors to execute independent instructions while waiting for dependent ones to complete. By using reservation stations and reorder buffers, modern CPUs can track which instructions are ready to execute and which are waiting for operands. This approach minimizes idle cycles and enhances overall throughput by ensuring that the processor remains busy even when some instructions are stalled due to dependencies. However, it requires complex hardware mechanisms and can introduce additional latency when resolving dependencies.

Related terms

Data Hazard: A situation that arises when instructions that depend on each other are executed out of order, leading to incorrect results.

Control Dependency: A scenario where the execution of an instruction depends on the outcome of a previous instruction, often related to branching or jumps in code.

Speculative Execution: A technique used in modern processors where instructions are executed before it is known if they will be needed, based on predicted paths of execution.