pipelinehazards

Pipeline hazards refer to conditions that prevent a processor’s instruction pipeline from delivering a steady stream of results. They occur when the execution of one instruction cannot proceed to the next stage in the usual order because of dependencies, resource conflicts, or changes in control flow. Hazards can reduce instruction throughput and increase latency if not managed.

Data hazards arise when instructions depend on the results of earlier instructions that are not yet available. The main forms are read-after-write (RAW), write-after-read (WAR), and write-after-write (WAW). RAW hazards are the most common in in-order pipelines. Remedies include forwarding or bypassing data from later pipeline stages to earlier ones, and inserting stalls (bubbles) when necessary. Hazard detection logic and compiler techniques can minimize stalls by reordering instructions.

Control hazards occur when the pipeline fetches instructions without knowing whether a branch will be taken. Branch mispredictions can waste cycles as the pipeline is flushed and refilled. Mitigations include branch prediction schemes, speculative execution, delayed branching, and selective flushing of instructions from the pipeline when a prediction fails.

Structural hazards happen when two instructions simultaneously require the same hardware resource, such as a single arithmetic unit or memory port. They are mitigated by duplicating resources, redesigning pipelines to allow more flexible use of units, or carefully scheduling instructions to avoid conflicts.

Modern CPUs often use out-of-order execution, deep pipelines, and sophisticated prediction and buffering to mask hazards. Effective hazard management is central to achieving high instruction throughput while maintaining correct program behavior.