2. Structural, data, and control hazards typically require a processor pipeline

Question

2. Structural, data, and control hazards typically require a processor pipeline to stall. Listed below are a series of optimization techniques implemented in a compiler or a processor pipeline designed to reduce or eliminate stalls due to these hazards. For each of the following optimization techniques, state which pipeline hazards it addresses and how it addresses them. Some optimization techniques may address more than one hazard.

(a) Branch prediction

(b) Instruction scheduling

(c) Delay slots

(d) Increasing availability of functional units (ALUs, adders, etc)

Explanation / Answer

Answer:

(a) Branch prediction:

It addresses control hazards by guessing the outcome of a branch instruction and then speculatively executes the instructions on one side of the branch to keep the pipeline moving. Predictions can be made in hardware or in software by the compiler.

(b) Instruction Scheduling

It addresses structural hazards and data hazards. It addresses data hazards by either moving instructions that are not dependent on an instruction, say A, before some instructions that depend on A and thus avoiding the stall that would have occurred otherwise. It addresses structural hazards by making sure instructions that use functional units that have limited number of instances are be scheduled far apart from each other and there is no unnecessary stall due to this. It can be done in hardware (superscalar processor) or statically by the compiler

(c) delay slots

It addresses control hazards. It helps to avoid a stall that would result due branch target identification during the decode stage by scheduling the execution of some other instruction which anyway has to execute irrespective of the branch condition.

(d) increasing availability of functional units (ALUs, adders etc)

It helps to avoid structural hazards. It is possible to run multiple instructions of the same type at the same time if we have replicated functional units

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