Pipeline Hazards: Definition & Types

Pipelining is a pivotal technique employed by CPUs to enhance efficiency and execution speed. Pipeline Hazards is an important part of the GATE Syllabus for Computer Science Engineering - CSE. 

By breaking down instruction execution into multiple stages, similar to an assembly line, pipelining allows simultaneous processing of different instructions, leading to improved throughput and performance.

• The beauty of pipelining comes with its own set of challenges in the form of pipeline hazards. 
• Hazards, also known as dependencies, are obstacles that can impede the smooth flow of instructions through the pipeline. 
• These dependencies arise due to conflicts in hardware resources, data dependencies between instructions

Key Terms: Pipeline, Hazards, Structural Data, Control, CPUs, Read After Write, Write After Read, Write After Write, Data Forwarding, ALUs

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What are Pipeline Hazards?

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Pipelining is a crucial technique in modern CPUs that enhances performance by breaking down instruction execution into multiple stages. This allows simultaneous handling of different instructions, akin to an assembly line. Hazards occur when instructions rely on the outcomes of previous instructions that have not yet completed their execution.

• There are three types of dependencies: Read After Write (RAW), Write After Read (WAR), and Write After Write (WAW) hazards. 
• Additionally, control hazards and structural hazards can further hinder smooth pipeline execution. 
• To tackle these challenges, CPUs employ techniques like forwarding, stalling, and branch prediction.
• Effectively managing hazards is vital to achieving higher instruction throughput and overall CPU performance. 
• By handling dependencies efficiently through techniques like forwarding, CPUs can mitigate the impact of hazards on the pipeline. 
• Stalling, inserting no-operations or "bubbles" when necessary, helps to resolve conflicts caused by hazards. 
• Branch prediction mechanisms aid in anticipating control hazards and reducing wasted clock cycles. 

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Types of Pipeline Hazards in Computer Architecture

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The various types of Pipeline Hazards are as follows:

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Structural Hazards

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In a pipelined CPU, structural hazards can occur due to conflicts over hardware resources like memory, registers, or ALUs. 

• When multiple instructions require access to the same resource simultaneously
• The hardware cannot handle all combinations in an overlapped pipelined execution. 
• One scenario of structural dependency involves a situation where two instructions in the pipeline try to access the same memory resource at the same clock cycle.

Solution 1: Introduce Stall

To resolve the structural hazard, a common approach is to introduce a "bubble" or stall in the pipeline.

• When a structural dependency is detected, the instruction causing the conflict is delayed, and subsequent instructions are also stalled until the hazard is resolved.
• This can lead to a significant increase in the number of clock cycles required to execute instructions, reducing overall CPU efficiency.
• The delay can propagate to multiple instructions, causing a noticeable impact on the pipeline's performance.

Better Solution: Increase Structural Resources

To mitigate the impact of structural hazards, one can increase the structural resources in the system.

• Options include increasing the number of pipeline stages or employing separate caches for instruction memory and data memory.
• Multiple levels of cache within the CPU can also improve performance.
• Another possibility is to have an exclusive ALU for address calculation or use register files with multiport access.

Modern Solutions

Modern CPUs often implement a combination of these techniques to handle dependencies efficiently.

• They may have deeper pipelines, which provide more stages to alleviate structural hazards and improve overall performance.
• Cache memories are commonly used to reduce the impact of memory-related dependencies.
• Register files with multiport access to enable simultaneous read and write operations on different registers.

Trade-offs

Increasing structural resources can enhance CPU performance but also leads to higher cost and complexity. Designers must make trade-offs to find the optimal solution that balances performance and cost.

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Data Hazards

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In pipelined CPUs, data hazards arise when an instruction's execution depends on the results of a previous instruction that is still being processed in the pipeline. 

• These dependencies can lead to incorrect results if not properly managed.
•  Data hazards are classified into three categories based on the order of READ or WRITE operations on registers:

(a) RAW (Read After Write) - Flow/True Data Dependency: Occurs when an instruction reads data that is produced by a previous instruction. 

• The dependent instruction needs to wait until the previous instruction completes its write operation 
• It stores the data in the register or memory.

(b) WAR (Write After Read) - Anti-Data Dependency: This rare case occurs when a subsequent instruction writes to a register before a previous instruction reads from it. It is more common in machines with complex and special instructions.

(c) WAW (Write After Write) - Output Data Dependency: This occurs when two parallel instructions write to the same register, and their order of execution matters.

• It is essential to ensure that the second instruction writes its result 
• After the first instruction has completed its write operation.

Handling Data Hazards

To mitigate data hazards in pipelined CPUs, various techniques are employed:

​​(a) Data Forwarding (Data Hazard Interlock): Data forwarding, also known as data hazard interlock

• It involves passing the result of a previous instruction directly to the functional unit that requires it.
• This technique makes the result available earlier to the dependent instruction, reducing the need for stalls and improving pipeline efficiency.

​​(b) Compiler Optimizations: The compiler can play a role in detecting data dependencies during the code optimization stage.

• It may reorder instructions or insert NOP (No Operation) instructions to reduce data hazards when generating executable code.

(c) Register Renaming: Modern CPUs use register renaming techniques to avoid WAW and WAR hazards.By allocating different physical registers for instructions that use the same logical register, hazards can be prevented.

(d) Out-of-Order Execution: Some CPUs employ out-of-order execution

• Which allows instructions to be executed in a sequence different from their original program order.
• This technique enables the processor to execute independent instructions concurrently, reducing stalls caused by data hazards.

(e) Multiple Stages of Pipelines: CPUs with deeper pipelines can also alleviate data hazards by breaking down instruction execution into more stages.

• Efficiently managing data hazards is crucial for maintaining correct instruction execution and improving the overall performance of pipelined CPUs. 
• A combination of hardware and software techniques is used to handle these hazards effectively.

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Control Hazards

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Control hazards, also known as branch hazards, occur in pipelined CPUs when the flow of program execution depends on the outcome of branch instructions. Branch instructions are used to alter the program flow based on certain conditions, such as loop iterations or conditional statements.

Solutions for Control Hazards

To handle control hazards efficiently, various techniques are employed:

​​(a) Stalling the Pipeline: Stall the pipeline as soon as a branch instruction is decoded, preventing any further instruction fetching until the branch decision is resolved. 

• This approach reduces pipeline throughput since around 30% of instructions in a program are often branch instructions
• Leading to a pipeline operating at about 50% capacity with stalling.

​​(b) Branch Prediction: Instead of stalling, allow the pipeline to proceed and predict the outcome of the branch instruction. 

• For example, loops tend to iterate multiple times before a branch condition is met. 
• Predict that the branch will not be taken, and if it turns out to be correct, the pipeline continues without disruption. 
• However, if the prediction is wrong, the pipeline needs to be flushed (undone), and the correct instructions need to be fetched.

(c) Dynamic Branch Prediction: Maintain a Branch Table Buffer (BTB), acting as a cache with entries for branch instruction addresses and their corresponding target branch addresses. 

• When a conditional branch instruction is encountered, the BTB is checked for a hit, and if found
• The corresponding target branch address is used for fetching the next instruction. 
• If the prediction fails, flushing is required.

(d) Reordering Instructions (Delayed Branch): The compiler can reorder instructions, positioning the branch instruction later in the sequence to bring useful, non-affected instructions earlier. 

• This way, the branch instruction fetch is delayed. 
• If no such instructions are available, NOP instructions (no operations) may be introduced to achieve delayed branching.

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Things to Remember

• Pipelining improves CPU performance by breaking down instruction execution into multiple stages, but hazards can impede its efficiency.
• Hazards occur due to dependencies between instructions, such as Read After Write (RAW), Write After Read (WAR), and Write After Write (WAW) hazards.
• Structural hazards arise from resource conflicts and can be resolved through stalling or increasing structural resources like caches and multiple ALUs.
• Data hazards are caused by dependencies on the results of previous instructions and can be managed using data forwarding
• Control hazards, or branch hazards, arise from branch instructions that decide program flow
• Solutions include stalling, branch prediction, and dynamic branch prediction.
• Handling pipeline hazards efficiently is crucial for achieving higher instruction throughput and overall CPU performance. 
• Modern CPUs use a combination of techniques to manage hazards and improve execution efficiency.