The Cortex-M4 processor implements a 3-stage pipeline to improve performance by allowing multiple instructions to be processed simultaneously. However, pipeline hazards can occur when the next instruction cannot execute in the following clock cycle, leading to bubbles and stalls in the pipeline. This article provides an in-depth explanation of the various types of hazards that can occur in the Cortex-M4 pipeline and how they are handled.
Overview of the Cortex-M4 Pipeline
The Cortex-M4 pipeline consists of three main stages:
- Fetch – Instruction is fetched from memory
- Decode – Instruction is decoded into control signals
- Execute – Instruction is executed
By separating the execution into multiple stages, multiple instructions can be in the pipeline simultaneously. For example, while one instruction is being executed, the next instruction can be decoded and a third instruction can be fetched from memory.
However, situations may arise where an instruction cannot proceed to the next pipeline stage due to dependencies or resource conflicts. These scenarios lead to pipeline stalls and bubbles, reducing overall performance. The main pipeline hazards in Cortex-M4 are:
- Structural Hazards
- Data Hazards
- Control Hazards
The following sections explain each of these hazards and how the Cortex-M4 architecture handles them.
Structural Hazards
Structural hazards occur when there is conflict in using the same hardware resource for two instructions in the pipeline. As the Cortex-M4 implements an in-order pipeline, these hazards are minimized compared to out-of-order pipelines. The main structural hazards are:
Write-After-Read (WAR)
WAR hazards occur when an instruction tries to write to a register before the previous instruction has read from it. For example: ADD R1, R2, R3 SUB R4, R1, R5
Here, the SUB instruction requires the value in R1, but ADD is still computing that value. The pipeline will stall the SUB instruction until ADD completes execution.
Write-After-Write (WAW)
WAW hazards occur when two instructions try to write to the same register. For example: ADD R1, R2, R3 SUB R1, R4, R5
Here, both instructions are trying to write to R1. The pipeline will stall the second instruction until the first instruction completes writing R1.
Read-After-Write (RAW)
RAW hazards happen when an instruction tries to read a register before the previous instruction writes to it. For example: ADD R1, R2, R3 MUL R5, R1, R4
The MUL instruction needs the value in R1, but ADD has not completed writing it yet. The pipeline will stall MUL until ADD finishes executing.
Data Hazards
Data hazards occur when instructions that execute out-of-order require data from each other. As Cortex-M4 uses an in-order pipeline, it does not have data hazards between instructions. However, data hazards can still occur due to pipelining between different pipeline stages:
Read-After-Write (RAW)
This is the most common data hazard in pipelined processors. It happens when an instruction needs to read a register before the previous instruction writes to it. For example: ADD R1, R2, R3 SUB R4, R1, R5
The SUB instruction requires R1 but ADD has not written to it yet. Cortex-M4 inserts a pipeline bubble and stalls SUB to resolve this hazard.
Write-After-Read (WAR)
WAR hazards during pipelining occur when an instruction writes to a register before the next instruction reads it. For example: LDR R1, [R2] STR R3, [R1]
Here, STR requires the address in R1 before LDR has loaded it from memory. The pipeline will stall STR to prevent incorrect execution.
Control Hazards
Control hazards occur due to conditional changes in program flow like branches and jumps. As Cortex-M4 uses static branch prediction, these hazards are minimized compared to processors with dynamic prediction.
Branches
When a branch instruction is encountered, Cortex-M4 always predicts that the branch will not be taken. If the branch is actually taken, the instructions already fetched down the not-taken path will be flushed and the correct instructions are fetched from the taken path. This causes a pipeline flush and stall.
Loads
The Cortex-M4 core can speculatively load instructions and data from memory before the addresses are confirmed. If the speculation fails, the pipeline is flushed leading to a stall. For example: LDR R1, [R2] ADD R3, R1, R4
If the load address R2 is incorrect, the ADD instruction will be flushed from the pipeline.
Minimizing Pipeline Hazards
The following techniques can help reduce pipeline hazards in Cortex-M4 code:
- Rearrange code to avoid data dependencies between consecutive instructions
- Separate conflicting writes and reads to the same register
- Use forwarding paths to avoid stalls
- Optimize branching code to improve static prediction
- Rewrite critical code sections in assembly to finely control hazards
- Use compiler optimizations like loop unrolling to reduce dependencies
Tools for Analyzing Pipeline Hazards
ARM provides several tools to help analyze and debug pipeline issues in Cortex-M4:
- Pipeline modeling in ARM DS-5 Development Studio
- Cycle counting and stalling report in ARM Streamline Performance Analyzer
- Simulation based profiling in ARM Model Debugger
- Trace capabilities in CoreSight debug components
These tools provide valuable insight into the dynamic pipeline behavior and help identify exact causes of stalls or bubbles. The results can be used to fine-tune code for maximum pipeline efficiency.
Conclusion
Efficient pipelining is critical to achieving maximum performance in Cortex-M4 based systems. Hazards such as RAW, WAW, and control flow changes can significantly impact the pipeline. Techniques like rearranging instructions, branch optimization, and using compiler features help minimize stalls and bubbles. Profiling tools provide further assistance in pinpointing pipeline inefficiencies during code development and optimization.
Overall, a strong understanding of the Cortex-M4 pipeline and common hazards is essential for firmware developers to exploit the full capabilities of these microcontrollers. Meticulous pipeline management ensures that Cortex-M4 systems consistently deliver the expected high performance levels for today’s complex embedded applications.
The Cortex-M4 architecture implements both simple techniques like static branch prediction as well as more advanced mechanisms like speculative loading to keep the 3-stage pipeline running efficiently. Studying pipeline behavior using ARM-provided tools provides actionable inputs to fine-tune code for the highest efficiency. With hazards properly managed, developers can unleash the full power of the Cortex-M4 core to build high-performance embedded systems.
In summary, pipeline hazards are performance limiting issues in pipelined architectures like Cortex-M4 that reduce ideal instruction throughput. This article has provided a comprehensive overview of structural, data and control hazards as well as techniques to analyze and minimize them. Efficient pipeline management ensures Cortex-M4 microcontrollers can meet the increasing demands of modern embedded applications.
