cpu-pipelines-and-hazards

name	cpu-pipelines-and-hazards
description	CPU pipeline skill for hazards, forwarding, and stalls. Use when explaining pipeline stages, data/control hazards, forwarding paths, or branch stalls in performance analysis. Activates on queries about pipeline hazard, data hazard, control hazard, forwarding, pipeline stall, or superscalar basics.

CPU Pipelines and Hazards

Purpose

Explain classic and modern CPU pipeline concepts: stages, data and control hazards, forwarding/bypassing, stalls, and branch handling — foundational for optimization and understanding microarchitecture counters.

When to Use

• Interpreting pipeline stall metrics from skills/profilers/intel-vtune-amd-uprof
• Teaching why instruction order affects throughput
• Relating assembly scheduling to hardware behavior
• Debugging unexpected performance cliffs in hot loops

Workflow

1. Five-stage classic pipeline (MIPS-style mental model)

IF → ID → EX → MEM → WB

Overlapped execution: instruction N in EX while N+1 in ID.

2. Data hazards

Type	Example	Mitigation
RAW (true)	add r1,r2,r3 then sub r4,r1,r5	Forwarding from EX/MEM/WB
WAR / WAW	Rare in in-order; relevant in OoO rename	Register renaming

Without forwarding:

stall until writeback completes

3. Control hazards (branches)

Branch in ID → target unknown until EX
├── Predict taken/not-taken (static or dynamic)
├── Flush wrong-path instructions on mispredict
└── Penalty = pipeline depth (varies by CPU)

See skills/computer-architecture/branch-prediction-and-speculation.

4. Structural hazards

Limited functional units (single memory port) cause stalls even without dependencies.

5. Practical optimization hints

/* Bad — tight dependency chain */
for (int i = 0; i < n; i++)
    acc = acc + data[i];   /* each iter waits on acc */

/* Better — multiple accumulators (ILP) */
acc0 = acc1 = 0;
for (int i = 0; i < n; i += 2) {
    acc0 += data[i];
    acc1 += data[i+1];
}
acc = acc0 + acc1;

Pair with skills/low-level-programming/cpu-cache-opt — memory often dominates.

6. Reading uops / ports (x86)

perf stat -e instructions,cycles,stalls-frontend,stalls-backend ./app

VTune "Microarchitecture Exploration" maps to pipeline slots.

7. Agent usage

/cpu-pipelines-and-hazards Explain RAW hazard in this ARM assembly loop and how to break it

Common Problems

Symptom	Cause	Fix
High stalls-frontend	I-cache misses / branch mispredict	Align hot loop; see branch skill
No speedup from unroll	Memory bound	Profile loads; prefetch
Wrong cycle model	Ignored OoO execution	Use perf hardware counters
"NOP fixes it"	Timing-sensitive MMIO	Never tune device delays by NOP

Related Skills

• skills/computer-architecture/branch-prediction-and-speculation — mispredict cost
• skills/computer-architecture/memory-hierarchy-and-caches — load latency
• skills/low-level-programming/cpu-cache-opt — cache-line effects
• skills/profilers/intel-vtune-amd-uprof — pipeline analysis
• skills/low-level-programming/assembly-arm — instruction scheduling