FPU and Vector Units: Inputs and Outputs

Inputs and outputs contract

Inputs and Outputs for FPU and Vector Units centers on FP/vector utilization, latency overlap efficiency, and denormal handling penalties. Tie every claim to a measurable artifact and an owner-controlled action.

INPUTS
  - workload definition and target KPI
  - binary/compile flags/runtime/firmware metadata
  - microarchitecture and silicon assumptions
  - correctness and regression gates

OUTPUTS
  - evidence-backed bottleneck classification
  - owner-signed fix proposal
  - validation matrix with rollback thresholds

Ownership split

CPU OWNERSHIP LAYERS - FPU and Vector Units

artifact area     owner
----------------  ----------------------------
architecture    vector architect
RTL/microarch   FPU RTL owner
software/tools  math library owner

Rule: every regressed metric must map to an explicit owner and closure artifact.

CPU deep dive

Execution throughput depends on port balance, bypass quality, and realistic instruction mix assumptions.

Concept diagram

EXECUTION DATAPATH

issue -> ALU/FPU/vector/LSQ ports -> writeback -> retire

Metric graph

EXECUTION LOSS DRIVERS

port conflicts      █████
bypass hazards      ████
LSQ ordering stalls ███

Reports and artifacts

• port pressure heatmap

• pipeline hazard report

• ALU/FPU/vector utilization split

• LSQ ordering diagnostics

Mini case study

A compiler scheduling update over-concentrated uops on one port class, reducing effective multi-issue throughput.

Debug branches

• Map instruction classes to port availability

• Validate forwarding depth against dependency chains

• Inspect LSQ ordering events before widening pipes

Senior review question

Ask: which CPI/latency evidence proves this topic is truly closed beyond synthetic benchmarks?

Key takeaways

• Always connect microarchitectural counter changes to product workload outcomes.

• Lock binary, compiler, firmware, and thermal metadata before comparing CPU traces.

Common pitfalls

• Treating average IPC as sufficient proof while ignoring latency tails and outliers.

• Applying predictor or prefetch tweaks without first-failing-stage attribution.

• Declaring closure without reproducible perf, correctness, and power gates.

Handoff explanation

Inputs are broader than knob settings. CPU analysis inputs include workload mix, branch entropy, memory footprint, compiler revision, OS affinity policy, DVFS state, thermal envelope, and stepping.

Outputs must support action: FP/vector utilization, latency overlap efficiency, and denormal handling penalties, artifact packet (vector lane utilization map, FP latency histogram, and exception handling trace), bottleneck class, owner, expected effect, and rollback scope. "Performance improved" without this packet is not closure-ready.

The safest handoff is before/after evidence: environment tags, counters, traces, hypothesis, chosen change, rejected alternatives, and regression criteria.