DenseChip

RISC-V Design ยท All levels

Forwarding Network and Stall Control for Data Hazards: Debug Playbook

Debug Playbook for Forwarding Network and Stall Control for Data Hazards.

Debug playbook

Debug Playbook for Forwarding Network and Stall Control for Data Hazards is anchored on RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction.. Convert observations into mechanism-backed decisions with explicit ownership.

  1. Freeze workload seed, binary, compiler, firmware, and platform setup.

  2. Find earliest persistent mechanism failure in the timeline.

  3. Build one reduced reproducer for dominant hypothesis.

  4. Patch minimal fix with explicit rollback gate.

  5. Rerun correctness + performance + power matrix.

Decision tree

diagram
ROOT CAUSE TREE - Forwarding Network and Stall Control for Data Hazards

RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction. regressed
          |
   reproducible on fixed seed?
      /                 \
    no                   yes
    |                     |
env/tool drift       first failing domain?
                     /        |         \
                  decode    execute    memory/MMU
                    |         |            |
               control map  bypass/FU   TLB/walk/perm
                    |
         privilege/CSR side effects checked?

Stop at first confirmed mechanism, then assign explicit owner + fix proof.

Review memo template

diagram
RISC-V REVIEW MEMO - Pipeline Implementation / Forwarding Network and Stall Control for Data Hazards

1. Symptom
   - Failing metric: RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction.
   - Failing workload slice: <name>
   - Triggering mode: <privilege/virtualization/interrupt/cache/boot>
   - Build and runtime tags: <toolchain/firmware/RTL revision>

2. Mechanism hypothesis
   - Primary mechanism: Data hazards occur when a consumer instruction needs a value that has not reached the register file yet. Forwarding (bypass) paths resolve most ALU-to-ALU and ALU-to-branch dependencies by routing results from EX/MEM or MEM/WB back into EX operand muxes with priority logic for youngest producer wins. Some cases still require stalls, especially load-use hazards when data becomes valid only after memory response timing; hazard detection must freeze PC and IF/ID while injecting a controlled bubble into downstream control. Correctness depends on tight coordination between forwarding select logic, stall gating, and kill handling so that wrong-path or excepting instructions never source or write stale values. Robust implementations pair dependency matrices with constrained-random instruction streams and assertion-based checks on operand freshness.
   - Competing hypotheses: <decode control / hazard / MMU / privilege / integration>
   - Missing evidence: <trace/counter/proof/report>

3. Proposed action
   - Smallest reversible change: <RTL/compiler/runtime/config>
   - Expected movement: <throughput/latency/tail/perf-per-watt>
   - Regression risk: correctness, security, power, timing, software compatibility

4. Signoff
   - Required artifact: Forwarding select truth table, hazard detector pseudocode, and stall/bubble state machine timing diagram.
   - Required owners: CPU microarchitecture lead, RTL implementation owner, verification lead, formal verification owner, silicon validation owner
   - Final decision: ship, bounded rollout, rollback, or escalate
Practice this lesson