Forwarding Network and Stall Control for Data Hazards: Interview Drills

PROMPT
You observe RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction. on Forwarding Network and Stall Control for Data Hazards. Walk through root cause and release decision.

STRONG ANSWER
1. Defines workload and earliest failing mechanism.
2. Explains 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.
3. Requests proving artifact: Forwarding select truth table, hazard detector pseudocode, and stall/bubble state machine timing diagram.
4. Proposes bounded fix + owner + rollback-safe validation.

WEAK ANSWER
Suggests generic optimization ideas without mechanism proof or owner accountability.

RISC-V EVIDENCE MATRIX - Forwarding Network and Stall Control for Data Hazards

+--------------------------+--------------------------------+--------------------------------+---------------------------+
| Evidence                 | Tells you                      | Does not prove                 | Next action               |
+--------------------------+--------------------------------+--------------------------------+---------------------------+
| perf counter timeline    | where regression appears       | exact mechanism causality      | correlate with trace      |
| decode/control dump      | control intent per instruction | pipeline side-effect ordering  | inspect retire semantics  |
| trap + CSR logs          | privilege/fault behavior       | performance bottleneck alone   | pair with CPI buckets     |
| MMU/TLB walk trace       | translation behavior           | full system QoS impact         | test mixed workloads      |
| post-fix trend graph     | movement after fix             | long-term stability            | run stress matrix         |
+--------------------------+--------------------------------+--------------------------------+---------------------------+