RISC-V Design · All levels

Hazard Detection Basics in Decode: Worked Example

Worked Example for Hazard Detection Basics in Decode.

Worked example

Worked Example for Hazard Detection Basics in Decode is anchored on RAW stall cycles per kilo-instruction and percentage of avoidable stalls removed by forwarding.. Convert observations into mechanism-backed decisions with explicit ownership.

A regression flags RAW stall cycles per kilo-instruction and percentage of avoidable stalls removed by forwarding.. Strong closure isolates first failing mechanism, proves causality, applies one bounded change, and validates blast radius.

System view

diagram

RISC-V PIPELINE DIAGRAM - Hazard Detection Basics in Decode

PC -> IF -> ID -> EX -> MEM -> WB
      |     |      |      |      |
  i-cache decode  ALU/BR  LSU    regfile write
              \   |
               +-> branch resolve + redirect

Hot paths:
  - branch + load-use dependencies in ID/EX
  - memory latency stretching MEM stage
  - writeback arbitration for integer/vector units

Focus: keep control hazards predictable

Evidence matrix

diagram

RISC-V EVIDENCE MATRIX - Hazard Detection Basics in Decode

+--------------------------+--------------------------------+--------------------------------+---------------------------+
| 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         |
+--------------------------+--------------------------------+--------------------------------+---------------------------+

Capture baseline and failing traces under fixed metadata tags.
Classify stage loss and dominant mechanism.
Collect Hazard matrix covering producer-consumer distance, bypass eligibility, and required interlock action for each instruction class..
Apply one bounded fix with owner signoff.
Run validation matrix and decide ship/rollback.

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