RISC-V Design · All levels

Branch Handling, Pipeline Flush, and Redirect Timing: Worked Example

Worked Example for Branch Handling, Pipeline Flush, and Redirect Timing.

Worked example

Worked Example for Branch Handling, Pipeline Flush, and Redirect Timing is anchored on Branch mispredict penalty in cycles, wrong-path retire count (target zero), and redirect latency from resolve to fetch restart.. Convert observations into mechanism-backed decisions with explicit ownership.

A regression flags Branch mispredict penalty in cycles, wrong-path retire count (target zero), and redirect latency from resolve to fetch restart.. Strong closure isolates first failing mechanism, proves causality, applies one bounded change, and validates blast radius.

System view

diagram

RISC-V PIPELINE DIAGRAM - Branch Handling, Pipeline Flush, and Redirect Timing

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 - Branch Handling, Pipeline Flush, and Redirect Timing

+--------------------------+--------------------------------+--------------------------------+---------------------------+
| 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 Control-hazard recovery matrix covering resolve stage, flush depth, redirect source, and exception interaction cases..
Apply one bounded fix with owner signoff.
Run validation matrix and decide ship/rollback.

Practice this lesson