RISC-V Design · All levels

Branch Handling, Pipeline Flush, and Redirect Timing: Debug Playbook

Debug Playbook for Branch Handling, Pipeline Flush, and Redirect Timing.

Debug playbook

Debug Playbook 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.

Freeze workload seed, binary, compiler, firmware, and platform setup.
Find earliest persistent mechanism failure in the timeline.
Build one reduced reproducer for dominant hypothesis.
Patch minimal fix with explicit rollback gate.
Rerun correctness + performance + power matrix.

Decision tree

diagram

ROOT CAUSE TREE - Branch Handling, Pipeline Flush, and Redirect Timing

Branch mispredict penalty in cycles, wrong-path retire count (target zero), and redirect latency from resolve to fetch restart. 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 / Branch Handling, Pipeline Flush, and Redirect Timing

1. Symptom
   - Failing metric: Branch mispredict penalty in cycles, wrong-path retire count (target zero), and redirect latency from resolve to fetch restart.
   - Failing workload slice: <name>
   - Triggering mode: <privilege/virtualization/interrupt/cache/boot>
   - Build and runtime tags: <toolchain/firmware/RTL revision>

2. Mechanism hypothesis
   - Primary mechanism: Control hazards arise because branch outcome and target are known after some fetch/decode work has already happened. A simple baseline resolves branches in EX and flushes younger stages on taken or mispredicted decisions, but latency can be reduced by early compare or target adder placement if timing budget allows. Redirect logic must atomically cancel younger side effects, update fetch PC, and preserve precise exception behavior when branch and fault events race. Even without advanced prediction, a static policy (such as backward-taken/forward-not-taken) changes CPI materially and needs clean interface contracts between fetch, decode, and execute for prediction bits and recovery tags. Verification needs focused stress on nested branches, branch-after-load dependencies, and simultaneous flush-plus-stall interactions to prevent ghost writes and replay bugs.
   - 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: Control-hazard recovery matrix covering resolve stage, flush depth, redirect source, and exception interaction cases.
   - Required owners: CPU microarchitecture lead, frontend and fetch owner, RTL implementation owner, verification lead, performance modeling owner
   - Final decision: ship, bounded rollout, rollback, or escalate

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