verilog-dead-code-root-cause

name: verilog-dead-code-root-cause

description: Use this skill when the user provides Verilog/SystemVerilog source files or a source directory plus a top module, and wants a concise Chinese JSON diagnosis for unreachable procedural branches, static dead-code conditions, or logic that disappears before/after Yosys proc/opt, especially cases where for-loop unfolding or constant loop variables make if/case branches impossible. license: MIT

metadata:

author: zk

version: "1.1"

Verilog Dead Code Root Cause

When to Use

• The user asks whether a Verilog branch, assignment, signal, or block is dead code.

• The design has procedural always blocks, for loops, if/else, or case statements.

• The suspected issue is like for (k = 0; k < 3; ...) with conditions such as if (k == 3) or if (k > 3).

• The user wants to know whether before/after RTLIL diff is enough to recover the dead-code root.

• The user wants the final diagnosis saved as a concise Chinese JSON report.

Key Rule

Do not rely only on raw_proc.il -> opt_proc.il diff for procedural dead-code diagnosis.

Yosys may eliminate unreachable branches during Verilog elaboration or process lowering before the standard proc output. In those cases:

• pre_proc.il may still contain process and switch 1'0 / switch 1'1 with source locations.

• raw_proc.il may already have lost the unreachable branch.

• opt_proc.il may only show later cleanup, such as unused registers or internal logic being purged.

Workflow

1. Generate evidence files

Run from the lint_agent project root:

python skills/verilog-dead-code-root-cause/scripts/run_dead_code_trace.py --top <top_module> <source_or_dir> [more_sources...]

The script writes a new directory under reports/dead_code_<YYYYMMDD_HHMMSS>/ relative to the project root containing:

• dead_code_artifacts.json

• pre_proc.il

• raw_proc.il

• raw_proc_ifx_noopt.il

• opt_proc.il

Read ARTIFACTS_PATH=... from the script output and open dead_code_artifacts.json first.

2. Diagnose in this order

1. Read the source around the suspicious always, for, if, or case.

2. Read pre_proc.il and search for the source line, process, switch 1'0, and switch 1'1.

3. Read raw_proc.il to see what remains after proc.

4. Read opt_proc.il only to confirm additional cleanup after optimization.

5. Use raw_proc_ifx_noopt.il as a sanity check when you suspect default proc passes are hiding information.

3. What Counts as a Strong Dead-Code Finding

Report a likely real dead-code condition when several of these hold:

• Source has a statically impossible condition, such as loop variable ranges that cannot satisfy the branch.

• pre_proc.il shows a constant switch condition, especially switch 1'0 for an unreachable branch.

• The source location on the switch points back to the suspicious if or case.

• Signals assigned only inside the unreachable branch become unused or disappear later.

• raw_proc.il no longer contains a corresponding mux/comparator/assignment for the branch.

4. What Not to Claim

• Do not claim raw_proc -> opt_proc alone proves the original dead condition. It may only prove later cleanup.

• Do not treat every removed register as dead code; it may be removed because the module has no observable outputs.

• Do not report a branch as dead unless source-level ranges or pre_proc.il constant switches support it.

5. Write the final Chinese JSON diagnosis

• Write the final diagnosis to the report directory printed by the wrapper:

<REPORT_DIR>/dead_code_diagnosis.json

• The final assistant response should be brief and in Chinese: state the JSON report path and the number of dead-code findings. Do not duplicate the full report in prose unless the user asks.
• The JSON report must contain only core content:

{
  "summary": {
    "top_module": "top",
    "input_paths": ["rtl"],
    "artifact_dir": "reports/dead_code_20260428_153000",
    "dead_code_count": 1,
    "output_path": "reports/dead_code_20260428_153000/dead_code_diagnosis.json"
  },
  "findings": [
    {
      "id": "DC_001",
      "file": "rtl/top.sv",
      "line": 58,
      "condition": "if (k == 3)",
      "category": "源码级死代码",
      "root_cause": "for 循环中 k 的取值范围为 0 到 2，因此 k == 3 永远不成立。",
      "evidence": {
        "source_evidence": "源码中的循环范围与条件互斥",
        "pre_proc_evidence": "pre_proc.il 中对应分支表现为 switch 1'0",
        "raw_proc_evidence": "raw_proc.il 中已不存在该分支对应的 mux 或赋值",
        "opt_proc_evidence": "opt_proc.il 中仅保留后续清理结果"
      },
      "diagnosis": "该分支为静态不可达分支，应删除或修正循环范围/条件。",
      "fix_hint": "如果 k==3 是有效场景，应调整循环边界；否则删除该不可达分支。"
    }
  ]
}

Field rules:

• Keep JSON keys stable in English, but write all diagnosis text in Chinese.
• summary must contain only top module, inputs, artifact directory, count, and output path.
• findings must contain one entry per confirmed dead-code condition.
• Use category values 源码级死代码 or 优化清理.
• root_cause, diagnosis, and fix_hint must be concise and evidence-based.
• evidence must cite only facts from source code, pre_proc.il, raw_proc.il, raw_proc_ifx_noopt.il, and opt_proc.il.
• Do not include broad Yosys background or generic dead-code explanation.
• If no dead code is confirmed, write "findings": [] and set dead_code_count to 0.