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Replay fuzzer divergences to reproduce bugs, find root cause by comparing Ivy AST interpreter vs Boa bytecode execution

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Replay Divergence Skill

Overview

A divergence occurs when Ivy's AST interpreter produces a different result than Boa's bytecode execution for the same Vyper code. Divergences indicate bugs in either:

  • Ivy (AST interpreter bug)
  • Vyper compiler (codegen bug - bytecode doesn't match source semantics)
  • Boa/py-evm (rare execution bug)

Replaying Divergences

Use src/fuzzer/replay_divergence.py to reproduce a divergence:

source venv/bin/activate
PYTHONPATH=src python -m fuzzer.replay_divergence path/to/divergence.json

Exit codes:

  • 0: Divergence reproduced
  • 1: No divergence (Ivy and Boa now match)
  • 2: Usage error

Divergence JSON Structure

See trace-format.json for full schema. Key fields:

{
  "type": "deployment|execution|xfail",
  "step": 0,
  "divergent_runner": "boa:default",
  "ivy_call": {"success": true, "output": "0x..."},
  "boa_call": {"success": false, "error": "..."},
  "traces": [...]
}

Finding Root Cause

Step 1: Reproduce

PYTHONPATH=src python -m fuzzer.replay_divergence path/to/divergence.json

Step 2: Extract Source Code

The divergence JSON contains source_code in the deployment trace. Extract it to understand what code is being tested.

Step 3: Isolate the Bug

Create a minimal test case:

def test_divergence_minimal(get_contract):
    src = """
    # Extracted/minimized source from divergence
    @external
    def foo() -> uint256:
        return 42
    """
    c = get_contract(src)
    # Test the specific behavior that diverges
    assert c.foo() == 42

Step 4: Compare Ivy vs Boa

# Test with Ivy (via get_contract fixture)
from ivy.frontend.loader import loads
ivy_contract = loads(src)
ivy_result = ivy_contract.foo()

# Test with Boa directly
import boa
boa_contract = boa.loads(src)
boa_result = boa_contract.foo()

print(f"Ivy: {ivy_result}, Boa: {boa_result}")

Step 5: Determine Bug Location

  • If Ivy is wrong: Fix in src/ivy/ (interpreter bug)
  • If Boa is wrong: Likely Vyper compiler bug (report to Vyper team)
  • If both differ from spec: Check EVM semantics

Related Skills

  • Use boa skill to understand Boa's execution model
  • Use vyper skill to inspect compiler internals
  • Both boa and vyper source are in venv/ for inspection

Debugging Tips

  1. Add debug prints in Ivy's visitor methods (src/ivy/stmt.py, src/ivy/expr/expr.py)
  2. Compare storage dumps between Ivy and Boa
  3. Check if the divergence is in deployment vs execution phase
  4. Look for edge cases: overflow, empty arrays, default values