Bug: Compiler stack overflow crash on deeply nested expressions (~2900 levels)

[chamitro]

Summary

The Circom compiler crashes with a stack overflow when processing expressions nested approximately 2900+ levels deep. Instead of producing a clean error message, the compiler aborts with thread 'main' has overflowed its stack. The compiler should either handle the input or produce a clean error.

Environment

• Circom: 2.2.3 (latest master)
• OS: Ubuntu

Reproducer

The following Python script generates a test circuit with configurable nesting depth:

depth = 3000
expr = "a"
for _ in range(depth):
    expr = f"({expr} + 1)"

code = f"""pragma circom 2.0.0;
template T() {{
    signal input a;
    signal output out;
    var v = {expr};
    out <== a;
}}
component main = T();
"""

with open("test.circom", "w") as f:
    f.write(code)

python3 generate.py
circom test.circom --r1cs --wasm --output .
# CRASH: thread 'main' has overflowed its stack
# fatal runtime error: stack overflow, aborting

Results

Nesting depth	Circom	Noir	Cairo	Leo
1000	✅ OK	OK	CRASH (~450)	OK
2000	✅ OK	OK	CRASH	OK
2900	✅ OK	OK	CRASH	OK
3000	❌ CRASH	OK	CRASH	OK
5000	❌ CRASH	OK	CRASH	OK

The crash also occurs with very wide expressions (~5000 terms in a + a + a + ...).

Expected Behavior

The compiler should produce a clean error:

error: expression nesting depth exceeds maximum supported depth

A compiler should never crash on any input, regardless of how pathological it is.

Discovery Method

Found via differential testing tool that tests 4 ZK compilers with pathological circuit patterns.