EIP-7939: Count Leading Zeros (CLZ) Opcode

A Rust implementation demonstrating how the CLZ opcode works in Ethereum.

What is CLZ?

CLZ counts the number of leading zero bits in a 256-bit number. If the input is zero, it returns 256.

Why It Matters

Currently, implementing CLZ in Solidity costs ~184 gas and requires complex bitwise operations. Adding it as a native opcode provides:

• Lower gas costs for mathematical operations
• Reduced ZK proving costs
• Smaller bytecode size

Specification

• Opcode: 0x1e
• Gas Cost: 5
• Stack Input: 1 value (x)
• Stack Output: 1 value (leading zero count)

Use Cases

• Mathematical functions (sqrt, log, pow)
• Calldata compression/decompression
• Bitmap operations
• Post-quantum cryptography

Implementation

The core function uses a 4-limb approach for efficiency:

fn clz(x: U256) -> u32 {
    if x.is_zero() {
        return 256;
    }
    
    let limbs = x.0;
    
    if limbs[3] != 0 { return limbs[3].leading_zeros(); }
    if limbs[2] != 0 { return 64 + limbs[2].leading_zeros(); }
    if limbs[1] != 0 { return 128 + limbs[1].leading_zeros(); }
    if limbs[0] != 0 { return 192 + limbs[0].leading_zeros(); }
    
    256
}

Quick Start

# run the demo
cargo run

# run tests
cargo test

Test Results

running 8 tests
test tests::test_clz_zero ... ok
test tests::test_clz_one ... ok
test tests::test_clz_powers_of_two ... ok
test tests::test_clz_various_values ... ok
test tests::test_clz_max_value ... ok
test tests::test_evm_stack_clz ... ok
test tests::test_limb_boundaries ... ok
test tests::test_both_implementations_match ... ok

Examples

CLZ(0) = 256
CLZ(1) = 255
CLZ(256) = 247
CLZ(2^255) = 0
CLZ(2^256 - 1) = 0

Reference

Official EIP: https://eips.ethereum.org/EIPS/eip-7939

License

CC0