Deep EVM #17: Testing Huff Contracts — Foundry Fork Tests and Gas Assertions

Why Testing Huff Contracts Is Different

Huff is a low-level EVM assembly language that gives you direct control over the stack, memory, and storage. This power comes with a cost: there is no compiler to catch type errors, no SafeMath, and no automatic bounds checking. Every opcode you write is exactly what gets deployed. This makes testing not just important but absolutely critical.

Unlike Solidity where the compiler generates boilerplate for function dispatching, storage layout, and ABI encoding, Huff requires you to implement all of this manually. A single misplaced SWAP or incorrect jump destination can drain funds or brick a contract permanently.

In this article, we will build a complete testing strategy for Huff contracts using Foundry, covering unit tests, fork tests against mainnet state, gas assertions, and differential testing against a Solidity reference implementation.

Project Setup

First, let’s set up a Foundry project with Huff support. You need the Huff compiler (huffc) installed alongside Foundry:

curl -L get.huff.sh | bash
huffup
forge init huff-testing && cd huff-testing
forge install huff-language/foundry-huff

Configure foundry.toml to use the Huff compiler:

[profile.default]
src = "src"
out = "out"
libs = ["lib"]
ffi = true

[profile.default.fuzz]
runs = 10000
max_test_rejects = 100000

[profile.default.invariant]
runs = 256
depth = 128

The ffi = true flag is essential because foundry-huff uses FFI calls to invoke the Huff compiler during testing.

Writing a Huff Contract Under Test

Let’s write a simple ERC20-like token in Huff that we want to test:

// src/SimpleToken.huff
#define function balanceOf(address) view returns (uint256)
#define function transfer(address, uint256) nonpayable returns (bool)

#define constant BALANCES_SLOT = FREE_STORAGE_POINTER()

#define macro BALANCE_OF() = takes (0) returns (0) {
    0x04 calldataload           // [account]
    BALANCES_SLOT               // [slot, account]
    STORE_ELEMENT_FROM_KEYS(0x00) // [balance_slot]
    sload                       // [balance]
    0x00 mstore                 // []
    0x20 0x00 return
}

#define macro TRANSFER() = takes (0) returns (0) {
    0x24 calldataload           // [amount]
    0x04 calldataload           // [to, amount]
    caller                      // [from, to, amount]

    // Load sender balance
    dup1                        // [from, from, to, amount]
    BALANCES_SLOT
    STORE_ELEMENT_FROM_KEYS(0x00)
    sload                       // [from_bal, from, to, amount]

    // Check sufficient balance
    dup1                        // [from_bal, from_bal, from, to, amount]
    dup5                        // [amount, from_bal, from_bal, from, to, amount]
    gt                          // [amount>from_bal, from_bal, from, to, amount]
    fail jumpi

    // Subtract from sender
    dup4                        // [amount, from_bal, from, to, amount]
    swap1 sub                   // [new_from_bal, from, to, amount]
    dup2 BALANCES_SLOT
    STORE_ELEMENT_FROM_KEYS(0x00)
    sstore                      // [from, to, amount]

    // Add to recipient
    swap1                       // [to, from, amount]
    dup1 BALANCES_SLOT
    STORE_ELEMENT_FROM_KEYS(0x00)
    dup1 sload                  // [to_bal, to_slot, from, amount]
    dup4 add                    // [new_to_bal, to_slot, from, amount]
    swap1 sstore                // [from, amount]
    pop pop

    // Return true
    0x01 0x00 mstore
    0x20 0x00 return

    fail:
        0x00 0x00 revert
}

#define macro MAIN() = takes (0) returns (0) {
    0x00 calldataload 0xE0 shr
    dup1 0x70a08231 eq balanceOf jumpi
    dup1 0xa9059cbb eq transfer jumpi
    0x00 0x00 revert

    balanceOf:
        BALANCE_OF()
    transfer:
        TRANSFER()
}

Foundry Test Setup for Huff

The key to testing Huff in Foundry is using the HuffDeployer library from foundry-huff:

// test/SimpleToken.t.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

import "forge-std/Test.sol";
import "foundry-huff/HuffDeployer.sol";

interface ISimpleToken {
    function balanceOf(address account) external view returns (uint256);
    function transfer(address to, uint256 amount) external returns (bool);
}

contract SimpleTokenTest is Test {
    ISimpleToken token;
    address alice = makeAddr("alice");
    address bob = makeAddr("bob");

    function setUp() public {
        address deployed = HuffDeployer.deploy("SimpleToken");
        token = ISimpleToken(deployed);

        // Set initial balance for alice using vm.store
        bytes32 slot = keccak256(abi.encode(alice, uint256(0)));
        vm.store(address(token), slot, bytes32(uint256(1000e18)));
    }

    function test_balanceOf() public view {
        assertEq(token.balanceOf(alice), 1000e18);
        assertEq(token.balanceOf(bob), 0);
    }

    function test_transfer() public {
        vm.prank(alice);
        token.transfer(bob, 100e18);

        assertEq(token.balanceOf(alice), 900e18);
        assertEq(token.balanceOf(bob), 100e18);
    }

    function test_transferInsufficientBalance() public {
        vm.prank(bob);
        vm.expectRevert();
        token.transfer(alice, 1);
    }
}

Run with targeted matching:

forge test --match-contract SimpleTokenTest -vvv

The -vvv flag shows full stack traces on failure, which is essential for debugging Huff since error messages are not built into the bytecode.

Gas Snapshots and Regression Testing

Gas efficiency is the primary reason to write Huff. Foundry’s gas snapshot feature lets you track gas usage across test runs and catch regressions:

forge snapshot --match-contract SimpleTokenTest

This creates a .gas-snapshot file:

SimpleTokenTest:test_balanceOf() (gas: 5421)
SimpleTokenTest:test_transfer() (gas: 28934)
SimpleTokenTest:test_transferInsufficientBalance() (gas: 5102)

Compare against a baseline to detect regressions:

forge snapshot --diff .gas-snapshot

For CI, you can set a tolerance threshold:

forge snapshot --check .gas-snapshot --tolerance 1

This fails the build if any test’s gas usage increases by more than 1%. Commit the .gas-snapshot file to your repository so every PR is checked against the current baseline.

Differential Testing: Huff vs Solidity Reference

The most powerful testing technique for Huff is differential testing. Write a Solidity implementation that is known-correct, then verify your Huff contract produces identical results for all inputs:

contract DifferentialTest is Test {
    ISimpleToken huffToken;
    SolidityToken solToken;

    function setUp() public {
        huffToken = ISimpleToken(HuffDeployer.deploy("SimpleToken"));
        solToken = new SolidityToken();
    }

    function testFuzz_balanceOf_differential(
        address account,
        uint256 balance
    ) public {
        // Set same state in both contracts
        bytes32 huffSlot = keccak256(abi.encode(account, uint256(0)));
        vm.store(address(huffToken), huffSlot, bytes32(balance));
        deal(address(solToken), account, balance);

        // Compare outputs
        assertEq(
            huffToken.balanceOf(account),
            solToken.balanceOf(account),
            "balanceOf mismatch"
        );
    }
}

With fuzz.runs = 10000 in foundry.toml, Foundry generates 10,000 random inputs and verifies both implementations agree. This catches subtle bugs like off-by-one errors in stack manipulation or incorrect storage slot calculations.

Fork Testing Against Mainnet State

Fork testing lets you test your Huff contract against real mainnet state. This is invaluable for contracts that interact with existing protocols:

forge test --fork-url https://eth-mainnet.g.alchemy.com/v2/YOUR_KEY \
  --match-test testFork -vvv

function testFork_interactWithUniswap() public {
    // Your Huff contract can call real Uniswap contracts
    address WETH = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
    address DAI = 0x6B175474E89094C44Da98b954EedeAC495271d0F;

    // Test your Huff router against real pool state
    uint256 amountOut = IHuffRouter(address(huffRouter)).getAmountOut(
        1 ether, WETH, DAI
    );

    // Verify against Uniswap's own calculation
    assertTrue(amountOut > 0, "Should return non-zero amount");
}

Fork tests are slower (network calls) but catch integration issues that unit tests miss, such as incorrect interface encoding or unexpected state from real contracts.

Advanced: Testing Jump Tables and Dispatcher

Huff contracts typically implement their own function dispatcher. Test that unknown selectors revert and that every selector routes correctly:

function test_unknownSelectorReverts() public {
    (bool success,) = address(token).call(
        abi.encodeWithSelector(bytes4(0xdeadbeef))
    );
    assertFalse(success, "Unknown selector should revert");
}

function test_allSelectorsRoute() public view {
    // balanceOf(address)
    token.balanceOf(address(0));
    // transfer(address,uint256) -- should not revert with zero balance to zero
}

Gas Comparison: Huff vs Solidity vs Yul

Track gas differences across implementations to justify Huff complexity:

Operation	Solidity	Yul	Huff
balanceOf	2,604	2,341	2,187
transfer	29,412	27,891	26,534
Deploy	198,234	143,892	98,421

Huff typically saves 10-15% gas over Yul and 20-30% over Solidity for hot paths. For MEV bots and high-frequency operations, these savings compound to significant competitive advantages.

Conclusion

Testing Huff contracts requires a disciplined, multi-layered approach: unit tests for basic correctness, fuzz tests for edge cases, differential tests against Solidity references for equivalence, fork tests for real-world integration, and gas snapshots for performance regression. Foundry provides all these capabilities in a single toolkit.

The key insight is that lower-level code demands higher-level testing rigor. Every opcode you write manually is an opportunity for a subtle bug. Invest in comprehensive test suites, and the gas savings from Huff will be backed by the same confidence you have in well-tested Solidity.