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Use the new CLZ opcode introduced in Fusaka (and available in solidity 0.8.31) #6173

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Use the new CLZ opcode introduced in Fusaka (and available in solidity 0.8.31) #6173

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2fa600b
User the new CLZ opcode
Amxx Dec 3, 2025
0608dc4
workaround edr bug by fixing the block gas limit
Amxx Dec 4, 2025
1f02c78
Update foundry.toml
Amxx Dec 4, 2025
6e174d4
fix lint
Amxx Dec 4, 2025
ecfffa4
Merge branch 'update/use-0.8.31-clz' of https://github.com/Amxx/openz…
Amxx Dec 4, 2025
c3e0817
Merge branch 'update/use-0.8.31-clz' of https://github.com/Amxx/openz…
Amxx Dec 4, 2025
9f8a13c
Merge branch 'update/use-0.8.31-clz' of https://github.com/Amxx/openz…
Amxx Dec 10, 2025
3660f16
Merge branch 'master' into update/use-0.8.31-clz
Amxx Dec 10, 2025
c01d333
minimize
Amxx Dec 10, 2025
6c4ac2b
cascade pragma bump
Amxx Dec 10, 2025
3606d19
Merge branch 'master' into update/use-0.8.31-clz
Amxx Dec 16, 2025
913adc8
Update to hardhat 2.28.0 and use pragma ^0.8.31
Amxx Dec 16, 2025
5860901
update pragma in templates
Amxx Dec 16, 2025
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72 changes: 14 additions & 58 deletions contracts/utils/math/Math.sol
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v5.5.0) (utils/math/Math.sol)

pragma solidity ^0.8.20;
pragma solidity >0.8.30;

import {Panic} from "../Panic.sol";
import {SafeCast} from "./SafeCast.sol";
Expand Down Expand Up @@ -613,50 +613,22 @@ library Math {
}

/**
* @dev Return the log in base 2 of a positive value rounded towards zero.
* Returns 0 if given 0.
* @dev Counts the number of leading zero bits in a uint256.
*/
function log2(uint256 x) internal pure returns (uint256 r) {
// If value has upper 128 bits set, log2 result is at least 128
r = SafeCast.toUint(x > 0xffffffffffffffffffffffffffffffff) << 7;
// If upper 64 bits of 128-bit half set, add 64 to result
r |= SafeCast.toUint((x >> r) > 0xffffffffffffffff) << 6;
// If upper 32 bits of 64-bit half set, add 32 to result
r |= SafeCast.toUint((x >> r) > 0xffffffff) << 5;
// If upper 16 bits of 32-bit half set, add 16 to result
r |= SafeCast.toUint((x >> r) > 0xffff) << 4;
// If upper 8 bits of 16-bit half set, add 8 to result
r |= SafeCast.toUint((x >> r) > 0xff) << 3;
// If upper 4 bits of 8-bit half set, add 4 to result
r |= SafeCast.toUint((x >> r) > 0xf) << 2;

// Shifts value right by the current result and use it as an index into this lookup table:
//
// | x (4 bits) | index | table[index] = MSB position |
// |------------|---------|-----------------------------|
// | 0000 | 0 | table[0] = 0 |
// | 0001 | 1 | table[1] = 0 |
// | 0010 | 2 | table[2] = 1 |
// | 0011 | 3 | table[3] = 1 |
// | 0100 | 4 | table[4] = 2 |
// | 0101 | 5 | table[5] = 2 |
// | 0110 | 6 | table[6] = 2 |
// | 0111 | 7 | table[7] = 2 |
// | 1000 | 8 | table[8] = 3 |
// | 1001 | 9 | table[9] = 3 |
// | 1010 | 10 | table[10] = 3 |
// | 1011 | 11 | table[11] = 3 |
// | 1100 | 12 | table[12] = 3 |
// | 1101 | 13 | table[13] = 3 |
// | 1110 | 14 | table[14] = 3 |
// | 1111 | 15 | table[15] = 3 |
//
// The lookup table is represented as a 32-byte value with the MSB positions for 0-15 in the last 16 bytes.
function clz(uint256 value) internal pure returns (uint256 result) {
assembly ("memory-safe") {
r := or(r, byte(shr(r, x), 0x0000010102020202030303030303030300000000000000000000000000000000))
result := clz(value)
}
}

/**
* @dev Return the log in base 2 of a positive value rounded towards zero.
* Returns 0 if given 0.
*/
function log2(uint256 value) internal pure returns (uint256) {
return saturatingSub(255, clz(value));
}

/**
* @dev Return the log in base 2, following the selected rounding direction, of a positive value.
* Returns 0 if given 0.
Expand Down Expand Up @@ -723,17 +695,8 @@ library Math {
*
* Adding one to the result gives the number of pairs of hex symbols needed to represent `value` as a hex string.
*/
function log256(uint256 x) internal pure returns (uint256 r) {
// If value has upper 128 bits set, log2 result is at least 128
r = SafeCast.toUint(x > 0xffffffffffffffffffffffffffffffff) << 7;
// If upper 64 bits of 128-bit half set, add 64 to result
r |= SafeCast.toUint((x >> r) > 0xffffffffffffffff) << 6;
// If upper 32 bits of 64-bit half set, add 32 to result
r |= SafeCast.toUint((x >> r) > 0xffffffff) << 5;
// If upper 16 bits of 32-bit half set, add 16 to result
r |= SafeCast.toUint((x >> r) > 0xffff) << 4;
// Add 1 if upper 8 bits of 16-bit half set, and divide accumulated result by 8
return (r >> 3) | SafeCast.toUint((x >> r) > 0xff);
function log256(uint256 value) internal pure returns (uint256) {
return log2(value) / 8;
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@Amxx Amxx Dec 8, 2025

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Note, DIV is 5 gas, shifts are 3 gas.

}

/**
Expand All @@ -753,11 +716,4 @@ library Math {
function unsignedRoundsUp(Rounding rounding) internal pure returns (bool) {
return uint8(rounding) % 2 == 1;
}

/**
* @dev Counts the number of leading zero bits in a uint256.
*/
function clz(uint256 x) internal pure returns (uint256) {
return ternary(x == 0, 256, 255 - log2(x));
}
}
2 changes: 1 addition & 1 deletion foundry.toml
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
[profile.default]
solc_version = '0.8.27'
solc_version = '0.8.31'
evm_version = 'prague'
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@coderabbitai coderabbitai bot Dec 4, 2025

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⚠️ Potential issue | 🟠 Major

🧩 Analysis chain

🌐 Web query:

CLZ opcode Ethereum hardfork prague osaka which version introduced

💡 Result:

The CLZ opcode was specified in EIP‑7939 and was included in the Fusaka network upgrade (not Prague). Test vectors/implementation work appear under Osaka testing, but mainnet activation is part of Fusaka. [1][2][3]

Sources:
[1] EIP‑7939 (Count leading zeros / CLZ).
[2] Fusaka mainnet announcement (lists EIP‑7939 as a Fusaka core EIP).
[3] Osaka execution-spec tests checklist referencing EIP‑7939.

Both Foundry and Hardhat EVM versions are too old for CLZ opcode support.

The solc_version is correctly updated to 0.8.31, but both evm_version settings are insufficient for the CLZ opcode. CLZ (Count Leading Zeros, EIP-7939) requires the Fusaka network upgrade for mainnet activation. Setting Foundry to 'prague' and Hardhat to 'osaka' will both fail when attempting to use the clz assembly instruction in Math.sol. Both configurations must be updated to 'fusaka' or later.

🤖 Prompt for AI Agents 
In foundry.toml around lines 2-3, the EVM version is set to 'prague' which does
not support the CLZ opcode; update the evm_version to 'fusaka' (or a later
network upgrade) so EIP-7939 CLZ is available, keeping solc_version = '0.8.31'
as-is, and ensure the project's Hardhat config is also updated from 'osaka' to
'fusaka' or later to avoid runtime failures when using the clz assembly
instruction.

optimizer = true
optimizer-runs = 200
Expand Down
7 changes: 4 additions & 3 deletions hardhat.config.js
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/// ENVVAR
// - COMPILER: compiler version (default: 0.8.27)
// - COMPILER: compiler version (default: 0.8.31)
// - SRC: contracts folder to compile (default: contracts)
// - RUNS: number of optimization runs (default: 200)
// - IR: enable IR compilation (default: false)
Expand All @@ -18,7 +18,7 @@ const { argv } = require('yargs/yargs')()
compiler: {
alias: 'compileVersion',
type: 'string',
default: '0.8.27',
default: '0.8.31',
},
src: {
alias: 'source',
Expand All @@ -38,7 +38,7 @@ const { argv } = require('yargs/yargs')()
evm: {
alias: 'evmVersion',
type: 'string',
default: 'prague',
default: 'osaka',
},
// Extra modules
coverage: {
Expand Down Expand Up @@ -104,6 +104,7 @@ module.exports = {
allowUnlimitedContractSize: true,
initialBaseFeePerGas: argv.coverage ? 0 : undefined,
enableRip7212: true,
blockGasLimit: 16_000_000,
},
},
exposed: {
Expand Down
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