fhevm-solidity/docs/guides/gas.md

Gas estimation

FHE operations are typically more computationally expensive than classical operations due to their inherent complexity. As a reference, here is an approximation of the gas cost associated with each operation.

ebool

Function name	Gas
and/or/xor	26,000
not	30,000
decrypt	500,000

euint4

function name	Gas
add/sub	65,000
add/sub (scalar)	65,000
mul	150,000
mul (scalar)	88,000
div (scalar)	139,000
rem (scalar)	286,000
and/or/xor	32,000
shr/shl	116,000
shr/shl (scalar)	35,000
rotr/rotl	116,000
rotr/rotl (scalar)	35,000
eq/ne	51,000
ge/gt/le/lt	70,000
min/max	121,000
min/max (scalar)	121,000
neg	60,000
not	33,000
select	45,000
decrypt	500,000

euint8

Function name	Gas
add/sub	94,000
add/sub (scalar)	94,000
mul	197,000
mul (scalar)	159,000
div (scalar)	238,000
rem (scalar)	460,000
and/or/xor	34,000
shr/shl	133,000
shr/shl (scalar)	35,000
rotr/rotl	133,000
rotr/rotl (scalar)	35,000
eq/ne	53,000
ge/gt/le/lt	82,000
min/max	128,000
min/max (scalar)	128,000
neg	95,000
not	34,000
select	47,000
decrypt	500,000
randEuint8()	100,000

euint16

function name	euint16
add/sub	133,000
add/sub (scalar)	133,000
mul	262,000
mul (scalar)	208,000
div (scalar)	314,000
rem (scalar)	622,000
and/or/xor	34,000
shr/shl	153,000
shr/shl (scalar)	35,000
rotr/rotl	153,000
rotr/rotl (scalar)	35,000
eq/ne	54,000
ge/gt/le/lt	105,000
min/max	153,000
min/max (scalar)	150,000
neg	131,000
not	35,000
select	47,000
decrypt	500,000
randEuint16()	100,000

euint32

Function name	Gas fee
add/sub	162,000
add/sub (scalar)	162,000
mul	359,000
mul (scalar)	264,000
div (scalar)	398,000
rem (scalar)	805,000
and/or/xor	35,000
shr/shl	183,000
shr/shl (scalar)	35,000
rotr/rotl	183,000
rotr/rotl (scalar)	35,000
eq/ne	82,000
ge/gt/le/lt	128,000
min/max	183,000
min/max (scalar)	164,000
neg	160,000
not	36,000
select	50,000
decrypt	500,000
randEuint32()	100,000

euint64

Function name	Gas fee
add/sub	188,000
add/sub (scalar)	188,000
mul	641,000
mul (scalar)	356,000
div (scalar)	584,000
rem (scalar)	1,095,000
and/or/xor	38,000
shr/shl	227,000
shr/shl (scalar)	38,000
rotr/rotl	227,000
rotr/rotl (scalar)	38,000
eq/ne	86,000
ge/gt/le/lt	156,000
min/max	210,000
min/max (scalar)	192,000
neg	199,000
not	37,000
select	53,000
decrypt	500,000
randEuint64()	100,000

eaddress

Function name	Gas fee
eq/ne	90,000

Estimate gas

When you call estimate gas method, we can’t determine accurately the gas usage if your function uses TFHE.decrypt. During gas estimation, all TFHE.decrypt() will return 1.

What does it mean?

• require(TFHE.decrypt(ebool)); will be ok but require(!TFHE.decrypt(ebool)); will fail during estimation (revert transaction)
• A loop, where you expect a decrypt to be false to break, will never end in gas estimate method (and fails), since the decrypt will always return 1 (true)
• On the other hand, if your loop should last 2 or 3 cycles, until the value is 1, the estimation will be below.
• If you have branches (if/else) based on a decryption, the estimation will use the branch running when the decryption is 1

While it’s challenging to accurately estimate gas consumption when using TFHE.decrypt, we strongly encourage you to take this into consideration.

What can I do?

A possible solution is to overestimate your gas estimation. You can take this function (with ethers.js) as an example where we multiply the gas limit by 1.2.

export const createTransaction = async <A extends [...{ [I in keyof A]-?: A[I] | Typed }]>(
  method: TypedContractMethod<A>,
  ...params: A
) => {
  const gasLimit = await method.estimateGas(...params);
  const updatedParams: ContractMethodArgs<A> = [
    ...params,
    { gasLimit: Math.min(Math.round(+gasLimit.toString() * 1.2), 10000000) },
  ];
  return method(...updatedParams);
};

Gas limit

The current devnet has a gas limit of 10,000,000. If you send a transaction exceeding this limit, it won't be executed. Consequently, your wallet won't be able to emit a new transaction. To address this, emit a new transaction with the same nonce but the correct gas limit. In Metamask, you can enforce the use of a specific nonce by enabling the feature in 'Advanced Settings'.