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'.

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