Paymaster gas optimization

[gonzaotc]

Note

AI-generated with the Solidity Gas Optimizer skill in council mode: 4 models (Composer, Fable 5, GPT-5.6 Sol, Grok 4.5) each ran the full audit independently, the runs were merged and adjudicated, and every finding below was re-measured on this branch. All tests pass, but it's encouraged to review before merging to confirm behavior is unaffected.

Run

• Measurement: Hardhat, solc 0.8.35, runs 200, via-IR false; runtime deltas from deterministic whole-transaction handleOps probes, since the gas reporter cannot attribute calls made through the EntryPoint predeploy
• Tests: Full project suite: 8,007 passing and 1 pending (pre-existing); paymaster suite 90 passing
• Source runs: 4 independent audits at db06a6ef / d017d3e5; the scope tree is byte-identical across both commits
• Scope:

  contracts/account/paymaster/
    Paymaster.sol
    extensions/
      PaymasterERC20.sol
      PaymasterERC20Guarantor.sol
      PaymasterERC721Owner.sol
      PaymasterSigner.sol
  

Results

Five independent gas optimization candidates survived adjudication, one commit each and keyed by the IDs below. It's recommended to review them one by one, via the commits view.

ID	Change	Function	Δ runtime /call	Δ deploy gas	Found by
GAS-H-01	parse both paymaster gas limits with one calldata load	PaymasterSigner._signableUserOpHash	−712	−20,547 / −20,558	Fable 5, GPT-5.6 Sol
GAS-H-02	skip the mulDiv 512-bit path when the product fits one word	PaymasterERC20._erc20Cost	−966 base, −1,932 guaranteed	+17,102	Fable 5, GPT-5.6 Sol
GAS-M-01	one-window timestamp decode	PaymasterSigner._decodePaymasterUserOp	−199	+876 / +864	Grok 4.5, Composer, GPT-5.6 Sol
GAS-L-01	unchecked refund subtraction under the EntryPoint invariant	PaymasterERC20._refund	−72 per postOp	within noise	Fable 5
GAS-L-02	branchless validation data via SafeCast.toUint	PaymasterERC721Owner._validatePaymasterUserOp	−12	−3,458 / −3,465	Fable 5

GAS-H-01 · parse both paymaster gas limits with one calldata load (PaymasterSigner._signableUserOpHash)

• What it does: replaces the two ERC4337Utils accessor calls with one length < 52 check and one 32-byte load of paymasterAndData[20:52], split on the stack into the verification and postOp gas limits; the encoded hash is byte-identical, including the zero fallback for short data.
• Why it's cheaper: each accessor re-derived the calldata slice, re-checked the length, and loaded overlapping bytes of the same word; one load removes the duplicated bounds checks and slice construction, and dropping the accessor code removes about 20.5k of deployment bytecode.
• Tradeoff: the [20:52] offsets and the 128-bit split are spelled out locally in EIP-712 hashing code, duplicating layout knowledge the ERC4337Utils accessors centralize. Cherry-picks independently.
• Found by: Fable 5 and GPT-5.6 Sol, independently at the same base commit; the deploy delta was reproduced by both runs and again on this branch.

GAS-H-02 · skip the mulDiv 512-bit path when the product fits one word (PaymasterERC20._erc20Cost)

• What it does: branches on high == 0 from the already-computed mul512 and finishes the common case with Math.ceilDiv, keeping mulDiv for the genuine multi-word case and preserving the type(uint256).max saturation sentinel, including the denominator == 0 edge.
• Why it's cheaper: the overflow guard already computes the full 512-bit product and Math.mulDiv recomputes it internally; the one-word case needs only a ceiling division. The function runs twice per sponsored operation and four times per guaranteed one.
• Tradeoff: a second ceiling-division path in money math that must stay congruent with mulDiv (the equivalence at the high == 0 boundary and both sentinel edges was independently verified in review), plus +17.1k deployment gas per concrete paymaster. Cherry-picks independently.
• Found by: Fable 5 (measured); GPT-5.6 Sol raised the same waste as an advisory proposing a shared single-pass mulDiv primitive in Math, which remains the API-level alternative.

GAS-M-01 · one-window timestamp decode (PaymasterSigner._decodePaymasterUserOp)

• What it does: reads the 12-byte validity window once as bytes12 and splits it with casts (uint48(bytes6(validity)), uint48(bytes6(validity << 48))) instead of two adjacent bounds-checked 6-byte slices; the short-data path is unchanged.
• Why it's cheaper: one calldata slice and one bounds check replace two of each, on the decode executed by every signed user operation.
• Tradeoff: roughly 870 gas more at deployment and one shift-based byte offset in place of two explicit slice ranges, an idiom this file already uses for the gas-limits split. Cherry-picks independently.
• Found by: Grok 4.5, Composer, and GPT-5.6 Sol (whose isolated-function probe measured it flat; the whole-transaction re-measurement on this branch shows −199 per signed operation).

GAS-L-01 · unchecked refund subtraction under the EntryPoint invariant (PaymasterERC20._refund)

• What it does: wraps prefundAmount - actualAmount_ in unchecked.
• Why it's cheaper: the EntryPoint guarantees actualGasCost <= maxCost, so actualAmount_ <= prefundAmount holds and the compiler's underflow check is unreachable; removing it saves 72 gas on every postOp refund.
• Tradeoff: the no-underflow guarantee moves from local code to the EntryPoint invariant; a non-compliant caller that breaks it now produces a wrapped amount that fails inside trySafeTransfer (reverting with PaymasterERC20FailedRefund) instead of a Panic. Cherry-picks independently.
• Found by: Fable 5.

GAS-L-02 · branchless validation data via SafeCast.toUint (PaymasterERC721Owner._validatePaymasterUserOp)

• What it does: returns SafeCast.toUint(token().balanceOf(userOp.sender) == 0) instead of a ternary selecting between SIG_VALIDATION_FAILED and SIG_VALIDATION_SUCCESS.
• Why it's cheaper: the two constants are exactly 1 and 0, so the conditional jump becomes a bool-to-uint cast, saving 12 gas per operation and 3.4k deployment gas.
• Tradeoff: the named constants leave the return expression; the failure-to-1 mapping is documented by an inline comment and relies on the constants' numeric values. Cherry-picks independently.
• Found by: Fable 5.

🤖 Generated with Claude Code

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