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Simple Summary

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A bug was reported https://github.com/Synthetixio/synthetix/issues/243\nwhich the position of emit would reduce the gas estimation of trading SNX via uniswap down from 900K to 100K

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Motivation

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Solidity passes 63/64 of gas in subcalls and it is burning all this on a failed subcall.

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Specification

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Move the emitTransfer to before the callTokenFallbackIfNeeded.

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In ExternStateToken.sol:\n// Emit a standard ERC20 transfer event\nemitTransfer(from, to, value);

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// If the recipient is a contract, we need to call tokenFallback on it so they can do ERC223\n// actions when receiving our tokens. Unlike the standard, however, we don't revert if the\n// recipient contract doesn't implement tokenFallback.\ncallTokenFallbackIfNeeded(from, to, value, data);

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Limit the gas to sub calls to either 200K or the amount of gas thats left if it is less than 200K

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In TokenFallbackCaller.sol:\nuint gasLimit = gasleft() < MAX_GAS_SUB_CALL ? gasleft() : MAX_GAS_SUB_CALL;\nrecipient.call.gas(gasLimit)(abi.encodeWithSignature(\"tokenFallback(address,uint256,bytes)\", sender, amount, data));

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Implementation

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https://github.com/Synthetixio/synthetix/blob/v2.12.2/contracts/ExternStateToken.sol#L134\nhttps://github.com/Synthetixio/synthetix/blob/v2.12.2/contracts/TokenFallbackCaller.sol#L52

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Copyright

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Copyright and related rights waived via CC0.

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