why use up all the gasLimit when use assembly and revert together?

Question

try to call erc20 token's transferFrom() using assembly(to save some gas), code like this:

pragma solidity ^0.4.24;

contract TestAssemblyAndRevert {
    function test(address from, address to, uint256 value) public {
        // a standard erc20 token
        address token = 0xedc2d4aca4f9b6a23904fbb0e513ea0668737643;

        // call transferFrom() of token using assembly
        assembly { // LineA
            // keccak256('transferFrom(address,address,uint256)') & 0xFFFFFFFF00000000000000000000000000000000000000000000000000000000
            mstore(0, 0x23b872dd00000000000000000000000000000000000000000000000000000000)

            // calldatacopy(t, f, s) copy s bytes from calldata at position f to mem at position t
            // copy from, to, value from calldata to memory
            calldatacopy(4, 4, 96)

            // call ERC20 Token contract transferFrom function
            let result := call(gas, token, 0, 0, 100, 0, 32)

            if eq(result, 1) {
                return(0, 0)
            }

            //revert(0, 0); // LineB
        }

        revert("TOKEN_TRANSFER_FROM_ERROR"); // LineC
    }
}

the token is a standard ERC20 token that when some spender trying to call transferFrom() without enough allowance, it will revert, in our case, this line:

let result := call(gas, token, 0, 0, 100, 0, 32)

result will be 0.

The thing surprise me is that when this happens, the transaction will use up all the gasLimit. why is that?

I tried several other cases, none of them will use up the gas:

1. if I comment out the whole assembly code block, or
2. if I keep the assembly code block, but comment out: revert("TOKEN_TRANSFER_FROM_ERROR"), LineC
3. if I comment out LineC, keep the assembly block and uncomment LineB

Answer 1

Turned out that its caused by messing up the memory, if we use the free memory pointer: let ptr := mload(0x40), the burn-all-gas problem will disappear.

pragma solidity ^0.4.24;

contract TestAssemblyAndRevert {
    function test(address from, address to, uint256 value) public {
        // a standard erc20 token
        address token = 0xedc2d4aca4f9b6a23904fbb0e513ea0668737643;

        // call transferFrom() of token using assembly
        assembly {
            let ptr := mload(0x40)

            // keccak256('transferFrom(address,address,uint256)') & 0xFFFFFFFF00000000000000000000000000000000000000000000000000000000
            mstore(ptr, 0x23b872dd00000000000000000000000000000000000000000000000000000000)

            // calldatacopy(t, f, s) copy s bytes from calldata at position f to mem at position t
            // copy from, to, value from calldata to memory
            calldatacopy(add(ptr, 4), 4, 96)

            // call ERC20 Token contract transferFrom function
            let result := call(gas, token, 0, ptr, 100, ptr, 32)

            if eq(result, 1) {
                return(0, 0)
            }
        }

        revert("TOKEN_TRANSFER_FROM_ERROR");
    }
}