Contract creation gas cost depends on code in library function?

Question

I have a contract fooContract which is using a fooLib library, calling fooLib.foo(LibStruct storage param)

The transaction cost to create fooContract seems to depend on the code in the fooLib.foo() function despite the params are unchanged.

Ie. using solidity browser fooContract create costs 3,185,059. if I comment out the code in fooLib.foo(), the fooContract create cost goes down to 2,816,000 .

As far as I understood libs are a way to move out functionality for contracts too large for block gas limit.

Am I missing a point here?

Answer 1

I suspect not using a library the way it's intended to achieve gas costs in your contract. I assume what you're doing is something like:

library fooLib {
    // foo code
}

contract fooContract is fooLib {
    // foo code
}

In which case when you create fooContract you have to pay the compilation costs for fooLib. What you want to do is deploy fooLib on it's own, specify how to interact with the library via an interface, and specify the library address to link to with the solc compiler during compilation of fooContract. By doing this you would avoid having to pay the compilation cost of fooLib when deploying fooContract and instead just pay the transaction fees when utilizing the library.

The following stackexchange post has a solid detailed description of this process:

• What are the steps to compile and deploy a library in Solidity?

A very good aragon article on this:

• https://blog.aragon.one/library-driven-development-in-solidity-2bebcaf88736

Answer 2

The internal functions in libraries are pulled into the calling contract at compile time and a regular JUMP call is used instead of a DELEGATECALL. From Solidity docs: http://solidity.readthedocs.io/en/v0.4.21/contracts.html#libraries