Compiler opton: target EVM version

[axic]

I think we should add an option to select the target EVM version (frontier, homestead, metropolis, etc.)

Maybe this isn't such a big issue anymore for Frontier as everyone moved on to Homestead, but by the time we move on to Metropolis, Homestead will have a much bigger user base.

Key differences:

• Homestead:
  • delegatecall
• Metropolis
  • new gas costs (Long-term gas cost changes for IO-heavy operations to mitigate transaction spam attacks EIPs#150)
  • new opcodes (e.g. EVM opcodes: bitwise shifting EIPs#145)
  • new behaviour (e.g. EIP 5: Gas costs for return values EIPs#8)

Alternatively we might put this into a pragma instead of a compiler option.

[axic]

Probably we would need both:

• the actual target version used in the compiler
• pragma for defining the requirement in a given source code (that covers what inline assembly instructions are valid for that file and what the user expects in terms of gas usage; if the pragma is lacking, it accepts any version defined by the compiler):

pragma evm homestead;
pragma evm metropolis;

Then, we have two options:

• reject compilation if different evm versions are requested
• use the highest common denominator (I see this as a no go, as it is hard to decide what changes are compatible)

[axic]

I think we should have the following components:

1. target EVM version as a compiler flag
2. target gas table (referred to by hardfork names) as a compiler flag
3. required EVM version as a pragma following semver

EVM versioning is simple if we follow ethereum/EIPs#178 as the current semver handler can be reused:

pragma evm ^1.1; // to select homestead
pragma evm ^2.0; // to select EIP150

[chriseth]

I would say the default should be to target the EVM that is active on the mainnet. Pragmas can revert specific features to older EVM versions.

[axic]

Latest discussion have lead to considering pragma target byzantium, but eventually it turned out that due pragma being SourceUnit-local that would cause a headache during inheritance for users.

It seems to be a much better option to have a compiler option instead.

This should be presented to the user as:

• --evm-version byzantium on the the commandline
• settings: { evmVersion: "byzantium" } in jsonio

The evm versions are lowercase and we define homestead and byzantium as the two options for now. We default to byzantium.

The evmVersion must also be present in the settings in the metadata.

[chriseth]

Added the "annoys users" label because this makes it impossible to use returndatacopy and others without warning.