ABI/"Switch statement" for Contracts

[sezna]

The ASM preamble for contracts' function selector needs to be written. This will probably go somewhere near the build_preamble function here

The hashed function identifiers from #96 and the type of syntax tree would need to be passed in. This could either be in build_preamble itself, or a nearby function that is only called on contract-type ASTs.

[sezna]

cc @adlerjohn for confirmation -- the "input" to the selector comes from the first four bytes of in (addr, size)s in the call frame, so it can be accessed with:

addi $rA $fp i7;
lw $rB $rA i0;

The above follows my aforementioned assumption that the selector is in the first four bytes of the input data, which, if my counting is correct, is word 7.

(citing here)

[adlerjohn]

It depends on our convention. But using the first parameter exclusively as the selector is probably the most reasonable approach.

You can fetch it as 4 + 4 + 1*64 + 1 + 1 + 1 = 75 words offset from $fp, not 7, if my math is right. (Each row in the call frame table in the specs isn't one word.)

[sezna]

Ah, great, yeah I misinterpreted that as bits and thought they were word aligned. Perfect. So, then, let's decide that this can be our convention and move forward with it.

[sezna]

Should I put a revert opcode after the contract ABI switch statement? It seems correct to me, but I just want to double check.
And, if yes, what should the RVRT return?
cc @adlerjohn

[adlerjohn]

Ideally it should simply return with RET and let the caller handle the error, however there's no downside at this time of having a revert at the end of the switch if no cases are taken. You can just make it revert with $zero for now. Eventually we can standardize some error codes.