Calculate memory offsets in u64

[matklad]

EVM mload and mstore instructions seem relatively hot. Their arguments are u256, but this obviously can't work -- the memory needs to be limited somehow.

Reading the geth code, it seems the relevant bits are

• https://github.com/ethereum/go-ethereum/blob/b1e72f7ea998ad662166bcf23705ca59cf81e925/core/vm/interpreter.go#L200-L229
• https://github.com/ethereum/go-ethereum/blob/b1e72f7ea998ad662166bcf23705ca59cf81e925/core/vm/jump_table.go#L514

I think what that says is that:

• there's no intrinsic memory limit in EVM
• but there's a implicit memory limit due to gas cost -- extending memory costs gas, and you can't have enough gas to pay for 1<<64 bytes of memory
• so the actual offset calculation is carried out in u64

Switthing sputnik to use u64/usize for all offsets should speed it up somewhat significantly I think.

[matklad]

Ok, so the wins here are rather meaningful, about 6% of WASM gas.

I am in a bit of a doubt here though -- the correctness of the change would hinge on the fact that any memory offset larger than u32 is going to run out of gas, and, in sputnik, this is a non-local property, as gas counting and execution are very far apart from each other.

The design of oding seems to be more performant and correct here, as it simultanously executes the instruction and subtracts the gas cost, making this constraint local:

https://github.com/vorot93/evmodin/blob/e24f66db6292e4366caf57643b22238e8a2e2aa8/src/instructions/memory.rs#L66-L99

But refactoring sputnik to odin's design is probably a too big of a change...

[matklad]

PR implementing this: aurora-is-near/sputnikvm#9