dynamic gas cost instructions

[magik6k]

On top of #10

Rewrite of #4

This PR makes it possible to define much more advanced gas cost rules

/// An interface that describes instruction costs.
pub trait Rules {
    /// Returns the cost for the passed `instruction`.
    ///
    /// Returning an error can be used as a way to indicate that an instruction
    /// is forbidden
    fn instruction_cost(&self, instruction: &Operator) -> Result<InstructionCost>;
}

/// Dynamic costs instructions.
#[derive(Debug, PartialEq, Eq, Copy, Clone)]
pub enum InstructionCost {
    /// Charge fixed amount per instruction.
    Fixed(u64),

    /// Charge the specified amount of base miligas, plus miligas based on the
    /// last item on the stack. For memory.grow this is the number of pages.
    /// For memory.copy this is the number of bytes to copy.
    Linear(u64, NonZeroU32),
}

[Stebalien]

What if we just treat them as unsigned? I.e., just charge a lot of gas?

[Stebalien]

(if it's an i32, that is).

[magik6k]

That can get all kinds of messy with overflows, but I can dabble with that idea for a bit

[magik6k]

So apparently I64ExtendI32U already extends with all zero bits (so -1 becomes 0x00000000_ffffffff), so we can just do that and be fine.

You can see that with this demo: https://developer.mozilla.org/en-US/docs/WebAssembly/Reference/Control_flow/call

Put this as js:

var url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(
  fetch(url),
  {
    env: {
      greet: function(v) {
        console.log(v);
      }
    }
  }
);

And this as wat

(module
  (import "env" "greet" (func $greet (param i64)))

  (func
    i32.const -1
    i64.extend_i32_u
    call $greet
  )

  (start 1)
)

[Stebalien]

Yeah, there's a separate instruction for sign extending (I64ExtendI32I).

[Stebalien]

Actually, are you sure this is signed? Wasm usually doesn't distinguish between the two.

[Stebalien]

Ah, I guess 32 bits plus 31 bits because our gas is technically signed (although I'd like to remove that...). But yeah, this is reasonable.

[magik6k]

(rebased on master)