Optimize rpow(x,n,base)

[k06a]

1. Optimize rpow(x,n,base) for the case x != 0 && n == 0 (291 gas vs 383 gas)

   switch x case 0 {switch n case 0 {z := b} default {z := 0}}
   default {
      ...
   }

   =>

   switch n case 0 { z := b }
   default {
       switch x case 0 { z := 0 }
       default {
           ...
       }
   }

2. Simplify overflow check for squaring in rpow(x,n,base) (2355 gas vs 2388 gas for x = 2, n = 255)

   let xx := mul(x, x)
   if iszero(eq(div(xx, x), x)) { revert(0,0) }

   =>

   let xx := mul(x, x)
   if shr(128, x) { revert(0,0) }

[livnev]

@kmbarry1 can you get these optimisations to pass the spec? 😛

[kmbarry1]

I will give it a try today!

[kmbarry1]

Also, just to give "official" comment on this: looks good, but there are two things I want to check prior to merging:

1. gas for x != 0 && n != 0 is not worsened (I don't think it is, looks like it should be at least the same, maybe better, but want to check explicitly)
2. must successfully formally verify this

[gbalabasquer]

@kmbarry1 do we have any update on this regarding FV?

[kmbarry1]

Am getting through the backlog of changes that are definitely going in before analyzing this.