Function signextend

pub fn signextend<H>(interpreter: &mut Interpreter, _host: &mut H)
where
    H: Host + ?Sized,

Implements the SIGNEXTEND opcode as defined in the Ethereum Yellow Paper.

In the yellow paper SIGNEXTEND is defined to take two inputs, we will call them x and y, and produce one output. The first t bits of the output (numbering from the left, starting from 0) are equal to the t-th bit of y, where t is equal to 256 - 8(x + 1). The remaining bits of the output are equal to the corresponding bits of y. Note: if x >= 32 then the output is equal to y since t <= 0. To efficiently implement this algorithm in the case x < 32 we do the following. Let b be equal to the t-th bit of y and let s = 255 - t = 8x + 7 (this is effectively the same index as t, but numbering the bits from the right instead of the left). We can create a bit mask which is all zeros up to and including the t-th bit, and all ones afterwards by computing the quantity 2^s - 1. We can use this mask to compute the output depending on the value of b. If b == 1 then the yellow paper says the output should be all ones up to and including the t-th bit, followed by the remaining bits of y; this is equal to y | !mask where | is the bitwise OR and ! is bitwise negation. Similarly, if b == 0 then the yellow paper says the output should start with all zeros, then end with bits from b; this is equal to y & mask where & is bitwise AND.