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