How to write a constant time function to copy the most significant bit to all bits

Question

I\'d like to write a function, in C, which takes the MSB of uint8_t, and if it\'s set, returns 0xFF and if not 0x00. In short, which retur

Answer 1

Given the declaration of your uint8_t:

uint8_t x = // MSB is either 1 or 0 if `x` depending on the signed value of `x`

A cheat that assumes 2's complement for signed integers:

return (((uint16_t)(int8_t)x) >> 8) & 0xff;