x86-64 psABI says that only the least significant 8 bits are
significant if RAX has a return value of a function returning bool.
Likewise, it looks like only the least significant 1 or 2 bytes
are guaranteed to have a correct value for char and short return
values, respectively.

_Bool isn't just a 1-bit integer because when you convert a value
to bool, the result is 1 if the original value is non-zero. This
is contrary to the other small integral types, e.g. char, as you
can see below:

  char x  = 256; // x is 0
  _Bool y = 256; // y is 1