* In C: a byte is the minimal addressable unit; an N-byte object is made
up of N "unsigned char" objects, with successive addresses in terms of
incrementing an "unsigned char *" pointer. A byte is at least 8 bits.
* In GCC, at the level of GNU C APIs on the target, which generally
includes built-in functions: a byte (on the target) is made of
CHAR_TYPE_SIZE bits. In theory this could be more than BITS_PER_UNIT, or
that could be more than 8, though support for either of those cases would
be very bit-rotten (and I'm not sure there ever have been targets with
CHAR_TYPE_SIZE > BITS_PER_UNIT). Sizes passed to memcpy and memcmp are
sizes in units of CHAR_TYPE_SIZE bits.
* In GCC, at the RTL level: a byte (on the target) is a QImode object,
which is made of BITS_PER_UNIT bits. (HImode is always two bytes, SImode
four, etc., if those modes exist.) Support for BITS_PER_UNIT being more
than 8 is very bit-rotten.
* In GCC, on the host: GCC only supports hosts (and $build) where bytes
are 8-bit (though writing it as CHAR_BIT makes it clear that this 8 means
the number of bits in a host byte).
Internal interfaces e.g. representing the contents of strings or other
memory on the target may not currently be well-defined except when
BITS_PER_UNIT is 8. Cf. e.g.
<https://gcc.gnu.org/ml/gcc/2003-06/msg01159.html>. But the above should
at least give guidance as to whether BITS_PER_UNIT, CHAR_TYPE_SIZE (or
TYPE_PRECISION (char_type_node), preferred where possible to minimize
usage of target macros) or CHAR_BIT is logically right in a particular
place.