More on type sizes

[Richard Kenner]

    When a size is caluclated in bytes, we want to use TYPE_SIZE so that we
    get the expected overlow effects.
    When a size is calculated in bits, we don't want them.

Yes, I understand that, so let me rephrase my question: when do we want the
size of a type in bits?

    Note that not only the size, but also the offset of a bitfield has to
    be expressed in single bits - unless we want to use a representation
    as sum of a multiple of BITS_PER_UNIT plus a single-bit count that is
    smaller than BITS_PER_UNIT.

Well, we always used to do that, but the problem is that I believe this
calculation is now being done in "mixed mode": some in sizetype and some
in bitsizetype.  But the definition of DECL_FIELD_BITPOS is in
bitsizetype, so the calculation, on a 32-bit machine, will be done in 64 bits
if the value is a variable.

So I think we either have to always view it as a PLUS_EXPR of MULT_EXPR
of a CONVERT_EXPR of a sizetype vaule and a constant in bitsizetype (which I
think is a mess) or have two fields: DECL_POSITION, which is the position
in bytes (in sizetype) and a DECL_FIELD_BITPOS, which is a bitsizetype value
(currently always a constant less than BITS_PER_UNIT) and gets added to
DECL_POSITION after the appropriate multiplication and conversion.  I think
the latter is the best approach.  What do others think?