> OK, I see how the two are different now, but I still don't get why the > current version is right. If we say words are 16 bits for simplicity, > and we're storing VALUE == 0x0001_2345 into a 3-byte BLKmode bitfield, > surely we want to store 0x01_2345 rather than 0x00_0123?
Why? If you want this semantics, you need to use a bitfield with integer mode. In Ada, the following case is frequent: you have a record type with BLKmode and an alignment that causes it to padded (because the size is always a multiple of the alignment). You have a field in another record type whose type is the first record type but with a representation clause that prescribes the size without the padding. The Ada compiler must accept that. So the field is made a BLKmode bitfield, with DECL_SIZE (size w/o padding) smaller than TYPE_SIZE (size w/ padding). When you're assigning a value to the field, you drop the rightmost bytes in memory order, which are junk since padding. -- Eric Botcazou
