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Re: [PATCH] Fix PRs 52080, 52097 and 48124, rewrite bitfield expansion, enable the C++ memory model wrt bitfields everywhere
> But the bitfield group _does_ start on a byte boundary. At least
> that's what the new code in stor-layout ensures.
No, it doesn't, since it does the computation on a record by record basis.
Here we have a bitfield group that starts in a record and ends up in a nested
record.
> It's ok to assume the group starts on a byte boundary. But it's not
> ok to modify bits outside of the access, so the RMW cycle has to mask
> them properly.
We have 2 options:
1. the GCC 4.7 approach where get_bit_range returns 0 for bitstart, which
doesn't make much sense but is in keeping with store_bit_field.
2. the GCC 4.8 approach where get_bit_range attempts to return a more correct
value, but is currently wrong (bitregion_start=11, bitregion_end=18) because
the representative of the bitfield is wrong. The real representative of the
bitfield is outside the record type.
--
Eric Botcazou