http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50521
--- Comment #9 from Henrik Nordström <henrik at henriknordstrom dot net> 2011-10-28 07:32:48 UTC --- C standard does not define any of this It's all implementation and platform ABI dependent. The C standard does define not storage size of a bit-field other than that it's sufficiently large, or bit-fields of other types than _Bool and int (+qualifiers), or if bits outside the specific bit-field is accessed as side effect when operating on a bit-field. For example the ARM ABI defines volatile bitfield memory access in full detail as being equal to the base type of the bitfield, and I see now that it actually requires a double load in the mentioned test case. See http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf section 7.1.7.5.
