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SubjectRe: ARM unaligned MMIO access with attribute((packed))
On Wed, 2 Feb 2011, Richard Guenther wrote:

> The pointer conversions already invoke undefined behavior as specified by the
> C standard (6.3.2.3/7).

I would say: the conversions are undefined if the pointer is
insufficiently aligned for any of the pointer types involved (source,
destination or intermediate), where the appropriate alignment for a packed
type is 1. Thus, the conversion from packed to non-packed is OK iff the
pointer target is sufficiently aligned for the non-packed type.

In general from a sequence of casts the compiler is permitted to deduce
that the pointer is sufficiently aligned for whatever type in the sequence
has the greatest alignment requirement (the middle-end may not have that
information at present, but the front end could insert some form of
alignment assertion if useful for optimization). *But* that is what is
permitted in standards terms; it is not necessarily safe in practice. In
particular, on non-strict-alignment targets such as x86 people do in
practice assume that unaligned accesses are OK at the C level, not just
the assembly level (glibc does so, for example), so it might be a bad idea
to assume alignment in a way that would cause that to break.

--
Joseph S. Myers
joseph@codesourcery.com


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