ARM unaligned MMIO access with attribute((packed))

[Joseph S. Myers]

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 at codesourcery.com