Re: usb: descriptor structures have to be packed

On Wednesday 07 February 2007 2:14 pm, Phil Endecott wrote:
> David Brownell wrote:
> > On Wednesday 07 February 2007 10:41 am, Phil Endecott wr
> >> void inc(int* i) {
> >>    (*i)++;
> >> }
> 
> >> here's a quote from Paul Brook (http://gcc.gnu.org/ml/gcc-help/2006-12/msg00115.html):
> >> 
> >>    " the compiler is allowed to assume that the low 2 bits of an int* 
> >> are zero. "
> >
> > Unless it's declared as packed ... a fact which is 100% clear in the
> > definition of "packed".
> 
> Dave, how would you propose to re-write my "inc" function (above) to 
> declare i as a "packed int*"?  I have tried various things like

Did you try

	typedef int unaligned_int __attribute__((aligned(1)));

and suchlike?

I was a bit loose above.  The constraint is really about alignment,
and what's 100% clear is that there _is_ such a constraint on those
pointers, because they're from a struct declared as "packed".  The
normal GCC rules ("natural alignment" of pointer target) do not apply.

If GCC doesn't allow that, or doesn't recognize that a pointer to
an unaligned int can't be used where a "normal" pointer is needed,
then the issue would appear to be that the best GCC can do is warn
when you take the address of such an unaligned field.


> 
> void inc(int __attribute__((packed)) * i)
> void inc(int * __attribute__((packed)) i)
> void inc(int * i __attribute__((packed)))
> void inc(int __attribute__((aligned(1))) * i)
> void inc(int * __attribute__((aligned(1))) i)
> void inc(int * i __attribute__((aligned(1))))
> 
> and I get variously
> 
> warning: ‘packed’ attribute ignored
> error: alignment may not be specified for ‘i’
> 
> or no message at all, and I only ever get loadword-add-storeword code 
> generated.  My feeling is that there is no way to declare a "packed" or 
> "unaligned" pointer to an int, and so Paul is right.

He's clearly wrong about the semantics GCC has defined; that's
not debatable.  The documentation/specification is unambiguous.

What he may be right about is the semantics GCC _implemented_ which,
of course, are a horse of a different rainbow.  I have no idea what
kind of attributes pointers carry around inside GCC.

- Dave


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