[U-Boot] packed attribute problem

[Wolfgang Denk]

Dear Scott Wood,

In message <20101007135257.05a93975 at udp111988uds.am.freescale.net> you wrote:
>
> > It is a pretty common method to use a pointer to some struct (for
> > example, some form of PDU) and make it point to some I/O buffer.
> 
> Yes, but at that point we are not talking about well-defined C, but
> rather implementation-specific behavior.  There's nothing wrong with
> it, but the C standard is no longer authoritative on what happens in
> such cases.

Huch?  Which part of that is not well-defined (or even not
standard-conforming) C?

> Yes.  And there would also be performance complaints if each of those
> accesses were to trap and be emulated (even ignoring weird stuff like
> old ARM).  Thus it's nice to have some sort of pointer or data type
> annotation to tell the compiler to be careful.

I also complain about poor performance when instead of a single
instruction (a 32 bit load) at least 4 (8 bit) instructions need to be
executed.

> BTW, I see GCC splitting accesses to bitfields in a packed
> struct into bytes on powerpc, even with -mno-strict-align.

Indeed. Bitfields have always been evil.

Best regards,

Wolfgang Denk

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