[U-Boot] __attribute__((__packed__)) switching to byte-access on ARM
Guennadi Liakhovetski
lg at denx.de
Mon Aug 11 19:00:04 CEST 2008
On Mon, 11 Aug 2008, Kenneth Johansson wrote:
>
> On Mon, 2008-08-11 at 18:23 +0200, Guennadi Liakhovetski wrote:
>
> > I cannot think of any such examples... I know access-types can be changed
> > on the hardware, say, on a 16-bit bus a 32-bit access can be split into
> > two 16-bit accesses, but I currently cannot think of any examples of the
> > compiler doing such things. But, I think, I can imagine _why_ the compiler
> > might decide to do that - it cannot guarantee, that the structure is
> > 32-bit aligned, so, it is opting for the "safe" variant... So, the
> > question is: is the compiler right and the c-code was wrong, or the other
> > way round?
>
> Both are "right" :). C has a lot of implementation specific stuff that
> can make life interesting. Simple thing like if char is signed or
> unsigned sometimes is really important.
>
> If you have to make sure that the read/write is done in 16 bit the only
> option is to use assembler code.
I think, properly written mmapped-register access functions like readl()
work perfectly too.
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-0 Fax: +49-8142-66989-80 Email: office at denx.de
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