On Wed, Oct 25, 2000 at 09:20:38AM -0700, Chris G. Demetriou wrote:
> chris@buzzbee.freeserve.co.uk (Chris Gilbert) writes:
> > What is this rotating thing?  I don't remember coming across it before.  Is 
> > it documented anywhere?
> 
> Take a look in the ARM architecture reference manual.  It's documented
> behaviour of non-naturally aligned loads and stores.
> 
> something like:
> 
> 	0: 0xaa
> 	1: 0xbb
> 	3: 0xcc
> 	3: 0xdd
> 
> lw from 0:	0xddccbbaa
> lw from 1:	0xaaddccbb
> etc.
> 
> similarly for lh:
> 
> lh from 0:	0xbbaa
> lh from 1:	0xaabb
> 
> (i think i got the endianness right).
> 
> for most users unexpected results, but architecturally specified ones.
> 
> This comes up every so often on the list.  (I suspect my standard
> response is why Jason Thorpe said "unexpected" results in his recent
> post about unaligned accesses in the in-tree ssh...  People often say
> broken, or similar, but, weenie that i am, i'm prone to point out that
> they're actually quite correct and documented ... just unexpected.  8-)

And, sometimes, it can even be a win.  If you have an algorithm that wants
to do rotates, and you can structure it so that it does them by way of
unaligned loads/stores -- they're "free", they take no extra cycles, since
you had to get the data in/out anyway...

Unfortunately, the compiler probably isn't smart enough to do that unless
you give it some very explicit help, or just write the code in assembler.

-- 
Thor Lancelot Simon	                                      tls@rek.tjls.com
	the effort to perceive simply the cruel radiance of what is