Re: [Qemu-devel] [PATCH 0/3] Add and use bit rotate functions

[Richard Henderson]

On 09/12/2013 12:13 PM, Stefan Weil wrote:
> The first patch was already sent to qemu-devel and is only included
> here because patch 3 is based on it. Only patch 1 is needed for qemu-stable.
> 
> It looks like shift values of 0 or 32/64 work as expected because
> the compiler "knows" the pattern used to implement the rotate operation,
> so the code does not need special handling of some shift values.

Thinking about this closer, the only two behaviors I know of for
x >> n, n >= w, w = width of x, are: shift modulo w, or zero.

Both cases work for this usage:

   x << 0 | x >> 32
   = x | (x >> 0)       modulo
   = x | x
   = x

   = x | 0              zero
   = x

AFAIK we never actually observe missile launch as a side effect
of an out of range shift.  And with that in mind, I think the
existing implementation of rotate is fine.


r~