RE: warning: right shift count >= width of type

["Dave Korn" <dk@artimi.com>]

> -----Original Message-----
> From: Peter Barada  
> Sent: 29 November 2004 18:09

> > > ISTM reasonable that the result of a right-shift by 32 bits could
> > > be assumed to be the same thing you get if you 
> right-shift by 1 bit
> > > 32 times....
> >
> >The chip designers don't agree.
> 
> They *definitely* don't agree.  Most shift hardware is designed as a
> barrel shifter which is a large combinational logic block 
> that takes the
> n-bits of the shift count and using them determine what each bit of
> the result is supposed to be.  This allows a shift that takes only one
> clock, wheter the shift is arithmetic or logical, right or left, and a
> shift count from 0 up to the number of bits in the register-1.
> 
> Any shift count outside of that range is considered "undefined".

  No, I think it's more accurate to say that any shift count outside that
range is *unimplemented*.  "Undefined" is a concept from the C language
standard; mathematically, the operation is well defined, and in a practical
physical implementation, the operation is either "implemented" or
"unimplemented", but either way can hardly fail to be "defined", owing to
the deterministic nature of hardware implementations.
 
> n 1-bit shifts is equivient to 1 n-bit shift only if n is less than
> the size of the register in bits, at least for a hardware
> implementation described above.

  Similarly, I would say that 1 n-bit shift simply cannot be expressed on
such hardware, and it is meaningless to compare the outcome of a series of
operations that exist with some entirely hypothetical outcome of an
operation that simply does *not* exist.


    cheers, 
      DaveK
-- 
Can't think of a witty .sigline today....