[m-dev.] for review: change semantics of integer shifts

[Fergus Henderson]

On 15-Mar-1999, Zoltan Somogyi <zs at cs.mu.OZ.AU> wrote:
> 
> > I know that in ANSI C the result of `>>' is implementation defined (not
> > undefined) if the left operand is negative.  But that is only because
> > ANSI C was supposed to work efficiently even on machines using one's
> > complement arithmetic.  All modern machines use two's complement, and
> > so, I believe, all C implementations on these machines define `>>' for
> > negative left operands in the same way.  Thus there is no good reason for
> > Mercury to make `>>' or even `unchecked_right_shift' have undefined
> > behaviour in the case where the left hand operand is negative.
> 
> Depending on all C implementations on two's complement to define shift
> by a negative amount the same way would not be a good idea. On some machines,
> the shift amount is handled as unsigned, so what you think is a negative
> number will be treated as a large positive number.

I'm talking about shift *of* a negative amount (i.e. *left* operand negative),
not shift *by* a negative amount (i.e. *right* operand negative).

I agree that we should not depend on C implementations doing anything
sensible if the right-hand operand is negative or greater than or equal to
the word size.

Cheers,
	Fergus.

-- 
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