ntdll: Implement missing 64 bit shifts

[Mike Gibson]

Alexandre Julliard [julliard at winehq.org] writes:
> Mike Gibson <mike.gibson at storagecraft.com> writes:
> 
> > +/******************************************************************************
> > + *        _allshr   (NTDLL.@)
> > + *
> > + * Right arithmetic shift a 64 bit integer
> > + *
> > + * PARAMS
> > + *  a     [I] Initial number.
> > + *  count [I] Number of bits to shift by.
> > + *
> > + * RETURNS
> > + *  The value of a following the shift.
> > + */
> > +LONGLONG WINAPI _allshr( LONGLONG a, INT count )
> > +{
> > +    return a >> count;
> > +}
> 
> There's no guarantee that this would be an arithmetic shift, it's
> compiler-dependent.

RtlLargeIntegerArithmeticShift doesn't handle this case either, but of course, that's not the point.  Once we determine a solution, I'll fix it up as well.  The following code would work regardless or right shift behavior.  Of course, it's a bit slower.

/* create mask to extend the sign bit */
LONGLONG mask;
mask = ( LONGLONG ) 1 << 63;
mask &= a;
mask >>= count;

/* do the shift */
a >>= count;

/* add the mask */
a |= mask;

return a;

The vast majority of platforms seem to do arithmetic right shift.  It would be better to have a #define that indicates what the right shifting behavior is.  Only on those platforms where right shift isn't arithmetic would the slower code be used.

AFAIK, this sort of thing is determined in the configure step.  I can write a test for it, but I have a suspicion someone may have already done this.  Anyone know?  If not, I would appreciate some pointers (or links) on how to go about adding such a test.

Mike Gibson