On Tuesday 21 March 2006 17:15, Jeffrey A Law wrote:
> On Tue, 2006-03-21 at 10:14 +0100, Duncan Sands wrote:
>
> > Hi Jeff, on the subject of seeing through typecasts, I was playing around
> > with VRP and noticed that the following "if" statement is not eliminated:
> >
> > int u (unsigned char c) {
> > int i = c;
> >
> > if (i < 0 || i > 255)
> > return -1; /* never taken */
> > else
> > return 0;
> > }
> >
> > Is it supposed to be?
> Depends...
>
> The "problem" is the parameter is marked a VR_VARYING (as it
> should be).
Should it be? I was surprised to see that all ranges are initialised
to VR_VARYING in the vrp pass, since many types have natural ranges
associated with them, for example [0, 255] for the above unsigned char;
starting off with this natural range is, well, natural, and surely
simplifies a bunch of code, by making things more uniform, eg: no need
to special case unsigned values, type conversions, ...
> Unfortunately, the VR_VARYING source operand prevents
> us from extracting a range for the result of the typecast.
>
> Sigh. We've got this "interesting" problem in VRP, namely that
> we can sometimes extract ranges for operations on VR_VARYING
> objects, but generally we punt. To compensate we often create
> a range, say 0..MAXINT for an unsigned object -- that's effectively
> a VARYING range, but the rest of VRP doesn't pessimize so much
> when presented with 0..MAXINT vs VARYING.
>
> If we weren't so pessimistic with VR_VARYING or we always created
> ranges, even if they cover TYPE_MIN .. TYPE_MAX then this wouldn't
> be a problem.
>
> I'll see what I can cobble together...
That would be great.
All the best,
Duncan.
