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). 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...
Jeff
