> > I think that it is easy for back end to make good use of
> > TYPE_MIN_VALUE/TYPE_MAX_VALUE. Namely, consider the assignment
> >
> > x := y + z * w;
> >
> > where variables y, z and w have values in the interval [0,7] and
> > x have values in [0,1000]. Pascal converts the above to the
> > following C like code:
> >
> > int tmp = (int) y + (int) z * (int) w;
> > x = (tmp < 0 || tmp > 1000)? (Range_Check_Error (), 0) : tmp;
> >
> > I expect VRP to deduce that tmp will have values in [0..56] and
> > eliminate range check. Also, it should be clear that in the
> > assigment above artithmetic can be done using any convenient
> > precision.
> VRP can certainly do this -- I added the ability to see through
> more typecasts a while back. In general, I've tried to improve
> the ability of our optimizers to eliminate or at least "see through"
> some typecasts. However, that capability is often very limited
> and the ability to see through type casts is not pervasive in
> the optimization pipeline.
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?
Thanks,
Duncan.
