This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Ada subtypes and base types


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



Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]