Re: anti-ranges of signed variables

[Martin Sebor <msebor at gmail dot com>]

On 11/11/2016 12:30 PM, Martin Sebor wrote:
> On 11/11/2016 12:12 PM, Andrew Pinski wrote:
> > On Fri, Nov 11, 2016 at 10:51 AM, Martin Sebor <msebor@gmail.com> wrote:
> > > On 11/11/2016 10:53 AM, Richard Biener wrote:
> > > > On November 11, 2016 6:34:37 PM GMT+01:00, Martin Sebor
> > > > <msebor@gmail.com>
> > > > wrote:
> > > > > I noticed that variables of signed integer types that are constrained
> > > > > to a specific subrange of values of the type like so:
> > > > >
> > > > >      [-TYPE_MAX + N, N]
> > > > >
> > > > > are reported by get_range_info as the anti-range
> > > > >
> > > > >      [-TYPE_MAX, TYPE_MIN - 1]
> > > > >
> > > > > for all positive N of the type regardless of the variable's actual
> > > > > range.  Basically, such variables are treated the same as variables
> > > > > of the same type that have no range info associated with them at all
> > > > > (such as function arguments or global variables).
> > > > >
> > > > > For example, while a signed char variable between -1 and 126 is
> > > > > represented by
> > > > >
> > > > >      VR_ANTI_RANGE [127, -2]
> > > >
> > > >
> > > > ? I'd expect [-1, 126].  And certainly never range-min > range-max
> > >
> > >
> > > Okay.  With this code:
> > >
> > >   void f (void *d, const void *s, signed char i)
> > >   {
> > >     if (i < -1 || 126 < i) i = -1;
> > >     __builtin_memcpy (d, s, i);
> > >   }
> > >
> > > I see the following in the output of -fdump-tree-vrp:
> > >
> > >   prephitmp_11: ~[127, 18446744073709551614]
> >
> >
> > prephitmp_11 is an unsigned type as the last argument to memcpy is
> > size_t which is an unsigned type. We had this discussion in bugzilla
> > about this same thing when we were talking about pointer plus.
>
> Right.  But that wasn't my question (the reported range above
> correctly reflects the variable's range in the code).
>
> My question is: why is the range for i in
>
>   if (i < -3 || 124 < i) i = -1;
>
> the same as in
>
>   if (i < -4 || 123 < i) i = -1;
>
> In both cases it's represented as
>
>   prephitmp_12: ~[128, 18446744073709551487]
>
> which is unrelated to i's actual range.  This seems to be true
> for all signed types in the range [-TYPE_MAX + N, N].

There are still very few clients of get_range_info in GCC so it
took me some time to figure out how to demonstrate this effect
in a standalone test case with unmodified GCC.  It finally dawned
on me that I could use Aldy's -Walloca-larger-than to reproduce
it.  I raised bug 78327 with the details.

Martin