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Re: anti-ranges of signed variables

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 <> wrote:
On 11/11/2016 10:53 AM, Richard Biener wrote:

On November 11, 2016 6:34:37 PM GMT+01:00, Martin Sebor

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.


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