[Bug c/63303] Pointer subtraction is broken when using -fsanitize=undefined

["mikulas at artax dot karlin.mff.cuni.cz" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63303

--- Comment #6 from mikulas at artax dot karlin.mff.cuni.cz ---
"you really can't have an object bigger than half of the address space in
C/C++" - where does the standard claim this? If this is true, we should change
malloc so that it doesn't allocate 2GiB or larger objects.


Regarding pointer difference, the C standard says this:

When two pointers are subtracted, both shall point to elements of the same
array object, or one past the last element of the array object; the result is
the difference of the subscripts of the two array elements. The size of the
result is implementation-defined, and its type (a signed integer type) is
ptrdiff_t defined in the <stddef.h> header. If the result is not representable
in an object of that type, the behavior is undefined. In other words, if the
expressions P and Q point to, respectively, the i-th and j-th elements of an
array object, the expression (P)-(Q) has the value i−j provided the value fits
in an object of type ptrdiff_t.

So: p points to the beginning, q points one past the last element, so the first
condition is valid.

The result is the difference of the subscripts of those two array elements:
0x50000000 - 0 = 0x50000000 - this is clearly representable in the type
ptrdiff_t, so 0x50000000 result should be returned.
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From: "krebbel at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug debug/63300] 'const volatile' sometimes stripped in debug info
Date: Fri, 19 Sep 2014 16:20:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc300

Andreas Krebbel <krebbel at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-09-19
                 CC|                            |mark at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Andreas Krebbel <krebbel at gcc dot gnu.org> ---
Reghunt indicates that this is caused by r214143.

For a const volatile type none of the following IFs is triggered:

  if ((cv_quals & TYPE_QUAL_CONST)
      /* If there are multiple type modifiers, prefer a path which
     leads to a qualified type.  */
      && (((cv_quals & ~TYPE_QUAL_CONST) == TYPE_UNQUALIFIED)
      || get_qualified_type (type, cv_quals) == NULL_TREE
      || (get_qualified_type (type, cv_quals & ~TYPE_QUAL_CONST)
          != NULL_TREE)))
    {
      mod_type_die = new_die (DW_TAG_const_type, mod_scope, type);
      sub_die = modified_type_die (type, cv_quals & ~TYPE_QUAL_CONST,
                   context_die);
    }
  else if ((cv_quals & TYPE_QUAL_VOLATILE)
       && (((cv_quals & ~TYPE_QUAL_VOLATILE) == TYPE_UNQUALIFIED)
           || get_qualified_type (type, cv_quals) == NULL_TREE
           || (get_qualified_type (type, cv_quals & ~TYPE_QUAL_VOLATILE)
           != NULL_TREE)))
    {
      mod_type_die = new_die (DW_TAG_volatile_type, mod_scope, type);
      sub_die = modified_type_die (type, cv_quals & ~TYPE_QUAL_VOLATILE,
                   context_die);
    }
  else if (cv_quals & TYPE_QUAL_RESTRICT)
    {
      mod_type_die = new_die (DW_TAG_restrict_type, mod_scope, type);
      sub_die = modified_type_die (type, cv_quals & ~TYPE_QUAL_RESTRICT,
                   context_die);
    }

So the DIE ends up with an unqualified type.