[Bug tree-optimization/85754] New: missing -Wrestrict on memcpy with non-constant offsets less than size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85754

            Bug ID: 85754
           Summary: missing -Wrestrict on memcpy with non-constant offsets
                    less than size
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When memcpy is used to copy sequences at offsets within the same array
-Wrestrict uses the full range of each of the offsets even when the offsets are
trivially related and.  When the distance between the offsets is less than the
size of the copy the copy is guaranteed to overlap, yet -Wrestrict is not
issued.  The checker could work harder and try to detect at least some of these
cases.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-vrp=/dev/stdout z.c
void f (char *p, int i)
{
  __builtin_memcpy (p + i, p + i + 4, 32);   // missing -Wrestrict
}

...
Value ranges after VRP:

_1: ~[2147483648, 18446744071562067967]
_2: ~[2147483652, 18446744071562067971]
...
f (char * p, int i)
{
  sizetype _1;
  sizetype _2;
  char * _3;
  char * _4;

  <bb 2> [local count: 1073741825]:
  _1 = (sizetype) i_5(D);
  _2 = _1 + 4;
  _3 = p_6(D) + _2;
  _4 = p_6(D) + _1;
  __builtin_memcpy (_4, _3, 32);
  return;

}