[PATCH] enhance -Warray-bounds to detect out-of-bounds offsets (PR 82455)

[Martin Sebor]

In my work on -Wrestrict, to issue meaningful warnings, I found
it important to detect both out of bounds array indices as well
as offsets in calls to restrict-qualified functions like strcpy.
GCC already detects some of these cases but my tests for
the enhanced warning exposed a few gaps.

The attached patch enhances -Warray-bounds to detect more instances
out-of-bounds indices and offsets to member arrays and non-array
members.  For example, it detects the out-of-bounds offset in the
call to strcpy below.

The patch is meant to be applied on top posted here but not yet
committed:
   https://gcc.gnu.org/ml/gcc-patches/2017-10/msg01304.html

Richard, since this also touches tree-vrp.c I look for your comments.

Jeff, this is the enhancement you were interested in when we spoke
last week.

Thanks
Martin

$ cat a.c && gcc -O2 -S -Wall a.c
   struct A { char a[4]; void (*pf)(void); };

   void f (struct A *p)
   {
     p->a[5] = 'x';            // existing -Warray-bounds

     strcpy (p->a + 6, "y");   // enhanced -Warray-bounds
   }

   a.c: In function ‘f’:
   a.c:7:3: warning: offset 6 is out of bounds of ‘char[4]’ [-Warray-bounds]
    strcpy (p->a + 6, "y");
    ^~~~~~~~~~~~~~~~~~~~~~
   a.c:1:17: note: member declared here
    struct A { char a[4]; void (*pf)(void); };
                    ^
   a.c:5:7: warning: array subscript 5 is above array bounds of 
‘char[4]’ [-Warray-bounds]
      p->a[5] = 'x';
      ~~~~^~~
-------------- next part --------------
A non-text attachment was scrubbed...
Name: gcc-82455.diff
Type: text/x-patch
Size: 28853 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20171029/d1d61336/attachment.bin>