[Bug rtl-optimization/52186] New: array out of bounds error when accessing last byte of a struct via char ptr

["alex at aivor dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52186

             Bug #: 52186
           Summary: array out of bounds error when accessing last byte of
                    a struct via char ptr
    Classification: Unclassified
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: alex@aivor.com


Created attachment 26624
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26624
source code

In some cases, casting a struct to an unsigned char and indexing the last byte
causes an array out of bounds error.  To illustrate:

 struct foobar foobar;
 unsigned char *k = (unsigned char*)foobar;
 if ( k[sizeof(struct foobar)-1] ) /* out of bounds? */
   return;

The above snippet is only to describe the problem.  See the attached file
"test.c" for an real example.  When test.c is compiled with -O2 it gives the
following error:

 test.c:21:11: error: array subscript is above array bounds

Small changes to the code such as changing a struct member type from 'unsigned
char' to 'char' avoid the error.  Other changes that avoid the error are
described in comments in test.c.

The error is flagged with and without '-fno-strict-aliasing'.

The file test.c is as simple as I could make it -- no external includes, etc.

Attached files:

 test.c -- source code

 test.* -- files created by '-save-temps'

 log -- stdout and stderr of the compile command

Compile command:

 gcc -v -save-temps -c test.c -Wall -Werror  -O2 -fno-strict-aliasing