[Bug c/42136] New: Inconsistent strict-aliasing warning with cast from char[]

[Bug c/42136] New: Inconsistent strict-aliasing warning with cast from char[]

[astrange at ithinksw dot com]

Source:
typedef union u { unsigned i; unsigned short s[2]; unsigned char c[4]; } u;

char c[4] __attribute__((aligned));
short s[2] __attribute__((aligned));

int f1()
{
        return ((union u*)s)->i;
}

int f2()
{
        return ((union u*)c)->i;
}

Using gcc 4.5:

> gcc -O3 -fstrict-aliasing -Wall -S wstrict_aliasing_char.c

wstrict_aliasing_char.c: In function 'f2':
wstrict_aliasing_char.c:13:17: warning: dereferencing type-punned pointer will
break strict-aliasing rules

I would expect either both or neither of the functions to warn, since pointer
casting to unions is given in the manual as something that violates
strict-aliasing, although gcc doesn't seem to actually take advantage of this.

Instead, it looks like the warning is hardcoded to apply to a cast from char
(c-common.c:1746 in r1554411):
          alias_set_type set1 =
            get_alias_set (TREE_TYPE (TREE_OPERAND (expr, 0)));
          alias_set_type set2 = get_alias_set (TREE_TYPE (type));

          if (set1 != set2 && set2 != 0
              && (set1 == 0 || !alias_sets_conflict_p (set1, set2)))
            {
              warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
                       "pointer will break strict-aliasing rules");
              return true;
            }

This came up during some x264 work, but it's taken care of now with some
__attribute__((may_alias)).


-- 
           Summary: Inconsistent strict-aliasing warning with cast from
                    char[]
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: astrange at ithinksw dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42136

[Bug c/42136] Inconsistent strict-aliasing warning with cast from char[]

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2009-11-21 22:56 -------
!alias_sets_conflict_p (set1, set2) should really be alias_set_subset_of
with all the added false positives from the very imprecise frontend code
that this would cause.

Thus, the cast from short[] also should warn.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |diagnostic
   Last reconfirmed|0000-00-00 00:00:00         |2009-11-21 22:56:31
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42136