Harald van Dijk <truedfx@xxxxxxxxxx> writes:
> int main() {
> int a = 1;
> short *b = (short *) (char *) &a;
> *b = 2;
> return a;
> }
>
> This simple program returns 1 when compiled with -O2
> -fstrict-aliasing, and 2 when compiled with -O2
> -fno-strict-aliasing. It does not produce any warnings with
> -Wstrict-aliasing=2. According to the documentation:
>
> `-Wstrict-aliasing=2'
> This option is only active when `-fstrict-aliasing' is active. It
> warns about all code which might break the strict aliasing rules
> that the compiler is using for optimization. This warning catches
> all cases, but it will also give a warning for some ambiguous
> cases that are safe.
>
> Is this a bug? If so, what is the bug? Is the warning missing, or is
> the optimization invalid? And are there any alternative options that
> really catch every aliasing problem?
There should be a warning here. The warning code is confused by the
double cast. If you cast directly to short *, you should get the
warning.
I'm going to test this patch to fix this problem. Anybody see
anything wrong with it?
Ian
2005-07-26 Ian Lance Taylor <ian@xxxxxxxx>
* c-typeck.c (build_c_cast): For warn_strict_aliasing warnings,
use STRIP_NOPS on the operand.
Index: c-typeck.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-typeck.c,v
retrieving revision 1.467
diff -p -c -r1.467 c-typeck.c
*** c-typeck.c 20 Jul 2005 01:18:06 -0000 1.467
--- c-typeck.c 27 Jul 2005 02:36:51 -0000
*************** build_c_cast (tree type, tree expr)
*** 3306,3334 ****
if (flag_strict_aliasing && warn_strict_aliasing
&& TREE_CODE (type) == POINTER_TYPE
! && TREE_CODE (otype) == POINTER_TYPE
! && TREE_CODE (expr) == ADDR_EXPR
! && (DECL_P (TREE_OPERAND (expr, 0))
! || TREE_CODE (TREE_OPERAND (expr, 0)) == COMPONENT_REF)
! && !VOID_TYPE_P (TREE_TYPE (type)))
! {
! /* Casting the address of an object to non void pointer. Warn
! if the cast breaks type based aliasing. */
! if (!COMPLETE_TYPE_P (TREE_TYPE (type)))
! warning (OPT_Wstrict_aliasing, "type-punning to incomplete type "
! "might break strict-aliasing rules");
! else
! {
! HOST_WIDE_INT set1 = get_alias_set (TREE_TYPE (TREE_OPERAND (expr, 0)));
! HOST_WIDE_INT set2 = get_alias_set (TREE_TYPE (type));
! if (!alias_sets_conflict_p (set1, set2))
! warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
! "pointer will break strict-aliasing rules");
! else if (warn_strict_aliasing > 1
! && !alias_sets_might_conflict_p (set1, set2))
! warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
! "pointer might break strict-aliasing rules");
}
}
--- 3306,3341 ----
if (flag_strict_aliasing && warn_strict_aliasing
&& TREE_CODE (type) == POINTER_TYPE
! && TREE_CODE (otype) == POINTER_TYPE)
! {
! tree e = expr;
! STRIP_NOPS (e);
! if (TREE_CODE (e) == ADDR_EXPR
! && (DECL_P (TREE_OPERAND (e, 0))
! || TREE_CODE (TREE_OPERAND (e, 0)) == COMPONENT_REF)
! && !VOID_TYPE_P (TREE_TYPE (type)))
! {
! /* Casting the address of an object to non void pointer.
! Warn if the cast breaks type based aliasing. */
! if (!COMPLETE_TYPE_P (TREE_TYPE (type)))
! warning (OPT_Wstrict_aliasing,
! "type-punning to incomplete type "
! "might break strict-aliasing rules");
! else
! {
! HOST_WIDE_INT set1, set2;
!
! set1 = get_alias_set (TREE_TYPE (TREE_OPERAND (e, 0)));
! set2 = get_alias_set (TREE_TYPE (type));
! if (!alias_sets_conflict_p (set1, set2))
! warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
! "pointer will break strict-aliasing rules");
! else if (warn_strict_aliasing > 1
! && !alias_sets_might_conflict_p (set1, set2))
! warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
! "pointer might break strict-aliasing rules");
! }
}
}
