Re: -fstrict-aliasing, -Wstrict-aliasing=2, and indirect conversions

[Ian Lance Taylor <ian at airs dot com>]

Harald van DÄk <truedfx@gentoo.org> 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@airs.com>

	* 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");
! 		}
  	    }
  	}