Re: [PATCH] Make -Wint-in-bool-context warn on suspicious shift ops

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 09/29/16 20:52, Bernd Edlinger wrote:
> On 09/29/16 20:03, Jason Merrill wrote:
>>
>> What do you think about dropping the TYPE_UNSIGNED exception as well?
>> I don't see what difference that makes.
>>
>
>
> If I drop that exception, then I could also drop the check for
> INTEGER_TYPE and the whole if, because I think other types can not
> happen, but if they are allowed they are as well bogus here.
>
> I can try a bootstrap and see if there are false positives.
>
> But I can do that as well in a follow-up patch, this should probably
> be done step by step, especially when it may trigger some false
> positives.
>
> I think I could also add more stuff, like unary + or - ?
> or maybe also binary +, -, * and / ?
>
> We already discussed making this a multi-level option,
> and maybe enabling the higher level explicitly in the
> boot-strap.
>
> As long as the warning continues to find more bugs than false
> positives, it is probably worth extending it to more cases.
>
> However unsigned integer shift are not undefined if they overflow.
>
> It is possible that this warning will then trigger also on valid
> code that does loop termination with unsigned int left shifting.
> I dont have a real example, but maybe  like this hypothetical C-code:
>
>   unsigned int x=1, bits=0;
>   while (x << bits) bits++;
>   printf("bits=%d\n", bits);
>
>
> Is it OK for everybody to warn for this on -Wall, or maybe only
> when -Wextra or for instance -Wint-in-bool-context=2 is used ?
>
>

Unfortunately, without that exception there is a false positive:

In file included from ../../gcc-trunk/gcc/ada/gcc-interface/decl.c:30:0:
../../gcc-trunk/gcc/ada/gcc-interface/decl.c: In function 'int 
adjust_packed(tree, tree, int)':
../../gcc-trunk/gcc/tree.h:1874:22: error: << on signed integer in 
boolean context [-Werror=int-in-bool-context]
       ? ((unsigned)1) << ((NODE)->type_common.align - 1) : 0)
         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../gcc-trunk/gcc/ada/gcc-interface/decl.c:6928:7: note: in expansion 
of macro 'TYPE_ALIGN'
    if (TYPE_ALIGN (record_type)
        ^~~~~~~~~~


But that did not happen with this version:

Index: c-common.c
===================================================================
--- c-common.c	(revision 240571)
+++ c-common.c	(working copy)
@@ -4655,6 +4655,14 @@ c_common_truthvalue_conversion (location_t locatio
  	return c_common_truthvalue_conversion (location,
  					       TREE_OPERAND (expr, 0));

+    case LSHIFT_EXPR:
+      /* Warn on signed integer left shift.  */
+      if (TREE_CODE (TREE_TYPE (expr)) == INTEGER_TYPE
+	  && !TYPE_UNSIGNED (TREE_TYPE (expr)))
+	warning_at (EXPR_LOCATION (expr), OPT_Wint_in_bool_context,
+		    "<< on signed integer in boolean context");
+      break;
+
      case COND_EXPR:
        if (warn_int_in_bool_context
  	  && !from_macro_definition_at (EXPR_LOCATION (expr)))


Is that version OK for you?


Bernd.