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.
