On 09/05/16 14:03, Marek Polacek wrote:
> On Fri, Sep 02, 2016 at 03:51:49PM +0000, Bernd Edlinger wrote:
>> Hi,
>>
>> > + r += !a == ~b;
>> > + r += !a == ~(int) b;
>>
>> I don't understand why ~b should not be warned at -Wall.
>
> Yeah, there was an RFE for this but I'm not finding it.
>
I certainly agree that the warning is compeletely misleading here:
test.c: In function 'f':
test.c:10:11: warning: logical not is only applied to the left hand side
of comparison [-Wlogical-not-parentheses]
r += !a == ~b;
^~
test.c:10:8: note: add parentheses around left hand side expression to
silence this warning
r += !a == ~b;
^~
( )
this will not fix it, but make it worse.
I think a better warning would be
warning: ~ on boolean value, did you mean ! ?
>> Frankly I don't even understand why the above statements are
>> completely optimized away.
>>
>> r += !a == ~b;
>> is optimized away, but
>>
>> b = ~b;
>> r += !a == b;
>>
>> Is not. Why?
>
> Something in ccp I suppose. But I didn't investigate closely because
> it's not really relevant to this patch, sorry.
>
Yes, I think the problem is that ~ on bool maps [0:1] to [-1:-2]
and thus the result is no longer boolean. But probaby the assignment
normalizes the result again: b = ~b; has the effect of setting b = 1.
A warning for "~ on boolean" would not be too difficult, but making a
fix-it will probably need more work:
Index: gcc/c/c-typeck.c
===================================================================
--- gcc/c/c-typeck.c (revision 239971)
+++ gcc/c/c-typeck.c (working copy)
@@ -4192,6 +4192,17 @@ build_unary_op (location_t location,
break;
case BIT_NOT_EXPR:
+ {
+ tree e = arg;
+
+ /* Warn if the condition has boolean value. */
+ while (TREE_CODE (e) == COMPOUND_EXPR)
+ e = TREE_OPERAND (e, 1);
+
+ if (TREE_CODE (TREE_TYPE (arg)) == BOOLEAN_TYPE
+ || truth_value_p (TREE_CODE (e)))
+ warning(0, "~ on boolean value, did you mean ! ?");
+ }
/* ~ works on integer types and non float vectors. */
if (typecode == INTEGER_TYPE
|| (typecode == VECTOR_TYPE
Index: gcc/cp/typeck.c
===================================================================
--- gcc/cp/typeck.c (revision 239971)
+++ gcc/cp/typeck.c (working copy)
@@ -5839,6 +5839,8 @@ cp_build_unary_op (enum tree_code code, tree xarg,
break;
case BIT_NOT_EXPR:
+ if (TREE_CODE (TREE_TYPE (arg)) == BOOLEAN_TYPE)
+ warning (0, "~ on boolean value, did you mean ! ?");
if (TREE_CODE (TREE_TYPE (arg)) == COMPLEX_TYPE)
{
code = CONJ_EXPR;
Bernd.
