[cfe-commits] Patch: Add a warning for NULL used in arithmetic operations

Eli Friedman eli.friedman at gmail.com
Wed Jun 15 14:48:46 PDT 2011

On Wed, Jun 15, 2011 at 2:31 PM, Richard Trieu <rtrieu at google.com> wrote:
> On Tue, Jun 14, 2011 at 7:10 AM, Douglas Gregor <dgregor at apple.com> wrote:
>> On Jun 13, 2011, at 5:00 PM, Richard Trieu wrote:
>> On Thu, May 26, 2011 at 3:10 AM, Chandler Carruth <chandlerc at google.com>
>> wrote:
>>> On Mon, May 2, 2011 at 5:48 PM, Richard Trieu <rtrieu at google.com> wrote:
>>>> Add warning when NULL constant value is used improperly in expressions.
>>> Some high-level comments:
>>> 1) Why only handle GNUNull null-pointer-constants? I think there is a
>>> good reason here (making them behave as much like nullptr as possible) but
>>> it would be good to document that in comments at least.
>> The other kinds of nulls are zero integer and c++0x nullptr.
>>  Integral zero is valid for these operations.  nullptr is a different type
>> which already produces an error in these cases.
>>> 2) What drives the set of invalid operations? I assume its those that
>>> nullptr would be invalid for? If so, can you provide standard citations
>>> against the draft standard? Also, is there no way to just turn on the C++0x
>>> errors for nullptr when we see the GNUNull in C++03, but down-grade them to
>>> warnings?
>> These operations are the ones where the null pointer would be used as an
>> integer instead of a pointer. I've also copied the test, but using nullptr
>> instead to show that they error in same places.
>> It should be possible to change the NULL into a nullptr and then run the
>> checks, but that would probably involve touching code in all the of
>> operation checking functions.  I feel that it would be better to keep this
>> code in one place instead of spread across so many functions.
>>> 3) Because these are warnings, we can't return ExprError; that changes
>>> the parse result.
>> Removed the early exit with ExprError.
>> +  bool LeftNull = Expr::NPCK_GNUNull ==
>> +      lhs.get()->isNullPointerConstant(Context,
>> +
>> Expr::NPC_ValueDependentIsNotNull);
>> +  bool RightNull = Expr::NPCK_GNUNull ==
>> +      rhs.get()->isNullPointerConstant(Context,
>> +
>> Expr::NPC_ValueDependentIsNotNull);
>> +
>> +  // Detect when a NULL constant is used improperly in an expression.
>> +  if (LeftNull || RightNull) {
>> I think you want:
>> if (LeftNull != RightNull) {
> No, I did want (LeftNull || RightNull) since something like (NULL * NULL)
> should be warned on.  I will add the check so that using two NULLs will only
> produce one warning.  The inequality check is already present for comparison
> operators.
>> here? At the very least, we shouldn't emit two warnings when both sides of
>> the operator are NULL.
>> +      // These are the operations that would not make sense with a null
>> pointer
>> +      // if the other expression the other expression is not a pointer.
>> +      if ((LeftNull != RightNull) &&
>> !lhs.get()->getType()->isPointerType() &&
>> +          !rhs.get()->getType()->isPointerType()) {
>> You also need to check for member pointers, block pointers, and
>> Objective-C pointers here.
>> - Doug
> Two points here.
> 1) In Objective-C test, I see that NULL is defined as (void*)0 which will
> not cause it to be picked up by the NULL checks for GNU null.  There are
> already warnings for improper conversion to integer so having a special case
> for Objective-C is not required.

Not so for ObjC++.

> 2) Should member pointers and block pointers behave the same as other
> pointers for relational and quality operators with NULL and nullptr?

Equality, yes.  Relational operators with member and block pointers
aren't legal.

> For
> NULL, the relational operators give an invalid operand error.  For nullptr,
> relational operators for both pointers and equality operators for block
> pointers give an invalid operand error.  This is different from other
> pointers which will not complain.  Is this proper behavior for these type of
> pointers?

Filed http://llvm.org/bugs/show_bug.cgi?id=10145 for the block
pointer+nullptr thing.


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