[cfe-commits] [patch] Warn on strange null pointers

[David Blaikie]

+patch

On Tue, Aug 7, 2012 at 3:50 PM, David Blaikie <dblaikie at gmail.com> wrote:
> Hi Richard,
>
>> +      else if (!isa<IntegerLiteral>(FromSansParens) &&
>> +               !isa<OpaqueValueExpr>(FromSansParens) &&
>> !isUnevaluatedContext())
>>
>> Could you split out another NPCK_ value instead of these checks? I'm
>> concerned about the constructs which Expr::isNullPointerConstant skips past
>> but this check doesn't.
>
> Makes a lot of sense. I've implemented an attempt at this - separating
> out NPCK_ZeroInteger into NPCK_ZeroLiteral and NPCK_ZeroExpression.
>
>> It would be nice to add a comment referencing core issue 903 somewhere in
>> here.
>
> Mentioned core issue 903 in the definition of NPCK_ZeroExpression.
>
>> On Mon, Aug 6, 2012 at 2:52 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>> On Fri, Jul 27, 2012 at 4:04 PM, Sean Silva <silvas at purdue.edu> wrote:
>>> > +    "initialization of pointer of type %0 to null from a non-trivial
>>> > zero "
>>> > +    "expression">, InGroup<UnusualNullConversion>;
>>> >
>>> > is this using a specific technical meaning for "non-trivial"? If not,
>>> > it would probably be better to say "suspicious zero expression" or
>>> > "unusual zero expression".
>>>
>>> Rephrased to:
>>>
>>> "initialization of pointer of type <foo> with an unusual null pointer
>>> expression"
>>
>>
>> Perhaps:
>>
>> warning: expression which evaluates to zero treated as a null pointer
>> constant of type %0
>
> Reworded as suggested.
>
>>> > +  bool isUnevaluatedContext() const {
>>> > +    return ExprEvalContexts.back().Context == Sema::Unevaluated;
>>> > +  }
>>> >
>>> > Given the comment, maybe name it "isCurrentContextUnevaluated()"?
>>> > Otherwise, when I see this called I ask myself "what is an unevaluated
>>> > context?".
>>>
>>> You may ask this, but the C++ standard talks about "unevaluated
>>> contexts" and so using the same terminology is hopefully helpful to
>>> those dealing with these things between both standard and
>>> implementation. While I agree the function could possibly be
>>> rephrased, I hesitate to remove/reorder the words "unevaluated
>>> context" from it - perhaps other reviews will have some
>>> opinions/pointers here.
>>
>>
>> I agree, this name is appropriate, since this directly represents a term
>> from the C++ standard.
>>
>>>
>>> > More generally, there seem to be a lot of "bare" uses of
>>> > ExprEvalContexts; it might pay to refactor this to be encapsulated a
>>> > bit better so that the invariants can be asserted and documented (or
>>> > at least put a FIXME above ExprEvalContexts). For example, it's a bit
>>> > weird to see something like
>>> >
>>> >    ExprEvalContexts.back().Context =
>>> >        ExprEvalContexts[ExprEvalContexts.size()-2].Context;
>>> > -  if (ExprEvalContexts.back().Context == Unevaluated)
>>> > +  if (isUnevaluatedContext())
>>> >
>>> > the `isUnevaluatedContext()` call is "encapsulated", but the
>>> > `ExprEvalContexts.back().Context =
>>> > ExprEvalContexts[ExprEvalContexts.size()-2].Context;` is not, and so
>>> > the connection between the two gets lost. I'm not entirely sure what a
>>> > good name for this unencapsulated statement would be, but something
>>> > like:
>>> >
>>> > ExprEvalContexts.copyCurrentContextFromEnclosing();
>>> > if (ExprEvalContexts.isCurrentContextUnevaluated())
>>> >   return E;
>>> > return TransformToPE(*this).TransformExpr(E);
>>> >
>>> > would make it a lot clearer what is going on, plus give nice places to
>>> > document/assert invariants. It seems like most uses of
>>> > ExprEvalContexts are either calls to .back(), so just getting a method
>>> > named `.getCurrentContext()` would be a win. What do you think? Sorry
>>> > that this is kind of tangential from your patch :)
>>>
>>> Yeah, it is rather tangential - happy enough to do it at some point
>>> (would rather not include it in the same patch - but it could go
>>> before or after) though. Again, perhaps other reviewers will weigh in
>>> with some thoughts/preferences here.
>>
>>
>> I think, since this function is directly manipulating ExprEvalContexts
>> anyway, it would be better to leave it as-is for now. The current half-way
>> change makes the code less clear. With that change, the
>> isUnevaluatedContext() refactoring LGTM.
>
> Committed as r161355.
>
> Thanks,
> - David
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