r223852 - AST: Don't assume two zero sized objects live at different addresses

[Richard Smith]

On Thu, Dec 11, 2014 at 1:02 PM, David Majnemer <david.majnemer at gmail.com>
wrote:

> On Thu, Dec 11, 2014 at 12:45 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Thu, Dec 11, 2014 at 11:47 AM, David Majnemer <
>> david.majnemer at gmail.com> wrote:
>>
>>> On Thu, Dec 11, 2014 at 11:28 AM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>>> On Tue, Dec 9, 2014 at 3:32 PM, David Majnemer <
>>>> david.majnemer at gmail.com> wrote:
>>>>
>>>>> Author: majnemer
>>>>> Date: Tue Dec  9 17:32:34 2014
>>>>> New Revision: 223852
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=223852&view=rev
>>>>> Log:
>>>>> AST: Don't assume two zero sized objects live at different addresses
>>>>>
>>>>> Zero sized objects may overlap with each other or any other object.
>>>>>
>>>>> This fixes PR21786.
>>>>>
>>>>> Modified:
>>>>>     cfe/trunk/lib/AST/ExprConstant.cpp
>>>>>     cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp
>>>>>
>>>>> Modified: cfe/trunk/lib/AST/ExprConstant.cpp
>>>>> URL:
>>>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/ExprConstant.cpp?rev=223852&r1=223851&r2=223852&view=diff
>>>>>
>>>>> ==============================================================================
>>>>> --- cfe/trunk/lib/AST/ExprConstant.cpp (original)
>>>>> +++ cfe/trunk/lib/AST/ExprConstant.cpp Tue Dec  9 17:32:34 2014
>>>>> @@ -1422,6 +1422,12 @@ static bool IsWeakLValue(const LValue &V
>>>>>    return Decl && Decl->isWeak();
>>>>>  }
>>>>>
>>>>> +static bool isZeroSized(const LValue &Value) {
>>>>> +  const ValueDecl *Decl = GetLValueBaseDecl(Value);
>>>>> +  return Decl && isa<VarDecl>(Decl) &&
>>>>> +         Decl->getASTContext().getTypeSize(Decl->getType()) == 0;
>>>>> +}
>>>>> +
>>>>>  static bool EvalPointerValueAsBool(const APValue &Value, bool
>>>>> &Result) {
>>>>>    // A null base expression indicates a null pointer.  These are
>>>>> always
>>>>>    // evaluatable, and they are false unless the offset is zero.
>>>>> @@ -6979,6 +6985,10 @@ bool IntExprEvaluator::VisitBinaryOperat
>>>>>              (RHSValue.Base && RHSValue.Offset.isZero() &&
>>>>>               isOnePastTheEndOfCompleteObject(Info.Ctx, LHSValue)))
>>>>>            return Error(E);
>>>>> +        // We can't tell whether an object is at the same address as
>>>>> another
>>>>> +        // zero sized object.
>>>>> +        if (isZeroSized(LHSValue) || isZeroSized(RHSValue))
>>>>> +          return Error(E);
>>>>>
>>>>
>>>> We can do better here: one of the pointers must be to a zero-sized
>>>> object, and the other must be a past-the-end pointer (where a pointer to a
>>>> zero-sized object is considered to be a past-the-end pointer).
>>>>
>>>
>>> Ah, clever.
>>>
>>>
>>>>
>>>>          // Pointers with different bases cannot represent the same
>>>>> object.
>>>>>          // (Note that clang defaults to -fmerge-all-constants, which
>>>>> can
>>>>>          // lead to inconsistent results for comparisons involving the
>>>>> address
>>>>>
>>>>> Modified: cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp
>>>>> URL:
>>>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp?rev=223852&r1=223851&r2=223852&view=diff
>>>>>
>>>>> ==============================================================================
>>>>> --- cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp (original)
>>>>> +++ cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp Tue Dec  9
>>>>> 17:32:34 2014
>>>>> @@ -1955,3 +1955,9 @@ namespace EmptyClass {
>>>>>    constexpr E2 e2b(e2); // expected-error {{constant expression}}
>>>>> expected-note{{read of non-const}} expected-note {{in call}}
>>>>>    constexpr E3 e3b(e3);
>>>>>  }
>>>>> +
>>>>> +namespace PR21786 {
>>>>> +  extern void (*start[])();
>>>>> +  extern void (*end[])();
>>>>> +  static_assert(&start != &end, ""); // expected-error {{constant
>>>>> expression}}
>>>>> +}
>>>>>
>>>>
>>>> This testcase looks like valid C++ code to me; the comparison is a
>>>> constant expression under the C++ rules and evaluates to true. I don't
>>>> think we can apply this check in this case, only when we have a complete
>>>> type that is zero-sized. That means we'll constant-fold equality
>>>> comparisons to 'false' even if they turn out to be true, but that seems to
>>>> be unavoidable.
>>>>
>>>
>>> I don't quite understand why we should fold that comparison to false,
>>> GCC and ICC both consider that expression to be non-constant.
>>>
>>
>> That doesn't make them right. =) C++ does not have zero-sized types, nor
>> the possibility of the above objects being at the same address. Per its
>> constant evaluation rules, the above expression *is* a constant expression,
>> and we are required to treat it as such. In this regard, zero-sized types
>> are not a conforming extension.
>>
>
> They are both (potentially) one-past-the-end objects though.  I think our
> hands are tied, seeing as how we use the constant expression evaluation to
> CodeGen if conditions and what-not.
>

I don't think it's so clear. No valid C or C++ program can have an array of
zero bound, and I think we should generally prioritize doing the right
thing on conforming code over giving better semantics to a language
extension. I think the question is, does any real code rely on this not
being constant-folded for incomplete arrays that turn out to have a bound
of zero?

In any case, the incomplete-type case should be restricted to incomplete
arrays, since incomplete class types can never have zero size in C++.
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