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

[David Majnemer]

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.
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