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

[David Majnemer]

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.
>

Actually, must the other be a past-the-end pointer?

#include <stdio.h>
extern "C" void (*__preinit_array_start[])();
void __attribute((section(".preinit_array"))) f() {}
int main() {
  printf("%p\n", &__preinit_array_start[0]);
  printf("%p\n", &f);
}

gives me:
0x600de0
0x600de0


>
>
>>
>>          // 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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