On Thu, Dec 11, 2014 at 1:12 PM, Richard Smith <[email protected]> wrote:
> On Thu, Dec 11, 2014 at 1:02 PM, David Majnemer <[email protected]> > wrote: > >> On Thu, Dec 11, 2014 at 12:45 PM, Richard Smith <[email protected]> >> wrote: >> >>> On Thu, Dec 11, 2014 at 11:47 AM, David Majnemer < >>> [email protected]> wrote: >>> >>>> On Thu, Dec 11, 2014 at 11:28 AM, Richard Smith <[email protected]> >>>> wrote: >>>> >>>>> On Tue, Dec 9, 2014 at 3:32 PM, David Majnemer < >>>>> [email protected]> 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? > I'm not entirely sure how we can answer that but I found the following after a minute of digging around the linux kernel: kernel_memsize = kernel_size + (_end - _edata); _end and _edata are two linker generated symbols. If people are subtracting these things, I can imagine that they are also comparing them. > > In any case, the incomplete-type case should be restricted to incomplete > arrays, since incomplete class types can never have zero size in C++. > I completely agree. In an ideal world, I'd stuff this zero-sized mumbo-jumbo under a hypothetical -fgcc-compatibility (or something similar).
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