On Thu, Dec 11, 2014 at 6:50 PM, David Majnemer <[email protected]> wrote:
> 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). > OK, so I think the compromise position is: An entity is considered as being possibly-zero-sized if either: 1) The type is incomplete and we're in C, or 2) The type is an array of unknown bound and we're in C++, or 3) The type is complete and its size is zero. We refuse to constant-fold an address comparison if one operand is possibly-zero-sized, and the other is either possibly-zero-sized or evaluates to the address of the start or end of an object of a complete type. Does that make sense to you? I think that's as close as we can get to the standard behavior in C and C++ without miscompiling address comparisons against zero-sized objects.
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