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.
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