PR5683: Issue a warning when subtracting pointers to types of zero size, and

treat such subtractions as being non-constant. Patch by Serge Pavlov! With a
few tweaks by me.

llvm-svn: 190439

2 files changed, 21 insertions, 0 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index b6602145e28..218ce8101b5 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -6570,6 +6570,15 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
         if (!HandleSizeof(Info, E->getExprLoc(), ElementType, ElementSize))
           return false;
 
+        // As an extension, a type may have zero size (empty struct or union in
+        // C, array of zero length). Pointer subtraction in such cases has
+        // undefined behavior, so is not constant.
+        if (ElementSize.isZero()) {
+          Info.Diag(E, diag::note_constexpr_pointer_subtraction_zero_size)
+            << ElementType;
+          return false;
+        }
+
         // FIXME: LLVM and GCC both compute LHSOffset - RHSOffset at runtime,
         // and produce incorrect results when it overflows. Such behavior
         // appears to be non-conforming, but is common, so perhaps we should
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index da834cc2519..918201c44cf 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -7041,6 +7041,18 @@ QualType Sema::CheckSubtractionOperands(ExprResult &LHS, ExprResult &RHS,
                                                LHS.get(), RHS.get()))
         return QualType();
 
+      // The pointee type may have zero size.  As an extension, a structure or
+      // union may have zero size or an array may have zero length.  In this
+      // case subtraction does not make sense.
+      if (!rpointee->isVoidType() && !rpointee->isFunctionType()) {
+        CharUnits ElementSize = Context.getTypeSizeInChars(rpointee);
+        if (ElementSize.isZero()) {
+          Diag(Loc,diag::warn_sub_ptr_zero_size_types)
+            << rpointee.getUnqualifiedType()
+            << LHS.get()->getSourceRange() << RHS.get()->getSourceRange();
+        }
+      }
+
       if (CompLHSTy) *CompLHSTy = LHS.get()->getType();
       return Context.getPointerDiffType();
     }