[PATCH] Fix to PR5683 - issue diagnostic for pointer subtraction with type of size zero.

[John McCall]

On Apr 9, 2013, at 6:37 AM, Serge Pavlov <sepavloff at gmail.com> wrote:
> Hi rsmith,
> 
> http://llvm-reviews.chandlerc.com/D637
> 
> CHANGE SINCE LAST DIFF
>  http://llvm-reviews.chandlerc.com/D637?vs=1554&id=1563#toc
> 
> Files:
>  include/clang/Basic/DiagnosticSemaKinds.td
>  lib/AST/ExprConstant.cpp
>  lib/Sema/SemaExpr.cpp
>  test/Sema/empty1.c
> 
> Index: include/clang/Basic/DiagnosticSemaKinds.td
> ===================================================================
> --- include/clang/Basic/DiagnosticSemaKinds.td
> +++ include/clang/Basic/DiagnosticSemaKinds.td
> @@ -3976,6 +3976,9 @@
> def warn_offsetof_non_standardlayout_type : ExtWarn<
>   "offset of on non-standard-layout type %0">, InGroup<InvalidOffsetof>;
> def err_offsetof_bitfield : Error<"cannot compute offset of bit-field %0">;
> +def warn_sub_ptr_zero_size_types : Warning<
> +  "subtraction of pointers to type %0 with zero size has undefined behavior">,
> +  InGroup<PointerArith>;
> 
> def warn_floatingpoint_eq : Warning<
>   "comparing floating point with == or != is unsafe">,
> Index: lib/AST/ExprConstant.cpp
> ===================================================================
> --- lib/AST/ExprConstant.cpp
> +++ lib/AST/ExprConstant.cpp
> @@ -5003,6 +5003,12 @@
>         if (!HandleSizeof(Info, E->getExprLoc(), ElementType, ElementSize))
>           return false;
> 
> +        // Empty struct or union in C has size 0 (GCC extension). Meaning of
> +        // pointer difference in such case is unspecified, so set ElementSize
> +        // to 1 to avoid division by zero.
> +        if (ElementSize.isZero())
> +            ElementSize = CharUnits::One();
> +
>         // 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
> Index: lib/Sema/SemaExpr.cpp
> ===================================================================
> --- lib/Sema/SemaExpr.cpp
> +++ lib/Sema/SemaExpr.cpp
> @@ -6734,6 +6734,20 @@
>                                                LHS.get(), RHS.get()))
>         return QualType();
> 
> +      if (!getLangOpts().CPlusPlus) {
> +        // If pointee type is a structure or union of zero size (GCC extension),
> +        // the subtraction does not make sense.
> +        if (!rpointee.getTypePtr()->isVoidType() &&
> +            !rpointee.getTypePtr()->isFunctionType()) {

None of the uses of getTypePtr() in this patch are necessary.  You can just
do rpointee->isVoidType(), etc.

John.