[PATCH] Fix to PR5683 - issue diagnostic for pointer subtraction with type of size zero.
Richard Smith
richard at metafoo.co.uk
Mon Apr 8 21:19:29 PDT 2013
On Mon, Apr 8, 2013 at 8:55 PM, Serge Pavlov <sepavloff at gmail.com> wrote:
> Patch updated
>
> Now correctly handles incomplete types and implements faster check
> (thanks to John McCall)
>
> Hi rsmith,
>
> http://llvm-reviews.chandlerc.com/D637
>
> CHANGE SINCE LAST DIFF
> http://llvm-reviews.chandlerc.com/D637?vs=1547&id=1554#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
> @@ -3971,6 +3971,9 @@
> 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 +6734,19 @@
> 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()->isRecordType()) {
>
This check still looks wrong to me. CodeGen has special cases for VLAs,
void, and function types; any other zero-size type should get a diagnostic.
Right now, it looks like you're missing at least the case of an array with
a zero-sized element type (such as an array of empty records). I think
that's the only other case for GNU C, but Objective-C might add some more
possibilities?
> + 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();
> }
> Index: test/Sema/empty1.c
> ===================================================================
> --- /dev/null
> +++ test/Sema/empty1.c
> @@ -0,0 +1,45 @@
> +// RUN: %clang_cc1 %s -fsyntax-only -verify
> +
> +struct empty {};
> +struct empty_2 {};
> +union empty_u {};
> +
> +struct empty* func_1p (struct empty* x) {
> + return x - 5;
> +}
> +
> +int func_1 () {
> + struct empty v[1];
> + return v - v; // expected-warning {{subtraction of pointers to type
> 'struct empty' with zero size has undefined behavior}}
> +}
> +
> +int func_2 (struct empty* x) {
> + return 1 + x - x; // expected-warning {{subtraction of pointers to
> type 'struct empty' with zero size has undefined behavior}}
> +}
> +
> +int func_3 (struct empty* x, struct empty* y) {
> + return x - y; // expected-warning {{subtraction of pointers to type
> 'struct empty' with zero size has undefined behavior}}
> +}
> +
> +int func_4 (struct empty* x, const struct empty* y) {
> + return x - y; // expected-warning {{subtraction of pointers to type
> 'struct empty' with zero size has undefined behavior}}
> +}
> +
> +int func_5 (volatile struct empty* x, const struct empty* y) {
> + return x - y; // expected-warning {{subtraction of pointers to type
> 'struct empty' with zero size has undefined behavior}}
> +}
> +
> +int func_6 (struct empty* x, struct empty_2* y) {
> + return x - y; // expected-error {{'struct empty *' and 'struct empty_2
> *' are not pointers to compatible types}}
> +}
> +
> +int func_7 () {
> + union empty_u v[1];
> + return v - v; // expected-warning {{subtraction of pointers to type
> 'union empty_u' with zero size has undefined behavior}}
> +}
> +
> +struct A; // expected-note {{forward declaration of 'struct A'}}
> +
> +int func_8 (struct A* x, struct A* y) {
> + return x - y; // expected-error {{arithmetic on a pointer to an
> incomplete type 'struct A'}}
> +}
>
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