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

[Eli Friedman]

On Mon, Aug 19, 2013 at 9:10 AM, Serge Pavlov <sepavloff at gmail.com> wrote:

> Friendly ping.
>
>
> 2013/6/22 Serge Pavlov <sepavloff at gmail.com>
>
>>   Updated patch.
>>
>> Hi rsmith,
>>
>> http://llvm-reviews.chandlerc.com/D637
>>
>> CHANGE SINCE LAST DIFF
>>   http://llvm-reviews.chandlerc.com/D637?vs=2518&id=2538#toc
>>
>> Files:
>>   include/clang/Basic/DiagnosticSemaKinds.td
>>   lib/AST/ExprConstant.cpp
>>   lib/Sema/SemaExpr.cpp
>>   test/Sema/empty1.c
>>   test/Sema/empty1.cpp
>>
>> Index: include/clang/Basic/DiagnosticSemaKinds.td
>> ===================================================================
>> --- include/clang/Basic/DiagnosticSemaKinds.td
>> +++ include/clang/Basic/DiagnosticSemaKinds.td
>> @@ -4113,6 +4113,9 @@
>>  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 of 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
>> @@ -6405,6 +6405,14 @@
>>          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).  Meaning of pointer difference in
>> such
>> +        // case is unspecified.
>> +        if (ElementSize.isZero()) {
>> +          Result = APValue(Info.Ctx.MakeIntValue(0, E->getType()));
>> +          return true;
>> +        }
>> +
>>          // 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
>> @@ -6870,6 +6870,18 @@
>>                                                 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();
>>      }
>> Index: test/Sema/empty1.c
>> ===================================================================
>> --- test/Sema/empty1.c
>> +++ test/Sema/empty1.c
>> @@ -36,3 +36,50 @@
>>    struct emp_1 f1;
>>    union emp_2 f2;
>>  };
>> +
>> +
>> +// Checks for pointer subtraction (PR15683)
>> +
>> +struct emp_1* func_1p (struct emp_1* x) {
>> +  return x - 5;
>> +}
>> +
>> +int func_1 () {
>> +  struct emp_1 v[1];
>> +  return v - v;  // expected-warning {{subtraction of pointers to type
>> 'struct emp_1' of zero size has undefined behavior}}
>> +}
>> +
>> +int func_2 (struct emp_1* x) {
>> +  return 1 + x - x;  // expected-warning {{subtraction of pointers to
>> type 'struct emp_1' of zero size has undefined behavior}}
>> +}
>> +
>> +int func_3 (struct emp_1* x, struct emp_1* y) {
>> +  return x - y;  // expected-warning {{subtraction of pointers to type
>> 'struct emp_1' of zero size has undefined behavior}}
>> +}
>> +
>> +int func_4 (struct emp_1* x, const struct emp_1* y) {
>> +  return x - y;  // expected-warning {{subtraction of pointers to type
>> 'struct emp_1' of zero size has undefined behavior}}
>> +}
>> +
>> +int func_5 (volatile struct emp_1* x, const struct emp_1* y) {
>> +  return x - y;  // expected-warning {{subtraction of pointers to type
>> 'struct emp_1' of zero size has undefined behavior}}
>> +}
>> +
>> +int func_6 () {
>> +  union emp_2 v[1];
>> +  return v - v;  // expected-warning {{subtraction of pointers to type
>> 'union emp_2' of zero size has undefined behavior}}
>> +}
>> +
>> +struct A;  // expected-note {{forward declaration of 'struct A'}}
>> +
>> +int func_7 (struct A* x, struct A* y) {
>> +  return x - y;  // expected-error {{arithmetic on a pointer to an
>> incomplete type 'struct A'}}
>> +}
>> +
>> +int func_8 (struct emp_1 (*x)[10], struct emp_1 (*y)[10]) {
>> +  return x - y;  // expected-warning {{subtraction of pointers to type
>> 'struct emp_1 [10]' of zero size has undefined behavior}}
>> +}
>> +
>> +int func_9 (struct emp_1 (*x)[], struct emp_1 (*y)[]) {
>> +  return x - y;  // expected-error {{arithmetic on a pointer to an
>> incomplete type 'struct emp_1 []'}}
>> +}
>> Index: test/Sema/empty1.cpp
>>
>>
C++-specific tests should go into test/SemaCXX/

Otherwise looks fine, but please let Richard take another look at the
ExprConstant.cpp changes first.

-Eli
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