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

[Serge Pavlov]

  Updated code in ExprConstant.cpp and test case.

Hi rsmith,

http://llvm-reviews.chandlerc.com/D637

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D637?vs=1580&id=2498#toc

Files:
  include/clang/Basic/DiagnosticASTKinds.td
  include/clang/Basic/DiagnosticSemaKinds.td
  lib/AST/ExprConstant.cpp
  lib/Sema/SemaExpr.cpp
  test/Sema/empty1.c

Index: include/clang/Basic/DiagnosticASTKinds.td
===================================================================
--- include/clang/Basic/DiagnosticASTKinds.td
+++ include/clang/Basic/DiagnosticASTKinds.td
@@ -46,6 +46,8 @@
   "floating point arithmetic produces %select{an infinity|a NaN}0">;
 def note_constexpr_pointer_subtraction_not_same_array : Note<
   "subtracted pointers are not elements of the same array">;
+def note_constexpr_pointer_subtraction_zero_size_types : Note<
+  "subtracted pointers point to elements of zero size type">;
 def note_constexpr_pointer_comparison_base_classes : Note<
   "comparison of addresses of subobjects of different base classes "
   "has unspecified value">;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -4112,6 +4112,9 @@
 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
@@ -6360,6 +6360,13 @@
         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.
+        if (ElementSize.isZero()) {
+          CCEDiag(E, diag::note_constexpr_pointer_subtraction_zero_size_types);
+          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
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -6865,6 +6865,19 @@
                                                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->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' with 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' with 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' with 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' with 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' with 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' with 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]' with 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 []'}}
+}
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