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

[Serge Pavlov]

  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()) {
+          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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