[Sema] Add -Wpointer-compare

This patch adds a warning that diagnoses comparisons of pointers to
'\0'. This is often indicative of a bug (e.g. the user might've
forgotten to dereference the pointer).

Patch by Elaina Guan!

Differential Revision: https://reviews.llvm.org/D65595

llvm-svn: 367940

1 files changed, 30 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 52fa932eee8..b407fb3c137 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -10443,6 +10443,32 @@ static QualType checkArithmeticOrEnumeralCompare(Sema &S, ExprResult &LHS,
   return S.Context.getLogicalOperationType();
 }
 
+void Sema::CheckPtrComparisonWithNullChar(ExprResult &E, ExprResult &NullE) {
+  if (!NullE.get()->getType()->isAnyPointerType())
+    return;
+  int NullValue = PP.isMacroDefined("NULL") ? 0 : 1;
+  if (!E.get()->getType()->isAnyPointerType() &&
+      E.get()->isNullPointerConstant(Context,
+                                     Expr::NPC_ValueDependentIsNotNull) ==
+        Expr::NPCK_ZeroExpression) {
+    if (const auto *CL = dyn_cast<CharacterLiteral>(E.get())) {
+      if (CL->getValue() == 0)
+        Diag(E.get()->getExprLoc(), diag::warn_pointer_compare)
+            << NullValue
+            << FixItHint::CreateReplacement(E.get()->getExprLoc(),
+                                            NullValue ? "NULL" : "(void *)0");
+    } else if (const auto *CE = dyn_cast<CStyleCastExpr>(E.get())) {
+        TypeSourceInfo *TI = CE->getTypeInfoAsWritten();
+        QualType T = Context.getCanonicalType(TI->getType()).getUnqualifiedType();
+        if (T == Context.CharTy)
+          Diag(E.get()->getExprLoc(), diag::warn_pointer_compare)
+              << NullValue
+              << FixItHint::CreateReplacement(E.get()->getExprLoc(),
+                                              NullValue ? "NULL" : "(void *)0");
+      }
+  }
+}
+
 // C99 6.5.8, C++ [expr.rel]
 QualType Sema::CheckCompareOperands(ExprResult &LHS, ExprResult &RHS,
                                     SourceLocation Loc,
@@ -10476,6 +10502,10 @@ QualType Sema::CheckCompareOperands(ExprResult &LHS, ExprResult &RHS,
   }
 
   checkArithmeticNull(*this, LHS, RHS, Loc, /*IsCompare=*/true);
+  if (!getLangOpts().CPlusPlus && BinaryOperator::isEqualityOp(Opc)) {
+    CheckPtrComparisonWithNullChar(LHS, RHS);
+    CheckPtrComparisonWithNullChar(RHS, LHS);
+  }
 
   // Handle vector comparisons separately.
   if (LHS.get()->getType()->isVectorType() ||