Warn on enum assignment to bitfields that can't fit all values

This adds -Wbitfield-enum-conversion, which warns on implicit
conversions that happen on bitfield assignment that change the value of
some enumerators.

Values of enum type typically take on a very small range of values, so
they are frequently stored in bitfields. Unfortunately, there is no
convenient way to calculate the minimum number of bits necessary to
store all possible values at compile time, so users usually hard code a
bitwidth that works today and widen it as necessary to pass basic
testing and validation. This is very error-prone, and leads to stale
widths as enums grow. This warning aims to catch such bugs.

This would have found two real bugs in clang and two instances of
questionable code. See r297680 and r297654 for the full description of
the issues.

This warning is currently disabled by default while we investigate its
usefulness outside of LLVM.

The major cause of false positives with this warning is this kind of
enum:
  enum E { W, X, Y, Z, SENTINEL_LAST };
The last enumerator is an invalid value used to validate inputs or size
an array. Depending on the prevalance of this style of enum across a
codebase, this warning may be more or less feasible to deploy. It also
has trouble on sentinel values such as ~0U.

Reviewers: rsmith, rtrieu, thakis

Reviewed By: thakis

Subscribers: hfinkel, voskresensky.vladimir, sashab, cfe-commits

Differential Revision: https://reviews.llvm.org/D30923

llvm-svn: 297761

clang/include/clang/Basic/DiagnosticGroups.td

@@ -34,6 +34,7 @@ def CXX14BinaryLiteral : DiagGroup<"c++14-binary-literal">;
 def GNUBinaryLiteral : DiagGroup<"gnu-binary-literal">;
 def GNUCompoundLiteralInitializer : DiagGroup<"gnu-compound-literal-initializer">;
 def BitFieldConstantConversion : DiagGroup<"bitfield-constant-conversion">;
+def BitFieldEnumConversion : DiagGroup<"bitfield-enum-conversion">;
 def BitFieldWidth : DiagGroup<"bitfield-width">;
 def ConstantConversion :
   DiagGroup<"constant-conversion", [ BitFieldConstantConversion ] >;
@@ -606,6 +607,7 @@ def Conversion : DiagGroup<"conversion",
                            [BoolConversion,
                             ConstantConversion,
                             EnumConversion,
+                            BitFieldEnumConversion,
                             FloatConversion,
                             Shorten64To32,
                             IntConversion,

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -4908,6 +4908,21 @@ def warn_bitfield_width_exceeds_type_width: Warning<
 def warn_anon_bitfield_width_exceeds_type_width : Warning<
   "width of anonymous bit-field (%0 bits) exceeds width of its type; value "
   "will be truncated to %1 bit%s1">, InGroup<BitFieldWidth>;
+def warn_bitfield_too_small_for_enum : Warning<
+  "bit-field %0 is not wide enough to store all enumerators of %1">,
+  InGroup<BitFieldEnumConversion>, DefaultIgnore;
+def note_widen_bitfield : Note<
+  "widen this field to %0 bits to store all values of %1">;
+def warn_unsigned_bitfield_assigned_signed_enum : Warning<
+  "assigning value of signed enum type %1 to unsigned bit-field %0; "
+  "negative enumerators of enum %1 will be converted to positive values">,
+  InGroup<BitFieldEnumConversion>, DefaultIgnore;
+def warn_signed_bitfield_enum_conversion : Warning<
+  "signed bit-field %0 needs an extra bit to represent the largest positive "
+  "enumerators of %1">,
+  InGroup<BitFieldEnumConversion>, DefaultIgnore;
+def note_change_bitfield_sign : Note<
+  "consider making the bitfield type %select{unsigned|signed}0">;
 
 def warn_missing_braces : Warning<
   "suggest braces around initialization of subobject">,

clang/lib/Sema/SemaChecking.cpp

@@ -8729,13 +8729,66 @@ bool AnalyzeBitFieldAssignment(Sema &S, FieldDecl *Bitfield, Expr *Init,
     return false;
 
   Expr *OriginalInit = Init->IgnoreParenImpCasts();
+  unsigned FieldWidth = Bitfield->getBitWidthValue(S.Context);
 
   llvm::APSInt Value;
-  if (!OriginalInit->EvaluateAsInt(Value, S.Context, Expr::SE_AllowSideEffects))
+  if (!OriginalInit->EvaluateAsInt(Value, S.Context,
+                                   Expr::SE_AllowSideEffects)) {
+    // The RHS is not constant.  If the RHS has an enum type, make sure the
+    // bitfield is wide enough to hold all the values of the enum without
+    // truncation.
+    if (const auto *EnumTy = OriginalInit->getType()->getAs<EnumType>()) {
+      EnumDecl *ED = EnumTy->getDecl();
+      bool SignedBitfield = BitfieldType->isSignedIntegerType();
+
+      // Enum types are implicitly signed on Windows, so check if there are any
+      // negative enumerators to see if the enum was intended to be signed or
+      // not.
+      bool SignedEnum = ED->getNumNegativeBits() > 0;
+
+      // Check for surprising sign changes when assigning enum values to a
+      // bitfield of different signedness.  If the bitfield is signed and we
+      // have exactly the right number of bits to store this unsigned enum,
+      // suggest changing the enum to an unsigned type. This typically happens
+      // on Windows where unfixed enums always use an underlying type of 'int'.
+      unsigned DiagID = 0;
+      if (SignedEnum && !SignedBitfield) {
+        DiagID = diag::warn_unsigned_bitfield_assigned_signed_enum;
+      } else if (SignedBitfield && !SignedEnum &&
+                 ED->getNumPositiveBits() == FieldWidth) {
+        DiagID = diag::warn_signed_bitfield_enum_conversion;
+      }
+
+      if (DiagID) {
+        S.Diag(InitLoc, DiagID) << Bitfield << ED;
+        TypeSourceInfo *TSI = Bitfield->getTypeSourceInfo();
+        SourceRange TypeRange =
+            TSI ? TSI->getTypeLoc().getSourceRange() : SourceRange();
+        S.Diag(Bitfield->getTypeSpecStartLoc(), diag::note_change_bitfield_sign)
+            << SignedEnum << TypeRange;
+      }
+
+      // Compute the required bitwidth. If the enum has negative values, we need
+      // one more bit than the normal number of positive bits to represent the
+      // sign bit.
+      unsigned BitsNeeded = SignedEnum ? std::max(ED->getNumPositiveBits() + 1,
+                                                  ED->getNumNegativeBits())
+                                       : ED->getNumPositiveBits();
+
+      // Check the bitwidth.
+      if (BitsNeeded > FieldWidth) {
+        Expr *WidthExpr = Bitfield->getBitWidth();
+        S.Diag(InitLoc, diag::warn_bitfield_too_small_for_enum)
+            << Bitfield << ED;
+        S.Diag(WidthExpr->getExprLoc(), diag::note_widen_bitfield)
+            << BitsNeeded << ED << WidthExpr->getSourceRange();
+      }
+    }
+
     return false;
+  }
 
   unsigned OriginalWidth = Value.getBitWidth();
-  unsigned FieldWidth = Bitfield->getBitWidthValue(S.Context);
 
   if (!Value.isSigned() || Value.isNegative())
     if (UnaryOperator *UO = dyn_cast<UnaryOperator>(OriginalInit))

clang/test/SemaCXX/warn-bitfield-enum-conversion.cpp

@@ -0,0 +1,59 @@
+// RUN: %clang_cc1 -std=c++11 -triple x86_64-windows-msvc -verify %s -Wbitfield-enum-conversion
+// RUN: %clang_cc1 -std=c++11 -triple x86_64-linux -verify %s -Wbitfield-enum-conversion
+
+enum TwoBits { Hi1 = 3 } two_bits;
+enum TwoBitsSigned { Lo2 = -2, Hi2 = 1 } two_bits_signed;
+enum ThreeBits { Hi3 = 7 } three_bits;
+enum ThreeBitsSigned { Lo4 = -4, Hi4 = 3 } three_bits_signed;
+enum TwoBitsFixed : unsigned { Hi5 = 3 } two_bits_fixed;
+
+struct Foo {
+  unsigned two_bits : 2;        // expected-note 2 {{widen this field to 3 bits}} expected-note 2 {{type signed}}
+  int two_bits_signed : 2;      // expected-note 2 {{widen this field to 3 bits}} expected-note 1 {{type unsigned}}
+  unsigned three_bits : 3;      // expected-note 2 {{type signed}}
+  int three_bits_signed : 3;    // expected-note 1 {{type unsigned}}
+
+#ifdef _WIN32
+  // expected-note@+2 {{type unsigned}}
+#endif
+  ThreeBits three_bits_enum : 3;
+  ThreeBits four_bits_enum : 4;
+};
+
+void f() {
+  Foo f;
+
+  f.two_bits = two_bits;
+  f.two_bits = two_bits_signed;            // expected-warning {{negative enumerators}}
+  f.two_bits = three_bits;                 // expected-warning {{not wide enough}}
+  f.two_bits = three_bits_signed;          // expected-warning {{negative enumerators}} expected-warning {{not wide enough}}
+  f.two_bits = two_bits_fixed;
+
+  f.two_bits_signed = two_bits;            // expected-warning {{needs an extra bit}}
+  f.two_bits_signed = two_bits_signed;
+  f.two_bits_signed = three_bits;          // expected-warning {{not wide enough}}
+  f.two_bits_signed = three_bits_signed;   // expected-warning {{not wide enough}}
+
+  f.three_bits = two_bits;
+  f.three_bits = two_bits_signed;          // expected-warning {{negative enumerators}}
+  f.three_bits = three_bits;
+  f.three_bits = three_bits_signed;        // expected-warning {{negative enumerators}}
+
+  f.three_bits_signed = two_bits;
+  f.three_bits_signed = two_bits_signed;
+  f.three_bits_signed = three_bits;        // expected-warning {{needs an extra bit}}
+  f.three_bits_signed = three_bits_signed;
+
+#ifdef _WIN32
+  // Enums on Windows are always implicitly 'int', which is signed, so you need
+  // an extra bit to store values that set the MSB. This is not true on SysV
+  // platforms like Linux.
+  // expected-warning@+2 {{needs an extra bit}}
+#endif
+  f.three_bits_enum = three_bits;
+  f.four_bits_enum = three_bits;
+
+  // Explicit casts suppress the warning.
+  f.two_bits = (unsigned)three_bits_signed;
+  f.two_bits = static_cast<unsigned>(three_bits_signed);
+}