[Clang] Add wraps attribute (for granular integer overflow handling)

clang/docs/ReleaseNotes.rst

@@ -249,6 +249,16 @@ Attribute Changes in Clang
 - Introduced a new attribute ``[[clang::coro_await_elidable]]`` on coroutine return types
   to express elideability at call sites where the coroutine is co_awaited as a prvalue.
 
+- Introduced ``__attribute((wraps))__`` which can be added to type or variable
+  declarations. Using an attributed type or variable in an arithmetic
+  expression will define the overflow behavior for that expression as having
+  two's complement wrap-around. These expressions cannot trigger integer
+  overflow warnings or sanitizer warnings. They also cannot be optimized away
+  by some eager UB optimizations.
+
+  This attribute is only valid for C, as there are built-in language
+  alternatives for other languages.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/include/clang/AST/Expr.h

@@ -4098,6 +4098,9 @@ class BinaryOperator : public Expr {
     return getFPFeaturesInEffect(LO).getAllowFEnvAccess();
   }
 
+  /// Does one of the subexpressions have the wraps attribute?
+  bool hasWrappingOperand(const ASTContext &Ctx) const;
+
 protected:
   BinaryOperator(const ASTContext &Ctx, Expr *lhs, Expr *rhs, Opcode opc,
                  QualType ResTy, ExprValueKind VK, ExprObjectKind OK,

clang/include/clang/AST/Type.h

@@ -1454,6 +1454,8 @@ class QualType {
     return getQualifiers().hasStrongOrWeakObjCLifetime();
   }
 
+  bool hasWrapsAttr() const;
+
   // true when Type is objc's weak and weak is enabled but ARC isn't.
   bool isNonWeakInMRRWithObjCWeak(const ASTContext &Context) const;
 

clang/include/clang/Basic/Attr.td

@@ -4796,3 +4796,10 @@ def ClspvLibclcBuiltin: InheritableAttr {
   let Documentation = [ClspvLibclcBuiltinDoc];
   let SimpleHandler = 1;
 }
+
+def Wraps : DeclOrTypeAttr {
+  let Spellings = [Clang<"wraps">];
+  let Subjects = SubjectList<[Var, TypedefName, Field]>;
+  let Documentation = [WrapsDocs];
+  let LangOpts = [COnly];
+}

clang/include/clang/Basic/AttrDocs.td

@@ -8446,3 +8446,72 @@ Declares that a function potentially allocates heap memory, and prevents any pot
 of ``nonallocating`` by the compiler.
   }];
 }
+
+def WrapsDocs : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+This attribute can be used with type or variable declarations to denote that
+arithmetic containing these marked components have defined overflow behavior.
+Specifically, the behavior is defined as being consistent with two's complement
+wrap-around. For the purposes of sanitizers or warnings that concern themselves
+with the definedness of integer arithmetic, they will cease to instrument or
+warn about arithmetic that directly involves a "wrapping" component.
+
+For example, ``-fsanitize=signed-integer-overflow`` or ``-Winteger-overflow``
+will not warn about suspicious overflowing arithmetic -- assuming correct usage
+of the wraps attribute.
+
+This example shows some basic usage of ``__attribute__((wraps))`` on a type
+definition when building with ``-fsanitize=signed-integer-overflow``
+
+.. code-block:: c
+
+  typedef int __attribute__((wraps)) wrapping_int;
+
+  void foo() {
+    wrapping_int a = INT_MAX;
+    ++a; // no sanitizer warning
+  }
+
+  int main() { foo(); }
+
+In the following example, we use ``__attribute__((wraps))`` on a variable to
+disable overflow instrumentation for arithmetic expressions it appears in. We
+do so with a popular overflow-checking pattern which we might not want to trip
+sanitizers (like ``-fsanitize=unsigned-integer-overflow``).
+
+.. code-block:: c
+
+  void foo(int offset) {
+    unsigned int A __attribute__((wraps)) = UINT_MAX;
+
+    // check for overflow using a common pattern, however we may accidentally
+    // perform a real overflow thus triggering sanitizers to step in. Since "A"
+    // is "wrapping", we can avoid sanitizer warnings.
+    if (A + offset < A) {
+      // handle overflow manually
+      // ...
+      return;
+    }
+
+    // now, handle non-overflow case ...
+  }
+
+The above example demonstrates some of the power and elegance this attribute
+provides. We can use code patterns we are already familiar with (like ``if (x +
+y < x)``) while gaining control over the overflow behavior on a case-by-case
+basis.
+
+When combined with ``-fwrapv``, this attribute can still be applied as normal
+but has no function apart from annotating types and variables for readers. This
+is because ``-fwrapv`` defines all arithmetic as being "wrapping", rendering
+this attribute's efforts redundant.
+
+When using this attribute without ``-fwrapv`` and without any sanitizers, it
+still has an impact on the definedness of arithmetic expressions containing
+wrapping components. Since the behavior of said expressions is now technically
+defined, the compiler will forgo some eager optimizations that are used on
+expressions containing UB.
+}];
+}
+

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1577,3 +1577,9 @@ def ExtractAPIMisuse : DiagGroup<"extractapi-misuse">;
 // Warnings about using the non-standard extension having an explicit specialization
 // with a storage class specifier.
 def ExplicitSpecializationStorageClass : DiagGroup<"explicit-specialization-storage-class">;
+
+// Warnings regarding the usage of __attribute__((wraps)) on non-integer types.
+def UselessWrapsAttr : DiagGroup<"useless-wraps-attribute">;
+
+// Warnings about the wraps attribute getting implicitly discarded
+def ImpDiscardedWrapsAttr : DiagGroup<"implicitly-discarded-wraps-attribute">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6633,6 +6633,13 @@ def warn_counted_by_attr_elt_type_unknown_size :
   Warning<err_counted_by_attr_pointee_unknown_size.Summary>,
   InGroup<BoundsSafetyCountedByEltTyUnknownSize>;
 
+def warn_wraps_attr_var_decl_type_not_integer : Warning<
+  "using attribute 'wraps' with non-integer type '%0' has no function">,
+  InGroup<UselessWrapsAttr>;
+def warn_wraps_attr_maybe_lost : Warning<
+  "'wraps' attribute may be implicitly discarded when converted to %0">,
+  InGroup<ImpDiscardedWrapsAttr>;
+
 let CategoryName = "ARC Semantic Issue" in {
 
 // ARC-mode diagnostics.

clang/lib/AST/Expr.cpp

@@ -2240,6 +2240,11 @@ bool BinaryOperator::isNullPointerArithmeticExtension(ASTContext &Ctx,
   return true;
 }
 
+bool BinaryOperator::hasWrappingOperand(const ASTContext &Ctx) const {
+  return getLHS()->getType().hasWrapsAttr() ||
+         getRHS()->getType().hasWrapsAttr();
+}
+
 SourceLocExpr::SourceLocExpr(const ASTContext &Ctx, SourceLocIdentKind Kind,
                              QualType ResultTy, SourceLocation BLoc,
                              SourceLocation RParenLoc,
@@ -4856,6 +4861,8 @@ BinaryOperator::BinaryOperator(const ASTContext &Ctx, Expr *lhs, Expr *rhs,
   if (hasStoredFPFeatures())
     setStoredFPFeatures(FPFeatures);
   setDependence(computeDependence(this));
+  if (hasWrappingOperand(Ctx))
+    setType(Ctx.getAttributedType(attr::Wraps, getType(), getType()));
 }
 
 BinaryOperator::BinaryOperator(const ASTContext &Ctx, Expr *lhs, Expr *rhs,
@@ -4874,6 +4881,8 @@ BinaryOperator::BinaryOperator(const ASTContext &Ctx, Expr *lhs, Expr *rhs,
   if (hasStoredFPFeatures())
     setStoredFPFeatures(FPFeatures);
   setDependence(computeDependence(this));
+  if (hasWrappingOperand(Ctx))
+    setType(Ctx.getAttributedType(attr::Wraps, getType(), getType()));
 }
 
 BinaryOperator *BinaryOperator::CreateEmpty(const ASTContext &C,

clang/lib/AST/ExprConstant.cpp

@@ -2809,7 +2809,7 @@ static bool CheckedIntArithmetic(EvalInfo &Info, const Expr *E,
   APSInt Value(Op(LHS.extend(BitWidth), RHS.extend(BitWidth)), false);
   Result = Value.trunc(LHS.getBitWidth());
   if (Result.extend(BitWidth) != Value) {
-    if (Info.checkingForUndefinedBehavior())
+    if (Info.checkingForUndefinedBehavior() && !E->getType().hasWrapsAttr())
       Info.Ctx.getDiagnostics().Report(E->getExprLoc(),
                                        diag::warn_integer_constant_overflow)
           << toString(Result, 10, Result.isSigned(), /*formatAsCLiteral=*/false,
@@ -14412,7 +14412,7 @@ bool IntExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
     if (!Result.isInt()) return Error(E);
     const APSInt &Value = Result.getInt();
     if (Value.isSigned() && Value.isMinSignedValue() && E->canOverflow()) {
-      if (Info.checkingForUndefinedBehavior())
+      if (Info.checkingForUndefinedBehavior() && !E->getType().hasWrapsAttr())
         Info.Ctx.getDiagnostics().Report(E->getExprLoc(),
                                          diag::warn_integer_constant_overflow)
             << toString(Value, 10, Value.isSigned(), /*formatAsCLiteral=*/false,

clang/lib/AST/Type.cpp

@@ -2850,6 +2850,10 @@ bool QualType::isWebAssemblyFuncrefType() const {
          getAddressSpace() == LangAS::wasm_funcref;
 }
 
+bool QualType::hasWrapsAttr() const {
+  return !isNull() && getTypePtr()->hasAttr(attr::Wraps);
+}
+
 QualType::PrimitiveDefaultInitializeKind
 QualType::isNonTrivialToPrimitiveDefaultInitialize() const {
   if (const auto *RT =

clang/lib/AST/TypePrinter.cpp

@@ -2024,6 +2024,9 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
   case attr::AArch64SVEPcs: OS << "aarch64_sve_pcs"; break;
   case attr::AMDGPUKernelCall: OS << "amdgpu_kernel"; break;
   case attr::IntelOclBicc: OS << "inteloclbicc"; break;
+  case attr::Wraps:
+    OS << "wraps";
+    break;
   case attr::PreserveMost:
     OS << "preserve_most";
     break;

clang/lib/CodeGen/CGExprScalar.cpp

@@ -158,6 +158,10 @@ struct BinOpInfo {
     }
     return false;
   }
+
+  /// Does the BinaryOperator have the wraps attribute?
+  /// If so, we can elide overflow sanitizer checks.
+  bool hasWrappingOperand() const { return E->getType().hasWrapsAttr(); }
 };
 
 static bool MustVisitNullValue(const Expr *E) {
@@ -750,7 +754,8 @@ class ScalarExprEmitter
 
   // Binary Operators.
   Value *EmitMul(const BinOpInfo &Ops) {
-    if (Ops.Ty->isSignedIntegerOrEnumerationType()) {
+    if (Ops.Ty->isSignedIntegerOrEnumerationType() &&
+        !Ops.hasWrappingOperand()) {
       switch (CGF.getLangOpts().getSignedOverflowBehavior()) {
       case LangOptions::SOB_Defined:
         if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
@@ -786,7 +791,8 @@ class ScalarExprEmitter
 
     if (Ops.Ty->isUnsignedIntegerType() &&
         CGF.SanOpts.has(SanitizerKind::UnsignedIntegerOverflow) &&
-        !CanElideOverflowCheck(CGF.getContext(), Ops))
+        !CanElideOverflowCheck(CGF.getContext(), Ops) &&
+        !Ops.hasWrappingOperand())
       return EmitOverflowCheckedBinOp(Ops);
 
     if (Ops.LHS->getType()->isFPOrFPVectorTy()) {
@@ -1118,7 +1124,7 @@ void ScalarExprEmitter::EmitIntegerTruncationCheck(Value *Src, QualType SrcType,
   // If the comparison result is 'i1 false', then the truncation was lossy.
 
   // Do we care about this type of truncation?
-  if (!CGF.SanOpts.has(Check.second.second))
+  if (!CGF.SanOpts.has(Check.second.second) || DstType.hasWrapsAttr())
     return;
 
   llvm::Constant *StaticArgs[] = {
@@ -1351,6 +1357,14 @@ void CodeGenFunction::EmitBitfieldConversionCheck(Value *Src, QualType SrcType,
   bool SrcSigned = SrcType->isSignedIntegerOrEnumerationType();
   bool DstSigned = DstType->isSignedIntegerOrEnumerationType();
 
+  bool SrcWraps = SrcType.hasWrapsAttr();
+  bool DstWraps = DstType.hasWrapsAttr();
+
+  // The wraps attribute should silence any sanitizer warnings
+  // regarding truncation or overflow
+  if (SrcWraps || DstWraps)
+    return;
+
   CodeGenFunction::SanitizerScope SanScope(this);
 
   std::pair<ScalarExprEmitter::ImplicitConversionCheckKind,
@@ -2926,6 +2940,9 @@ ScalarExprEmitter::EmitScalarPrePostIncDec(const UnaryOperator *E, LValue LV,
     bool excludeOverflowPattern =
         matchesPostDecrInWhile(E, isInc, isPre, CGF.getContext());
 
+    BinOpInfo Ops = createBinOpInfoFromIncDec(
+        E, value, isInc, E->getFPFeaturesInEffect(CGF.getLangOpts()));
+
     if (CGF.getContext().isPromotableIntegerType(type)) {
       promotedType = CGF.getContext().getPromotedIntegerType(type);
       assert(promotedType != type && "Shouldn't promote to the same type.");
@@ -2982,11 +2999,12 @@ ScalarExprEmitter::EmitScalarPrePostIncDec(const UnaryOperator *E, LValue LV,
       // Note that signed integer inc/dec with width less than int can't
       // overflow because of promotion rules; we're just eliding a few steps
       // here.
-    } else if (E->canOverflow() && type->isSignedIntegerOrEnumerationType()) {
+    } else if (E->canOverflow() && type->isSignedIntegerOrEnumerationType() &&
+               !Ops.hasWrappingOperand()) {
       value = EmitIncDecConsiderOverflowBehavior(E, value, isInc);
     } else if (E->canOverflow() && type->isUnsignedIntegerType() &&
                CGF.SanOpts.has(SanitizerKind::UnsignedIntegerOverflow) &&
-               !excludeOverflowPattern) {
+               !Ops.hasWrappingOperand() && !excludeOverflowPattern) {
       value = EmitOverflowCheckedBinOp(createBinOpInfoFromIncDec(
           E, value, isInc, E->getFPFeaturesInEffect(CGF.getLangOpts())));
     } else {
@@ -3775,7 +3793,8 @@ Value *ScalarExprEmitter::EmitDiv(const BinOpInfo &Ops) {
     if ((CGF.SanOpts.has(SanitizerKind::IntegerDivideByZero) ||
          CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow)) &&
         Ops.Ty->isIntegerType() &&
-        (Ops.mayHaveIntegerDivisionByZero() || Ops.mayHaveIntegerOverflow())) {
+        (Ops.mayHaveIntegerDivisionByZero() || Ops.mayHaveIntegerOverflow()) &&
+        !Ops.hasWrappingOperand()) {
       llvm::Value *Zero = llvm::Constant::getNullValue(ConvertType(Ops.Ty));
       EmitUndefinedBehaviorIntegerDivAndRemCheck(Ops, Zero, true);
     } else if (CGF.SanOpts.has(SanitizerKind::FloatDivideByZero) &&
@@ -3824,7 +3843,8 @@ Value *ScalarExprEmitter::EmitRem(const BinOpInfo &Ops) {
   if ((CGF.SanOpts.has(SanitizerKind::IntegerDivideByZero) ||
        CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow)) &&
       Ops.Ty->isIntegerType() &&
-      (Ops.mayHaveIntegerDivisionByZero() || Ops.mayHaveIntegerOverflow())) {
+      (Ops.mayHaveIntegerDivisionByZero() || Ops.mayHaveIntegerOverflow()) &&
+      !Ops.hasWrappingOperand()) {
     CodeGenFunction::SanitizerScope SanScope(&CGF);
     llvm::Value *Zero = llvm::Constant::getNullValue(ConvertType(Ops.Ty));
     EmitUndefinedBehaviorIntegerDivAndRemCheck(Ops, Zero, false);
@@ -4189,7 +4209,7 @@ Value *ScalarExprEmitter::EmitAdd(const BinOpInfo &op) {
       op.RHS->getType()->isPointerTy())
     return emitPointerArithmetic(CGF, op, CodeGenFunction::NotSubtraction);
 
-  if (op.Ty->isSignedIntegerOrEnumerationType()) {
+  if (op.Ty->isSignedIntegerOrEnumerationType() && !op.hasWrappingOperand()) {
     switch (CGF.getLangOpts().getSignedOverflowBehavior()) {
     case LangOptions::SOB_Defined:
       if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
@@ -4222,7 +4242,7 @@ Value *ScalarExprEmitter::EmitAdd(const BinOpInfo &op) {
 
   if (op.Ty->isUnsignedIntegerType() &&
       CGF.SanOpts.has(SanitizerKind::UnsignedIntegerOverflow) &&
-      !CanElideOverflowCheck(CGF.getContext(), op))
+      !CanElideOverflowCheck(CGF.getContext(), op) && !op.hasWrappingOperand())
     return EmitOverflowCheckedBinOp(op);
 
   if (op.LHS->getType()->isFPOrFPVectorTy()) {
@@ -4345,7 +4365,7 @@ Value *ScalarExprEmitter::EmitFixedPointBinOp(const BinOpInfo &op) {
 Value *ScalarExprEmitter::EmitSub(const BinOpInfo &op) {
   // The LHS is always a pointer if either side is.
   if (!op.LHS->getType()->isPointerTy()) {
-    if (op.Ty->isSignedIntegerOrEnumerationType()) {
+    if (op.Ty->isSignedIntegerOrEnumerationType() && !op.hasWrappingOperand()) {
       switch (CGF.getLangOpts().getSignedOverflowBehavior()) {
       case LangOptions::SOB_Defined:
         if (!CGF.SanOpts.has(SanitizerKind::SignedIntegerOverflow))
@@ -4378,7 +4398,8 @@ Value *ScalarExprEmitter::EmitSub(const BinOpInfo &op) {
 
     if (op.Ty->isUnsignedIntegerType() &&
         CGF.SanOpts.has(SanitizerKind::UnsignedIntegerOverflow) &&
-        !CanElideOverflowCheck(CGF.getContext(), op))
+        !CanElideOverflowCheck(CGF.getContext(), op) &&
+        !op.hasWrappingOperand())
       return EmitOverflowCheckedBinOp(op);
 
     if (op.LHS->getType()->isFPOrFPVectorTy()) {
@@ -4498,7 +4519,8 @@ Value *ScalarExprEmitter::EmitShl(const BinOpInfo &Ops) {
   bool SanitizeSignedBase = CGF.SanOpts.has(SanitizerKind::ShiftBase) &&
                             Ops.Ty->hasSignedIntegerRepresentation() &&
                             !CGF.getLangOpts().isSignedOverflowDefined() &&
-                            !CGF.getLangOpts().CPlusPlus20;
+                            !CGF.getLangOpts().CPlusPlus20 &&
+                            !Ops.hasWrappingOperand();
   bool SanitizeUnsignedBase =
       CGF.SanOpts.has(SanitizerKind::UnsignedShiftBase) &&
       Ops.Ty->hasUnsignedIntegerRepresentation();

clang/lib/Sema/Sema.cpp

@@ -735,6 +735,9 @@ ExprResult Sema::ImpCastExprToType(Expr *E, QualType Ty,
   QualType ExprTy = Context.getCanonicalType(E->getType());
   QualType TypeTy = Context.getCanonicalType(Ty);
 
+  if (E->getType().getTypePtr()->isIntegerType() && E->getType().hasWrapsAttr())
+    Ty = Context.getAttributedType(attr::Wraps, Ty, Ty);
+
   if (ExprTy == TypeTy)
     return E;