implement wraps attribute

Signed-off-by: Justin Stitt <[email protected]>

Author: JustinStitt

clang/docs/ReleaseNotes.rst

@@ -525,6 +525,16 @@ Attribute Changes in Clang
   The attributes declare constraints about a function's behavior pertaining to blocking and
   heap memory allocation.
 
+- 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 now applies syntax highlighting to the code snippets it

clang/include/clang/AST/Expr.h

@@ -4061,6 +4061,9 @@ 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

@@ -1457,6 +1457,8 @@ 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

@@ -4630,3 +4630,10 @@ 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

@@ -8176,3 +8176,71 @@ 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

@@ -1539,3 +1539,9 @@ def BitIntExtension : DiagGroup<"bit-int-extension">;
 
 // Warnings about misuse of ExtractAPI options.
 def ExtractAPIMisuse : DiagGroup<"extractapi-misuse">;
+
+// 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

@@ -6601,6 +6601,13 @@ 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

@@ -2237,6 +2237,11 @@ 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,
@@ -4774,6 +4779,8 @@ 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,
@@ -4791,6 +4798,8 @@ 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

@@ -2781,7 +2781,7 @@ 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,
@@ -14176,7 +14176,7 @@ 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

@@ -2875,6 +2875,10 @@ bool QualType::isTriviallyEqualityComparableType(
       CanonicalType, /*CheckIfTriviallyCopyable=*/false);
 }
 
+bool QualType::hasWrapsAttr() const {
+  return !isNull() && getTypePtr()->hasAttr(attr::Wraps);
+}
+
 bool QualType::isNonWeakInMRRWithObjCWeak(const ASTContext &Context) const {
   return !Context.getLangOpts().ObjCAutoRefCount &&
          Context.getLangOpts().ObjCWeak &&