diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -100,6 +100,11 @@
 - ``structs``, ``unions``, and ``arrays`` that are const may now be used as
   constant expressions.  This change is more consistent with the behavior of
   GCC.
+- Clang now supports the C-only attribute ``counted_by``. When applied to a
+  struct's flexible array member, it points to the struct field that holds the
+  number of elements in the flexible array member. This information can improve
+  the results of the array bound sanitizer and the
+  ``__builtin_dynamic_object_size`` builtin.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -4272,6 +4272,16 @@
     return field_begin() == field_end();
   }
 
+  FieldDecl *getLastField() {
+    FieldDecl *FD = nullptr;
+    for (FieldDecl *Field : fields())
+      FD = Field;
+    return FD;
+  }
+  const FieldDecl *getLastField() const {
+    return const_cast<RecordDecl *>(this)->getLastField();
+  }
+
   /// Note that the definition of this type is now complete.
   virtual void completeDefinition();
 
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -18,6 +18,7 @@
 #include "clang/AST/DeclarationName.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/LLVM.h"
+#include "clang/Basic/LangOptions.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/Specifiers.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -477,6 +478,15 @@
   // Return true if this is a FileContext Decl.
   bool isFileContextDecl() const;
 
+  /// Whether it resembles a flexible array member. This is a static member
+  /// because we want to be able to call it with a nullptr. That allows us to
+  /// perform non-Decl specific checks based on the object's type and strict
+  /// flex array level.
+  static bool isFlexibleArrayMemberLike(
+      ASTContext &Context, const Decl *D, QualType Ty,
+      LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel,
+      bool IgnoreTemplateOrMacroSubstitution);
+
   ASTContext &getASTContext() const LLVM_READONLY;
 
   /// Helper to get the language options from the ASTContext.
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -4234,3 +4234,21 @@
   let Subjects = SubjectList<[TypedefName], ErrorDiag>;
   let Documentation = [Undocumented];
 }
+
+def CountedBy : InheritableAttr {
+  let Spellings = [Clang<"counted_by">];
+  let Subjects = SubjectList<[Field]>;
+  let Args = [IdentifierArgument<"CountedByField">];
+  let Documentation = [CountedByDocs];
+  let LangOpts = [COnly];
+  // FIXME: This is ugly. Let using a DeclArgument would be nice, but a Decl
+  // isn't yet available due to the fact that we're still parsing the
+  // structure. Maybe that code could be changed sometime in the future.
+  code AdditionalMembers = [{
+    private:
+      SourceRange CountedByFieldLoc;
+    public:
+      SourceRange getCountedByFieldLoc() const { return CountedByFieldLoc; }
+      void setCountedByFieldLoc(SourceRange Loc) { CountedByFieldLoc = Loc; }
+  }];
+}
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -7193,3 +7193,69 @@
 (if any) of these ``cleanup`` callback functions.
 }];
 }
+
+def CountedByDocs : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+Clang supports the ``counted_by`` attribute on the flexible array member of a
+structure in C. The argument for the attribute is the name of a field member in
+the same structure holding the count of elements in the flexible array. This
+information can be used to improve the results of the array bound sanitizer and
+the ``__builtin_dynamic_object_size`` builtin.
+
+For example, the following code:
+
+.. code-block:: c
+
+  struct bar;
+
+  struct foo {
+    size_t count;
+    char other;
+    struct bar *array[] __attribute__((counted_by(count)));
+  };
+
+specifies that the flexible array member ``array`` has the number of elements
+allocated for it stored in ``count``. This establishes a relationship between
+``array`` and ``count``. Specifically, ``p->array`` must have at least
+``p->count`` number of elements available. It's the user's responsibility to
+ensure that this relationship is maintained through changes to the structure.
+
+In the following example, the allocated array erroneously has fewer elements
+than what's specified by ``p->count``. This would result in an out-of-bounds
+access not being detected.
+
+.. code-block:: c
+
+  #define SIZE_INCR 42
+
+  struct foo *p;
+
+  void foo_alloc(size_t count) {
+    p = malloc(MAX(sizeof(struct foo),
+                   offsetof(struct foo, array[0]) + count * sizeof(struct bar *)));
+    p->count = count + SIZE_INCR;
+  }
+
+The next example updates ``p->count``, breaking the relationship requirement
+that ``p->array`` must have at least ``p->count`` number of elements available:
+
+.. code-block:: c
+
+  #define SIZE_INCR 42
+
+  struct foo *p;
+
+  void foo_alloc(size_t count) {
+    p = malloc(MAX(sizeof(struct foo),
+                   offsetof(struct foo, array[0]) + count * sizeof(struct bar *)));
+    p->count = count;
+  }
+
+  void use_foo(int index) {
+    p->count += SIZE_INCR + 1; /* 'count' is now larger than the number of elements of 'array'. */
+    p->array[index] = 0;       /* the sanitizer can't properly check if this is an out-of-bounds access. */
+  }
+
+  }];
+}
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6365,6 +6365,19 @@
   "field %0 can overwrite instance variable %1 with variable sized type %2"
   " in superclass %3">, InGroup<ObjCFlexibleArray>;
 
+def err_counted_by_attr_not_on_flexible_array_member : Error<
+  "'counted_by' only applies to flexible array members">;
+def err_flexible_array_counted_by_attr_field_not_found : Error<
+  "field %0 in 'counted_by' not found">;
+def err_flexible_array_counted_by_attr_field_not_found_in_struct : Error<
+  "field %0 in 'counted_by' is not found in struct">;
+def err_flexible_array_counted_by_attr_refers_to_self : Error<
+  "field %0 in 'counted_by' cannot refer to the flexible array">;
+def err_flexible_array_counted_by_attr_field_not_integer : Error<
+  "field %0 in 'counted_by' is not a non-boolean integer type">;
+def note_flexible_array_counted_by_attr_field : Note<
+  "field %0 declared here">;
+
 let CategoryName = "ARC Semantic Issue" in {
 
 // ARC-mode diagnostics.
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -8975,6 +8975,10 @@
 public:
   AttrImporter(ASTImporter &I) : Importer(I), NImporter(I) {}
 
+  // Useful for accessing the imported attribute.
+  template <typename T> T *castAttrAs() { return cast<T>(ToAttr); }
+  template <typename T> const T *castAttrAs() const { return cast<T>(ToAttr); }
+
   // Create an "importer" for an attribute parameter.
   // Result of the 'value()' of that object is to be passed to the function
   // 'importAttr', in the order that is expected by the attribute class.
@@ -9181,6 +9185,15 @@
                   From->args_size());
     break;
   }
+  case attr::CountedBy: {
+    AI.cloneAttr(FromAttr);
+    const auto *ECA = cast<CountedByAttr>(FromAttr);
+    Expected<SourceRange> SR = Import(ECA->getCountedByFieldLoc()).get();
+    if (!SR)
+      return SR.takeError();
+    AI.castAttrAs<CountedByAttr>()->setCountedByFieldLoc(SR.get());
+    break;
+  }
 
   default: {
     // The default branch works for attributes that have no arguments to import.
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -29,7 +29,6 @@
 #include "clang/AST/Type.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/LLVM.h"
-#include "clang/Basic/LangOptions.h"
 #include "clang/Basic/Module.h"
 #include "clang/Basic/ObjCRuntime.h"
 #include "clang/Basic/PartialDiagnostic.h"
@@ -411,6 +410,81 @@
   return DC && DC->isFileContext();
 }
 
+bool Decl::isFlexibleArrayMemberLike(
+    ASTContext &Ctx, const Decl *D, QualType Ty,
+    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel,
+    bool IgnoreTemplateOrMacroSubstitution) {
+  // For compatibility with existing code, we treat arrays of length 0 or
+  // 1 as flexible array members.
+  const auto *CAT = Ctx.getAsConstantArrayType(Ty);
+  if (CAT) {
+    using FAMKind = LangOptions::StrictFlexArraysLevelKind;
+
+    llvm::APInt Size = CAT->getSize();
+    FAMKind StrictFlexArraysLevel =
+        Ctx.getLangOpts().getStrictFlexArraysLevel();
+
+    if (StrictFlexArraysLevel == FAMKind::IncompleteOnly)
+      return false;
+
+    // GCC extension, only allowed to represent a FAM.
+    if (Size.isZero())
+      return true;
+
+    if (StrictFlexArraysLevel == FAMKind::ZeroOrIncomplete && Size.uge(1))
+      return false;
+
+    if (StrictFlexArraysLevel == FAMKind::OneZeroOrIncomplete && Size.uge(2))
+      return false;
+  } else if (!Ctx.getAsIncompleteArrayType(Ty)) {
+    return false;
+  }
+
+  if (const auto *OID = dyn_cast_if_present<ObjCIvarDecl>(D))
+    return OID->getNextIvar() == nullptr;
+
+  const auto *FD = dyn_cast_if_present<FieldDecl>(D);
+  if (!FD)
+    return false;
+
+  if (CAT) {
+    // GCC treats an array memeber of a union as an FAM if the size is one or
+    // zero.
+    llvm::APInt Size = CAT->getSize();
+    if (FD->getParent()->isUnion() && (Size.isZero() || Size.isOne()))
+      return true;
+  }
+
+  // Don't consider sizes resulting from macro expansions or template argument
+  // substitution to form C89 tail-padded arrays.
+  if (IgnoreTemplateOrMacroSubstitution) {
+    TypeSourceInfo *TInfo = FD->getTypeSourceInfo();
+    while (TInfo) {
+      TypeLoc TL = TInfo->getTypeLoc();
+
+      // Look through typedefs.
+      if (TypedefTypeLoc TTL = TL.getAsAdjusted<TypedefTypeLoc>()) {
+        const TypedefNameDecl *TDL = TTL.getTypedefNameDecl();
+        TInfo = TDL->getTypeSourceInfo();
+        continue;
+      }
+
+      if (auto CTL = TL.getAs<ConstantArrayTypeLoc>()) {
+        const Expr *SizeExpr = dyn_cast<IntegerLiteral>(CTL.getSizeExpr());
+        if (!SizeExpr || SizeExpr->getExprLoc().isMacroID())
+          return false;
+      }
+
+      break;
+    }
+  }
+
+  // Test that the field is the last in the structure.
+  RecordDecl::field_iterator FI(
+      DeclContext::decl_iterator(const_cast<FieldDecl *>(FD)));
+  return ++FI == FD->getParent()->field_end();
+}
+
 TranslationUnitDecl *Decl::getTranslationUnitDecl() {
   if (auto *TUD = dyn_cast<TranslationUnitDecl>(this))
     return TUD;
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -205,85 +205,22 @@
 }
 
 bool Expr::isFlexibleArrayMemberLike(
-    ASTContext &Context,
+    ASTContext &Ctx,
     LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel,
     bool IgnoreTemplateOrMacroSubstitution) const {
-
-  // For compatibility with existing code, we treat arrays of length 0 or
-  // 1 as flexible array members.
-  const auto *CAT = Context.getAsConstantArrayType(getType());
-  if (CAT) {
-    llvm::APInt Size = CAT->getSize();
-
-    using FAMKind = LangOptions::StrictFlexArraysLevelKind;
-
-    if (StrictFlexArraysLevel == FAMKind::IncompleteOnly)
-      return false;
-
-    // GCC extension, only allowed to represent a FAM.
-    if (Size == 0)
-      return true;
-
-    if (StrictFlexArraysLevel == FAMKind::ZeroOrIncomplete && Size.uge(1))
-      return false;
-
-    if (StrictFlexArraysLevel == FAMKind::OneZeroOrIncomplete && Size.uge(2))
-      return false;
-  } else if (!Context.getAsIncompleteArrayType(getType()))
-    return false;
-
   const Expr *E = IgnoreParens();
+  const Decl *D = nullptr;
 
-  const NamedDecl *ND = nullptr;
-  if (const auto *DRE = dyn_cast<DeclRefExpr>(E))
-    ND = DRE->getDecl();
-  else if (const auto *ME = dyn_cast<MemberExpr>(E))
-    ND = ME->getMemberDecl();
+  if (const auto *ME = dyn_cast<MemberExpr>(E))
+    D = ME->getMemberDecl();
+  else if (const auto *DRE = dyn_cast<DeclRefExpr>(E))
+    D = DRE->getDecl();
   else if (const auto *IRE = dyn_cast<ObjCIvarRefExpr>(E))
-    return IRE->getDecl()->getNextIvar() == nullptr;
+    D = IRE->getDecl();
 
-  if (!ND)
-    return false;
-
-  // A flexible array member must be the last member in the class.
-  // FIXME: If the base type of the member expr is not FD->getParent(),
-  // this should not be treated as a flexible array member access.
-  if (const auto *FD = dyn_cast<FieldDecl>(ND)) {
-    // GCC treats an array memeber of a union as an FAM if the size is one or
-    // zero.
-    if (CAT) {
-      llvm::APInt Size = CAT->getSize();
-      if (FD->getParent()->isUnion() && (Size.isZero() || Size.isOne()))
-        return true;
-    }
-
-    // Don't consider sizes resulting from macro expansions or template argument
-    // substitution to form C89 tail-padded arrays.
-    if (IgnoreTemplateOrMacroSubstitution) {
-      TypeSourceInfo *TInfo = FD->getTypeSourceInfo();
-      while (TInfo) {
-        TypeLoc TL = TInfo->getTypeLoc();
-        // Look through typedefs.
-        if (TypedefTypeLoc TTL = TL.getAsAdjusted<TypedefTypeLoc>()) {
-          const TypedefNameDecl *TDL = TTL.getTypedefNameDecl();
-          TInfo = TDL->getTypeSourceInfo();
-          continue;
-        }
-        if (ConstantArrayTypeLoc CTL = TL.getAs<ConstantArrayTypeLoc>()) {
-          const Expr *SizeExpr = dyn_cast<IntegerLiteral>(CTL.getSizeExpr());
-          if (!SizeExpr || SizeExpr->getExprLoc().isMacroID())
-            return false;
-        }
-        break;
-      }
-    }
-
-    RecordDecl::field_iterator FI(
-        DeclContext::decl_iterator(const_cast<FieldDecl *>(FD)));
-    return ++FI == FD->getParent()->field_end();
-  }
-
-  return false;
+  return Decl::isFlexibleArrayMemberLike(Ctx, D, E->getType(),
+                                         StrictFlexArraysLevel,
+                                         IgnoreTemplateOrMacroSubstitution);
 }
 
 const ValueDecl *
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -853,6 +853,57 @@
     }
   }
 
+  if (IsDynamic) {
+    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
+        getLangOpts().getStrictFlexArraysLevel();
+    const Expr *Base = E->IgnoreParenImpCasts();
+
+    if (FieldDecl *FD = FindCountedByField(Base, StrictFlexArraysLevel)) {
+      const auto *ME = dyn_cast<MemberExpr>(Base);
+      llvm::Value *ObjectSize = nullptr;
+
+      if (!ME) {
+        const auto *DRE = dyn_cast<DeclRefExpr>(Base);
+        ValueDecl *VD = nullptr;
+
+        ObjectSize = ConstantInt::get(
+            ResType,
+            getContext().getTypeSize(DRE->getType()->getPointeeType()) / 8,
+            true);
+
+        if (auto *RD = DRE->getType()->getPointeeType()->getAsRecordDecl())
+          VD = RD->getLastField();
+
+        Expr *ICE = ImplicitCastExpr::Create(
+            getContext(), DRE->getType(), CK_LValueToRValue,
+            const_cast<Expr *>(cast<Expr>(DRE)), nullptr, VK_PRValue,
+            FPOptionsOverride());
+        ME = MemberExpr::CreateImplicit(getContext(), ICE, true, VD,
+                                        VD->getType(), VK_LValue, OK_Ordinary);
+      }
+
+      // At this point, we know that \p ME is a flexible array member.
+      const auto *ArrayTy = getContext().getAsArrayType(ME->getType());
+      unsigned Size = getContext().getTypeSize(ArrayTy->getElementType());
+
+      llvm::Value *CountField =
+          EmitAnyExprToTemp(MemberExpr::CreateImplicit(
+                                getContext(), const_cast<Expr *>(ME->getBase()),
+                                ME->isArrow(), FD, FD->getType(), VK_LValue,
+                                OK_Ordinary))
+              .getScalarVal();
+
+      llvm::Value *Mul = Builder.CreateMul(
+          CountField, llvm::ConstantInt::get(CountField->getType(), Size / 8));
+      Mul = Builder.CreateZExtOrTrunc(Mul, ResType);
+
+      if (ObjectSize)
+        return Builder.CreateAdd(ObjectSize, Mul);
+
+      return Mul;
+    }
+  }
+
   // LLVM can't handle Type=3 appropriately, and __builtin_object_size shouldn't
   // evaluate E for side-effects. In either case, we shouldn't lower to
   // @llvm.objectsize.
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -30,6 +30,7 @@
 #include "clang/Basic/CodeGenOptions.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Intrinsics.h"
@@ -925,16 +926,31 @@
     if (CE->getCastKind() == CK_ArrayToPointerDecay &&
         !CE->getSubExpr()->isFlexibleArrayMemberLike(CGF.getContext(),
                                                      StrictFlexArraysLevel)) {
+      CodeGenFunction::SanitizerScope SanScope(&CGF);
+
       IndexedType = CE->getSubExpr()->getType();
       const ArrayType *AT = IndexedType->castAsArrayTypeUnsafe();
       if (const auto *CAT = dyn_cast<ConstantArrayType>(AT))
         return CGF.Builder.getInt(CAT->getSize());
-      else if (const auto *VAT = dyn_cast<VariableArrayType>(AT))
+
+      if (const auto *VAT = dyn_cast<VariableArrayType>(AT))
         return CGF.getVLASize(VAT).NumElts;
       // Ignore pass_object_size here. It's not applicable on decayed pointers.
     }
+
+    if (FieldDecl *FD = CGF.FindCountedByField(Base, StrictFlexArraysLevel)) {
+      const auto *ME = dyn_cast<MemberExpr>(CE->getSubExpr());
+      IndexedType = Base->getType();
+      return CGF
+          .EmitAnyExprToTemp(MemberExpr::CreateImplicit(
+              CGF.getContext(), const_cast<Expr *>(ME->getBase()),
+              ME->isArrow(), FD, FD->getType(), VK_LValue, OK_Ordinary))
+          .getScalarVal();
+    }
   }
 
+  CodeGenFunction::SanitizerScope SanScope(&CGF);
+
   QualType EltTy{Base->getType()->getPointeeOrArrayElementType(), 0};
   if (llvm::Value *POS = CGF.LoadPassedObjectSize(Base, EltTy)) {
     IndexedType = Base->getType();
@@ -944,13 +960,53 @@
   return nullptr;
 }
 
+FieldDecl *CodeGenFunction::FindCountedByField(
+    const Expr *Base,
+    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel) {
+  const ValueDecl *VD = nullptr;
+
+  Base = Base->IgnoreParenImpCasts();
+
+  if (const auto *ME = dyn_cast<MemberExpr>(Base)) {
+    VD = dyn_cast<ValueDecl>(ME->getMemberDecl());
+  } else if (const auto *DRE = dyn_cast<DeclRefExpr>(Base)) {
+    // Pointing to the full structure.
+    VD = dyn_cast<ValueDecl>(DRE->getDecl());
+
+    QualType Ty = VD->getType();
+    if (Ty->isPointerType())
+      Ty = Ty->getPointeeType();
+
+    if (const auto *RD = Ty->getAsRecordDecl())
+      VD = RD->getLastField();
+  } else if (const auto *CE = dyn_cast<CastExpr>(Base)) {
+    if (const auto *ME = dyn_cast<MemberExpr>(CE->getSubExpr()))
+      VD = dyn_cast<ValueDecl>(ME->getMemberDecl());
+  }
+
+  const auto *FD = dyn_cast_if_present<FieldDecl>(VD);
+  if (!FD || !FD->getParent() ||
+      !Decl::isFlexibleArrayMemberLike(getContext(), FD, FD->getType(),
+                                       StrictFlexArraysLevel, true))
+    return nullptr;
+
+  const auto *CBA = FD->getAttr<CountedByAttr>();
+  if (!CBA)
+    return nullptr;
+
+  StringRef FieldName = CBA->getCountedByField()->getName();
+  auto It =
+      llvm::find_if(FD->getParent()->fields(), [&](const FieldDecl *Field) {
+        return FieldName == Field->getName();
+      });
+  return It != FD->getParent()->field_end() ? *It : nullptr;
+}
+
 void CodeGenFunction::EmitBoundsCheck(const Expr *E, const Expr *Base,
                                       llvm::Value *Index, QualType IndexType,
                                       bool Accessed) {
   assert(SanOpts.has(SanitizerKind::ArrayBounds) &&
          "should not be called unless adding bounds checks");
-  SanitizerScope SanScope(this);
-
   const LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
     getLangOpts().getStrictFlexArraysLevel();
 
@@ -960,6 +1016,8 @@
   if (!Bound)
     return;
 
+  SanitizerScope SanScope(this);
+
   bool IndexSigned = IndexType->isSignedIntegerOrEnumerationType();
   llvm::Value *IndexVal = Builder.CreateIntCast(Index, SizeTy, IndexSigned);
   llvm::Value *BoundVal = Builder.CreateIntCast(Bound, SizeTy, false);
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -3022,6 +3022,12 @@
   void EmitBoundsCheck(const Expr *E, const Expr *Base, llvm::Value *Index,
                        QualType IndexType, bool Accessed);
 
+  /// Find the FieldDecl specified in a FAM's "counted_by" attribute. Returns
+  /// \p nullptr if either the attribute or the field doesn't exist.
+  FieldDecl *FindCountedByField(
+      const Expr *Base,
+      LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel);
+
   llvm::Value *EmitScalarPrePostIncDec(const UnaryOperator *E, LValue LV,
                                        bool isInc, bool isPre);
   ComplexPairTy EmitComplexPrePostIncDec(const UnaryOperator *E, LValue LV,
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -17991,6 +17991,83 @@
          "Broken injected-class-name");
 }
 
+template <typename Functor>
+static const FieldDecl *FindFieldIf(const RecordDecl *RD, Functor &Pred) {
+  for (const Decl *D : RD->decls()) {
+    if (const auto *FD = dyn_cast<FieldDecl>(D); FD && Pred(FD))
+      return FD;
+
+    if (const auto *SubRD = dyn_cast<RecordDecl>(D))
+      if (const FieldDecl *FD = FindFieldIf(SubRD, Pred))
+        return FD;
+  }
+
+  return nullptr;
+}
+
+static void CheckCountedByAttr(Scope *Scope, Sema &S, const FieldDecl *FD) {
+  ASTContext &Ctx = S.getASTContext();
+  const RecordDecl *RD = FD->getParent();
+  const auto *ECA = FD->getAttr<CountedByAttr>();
+
+  auto Pred = [&](const Decl *D) {
+    if (const auto *Field = dyn_cast<FieldDecl>(D))
+      return Field->getName() == ECA->getCountedByField()->getName();
+    return false;
+  };
+  const FieldDecl *Field = FindFieldIf(RD, Pred);
+
+  LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
+      Ctx.getLangOpts().getStrictFlexArraysLevel();
+
+  if (!Field) {
+    // The "counted_by" field needs to exist within the struct.
+    DeclarationNameInfo NameInfo(ECA->getCountedByField(),
+                                 ECA->getCountedByFieldLoc().getBegin());
+    LookupResult Result(S, NameInfo, Sema::LookupOrdinaryName);
+
+    S.LookupName(Result, Scope);
+    if (Result.getResultKind() == LookupResult::Found) {
+      SourceRange SR = ECA->getCountedByFieldLoc();
+      S.Diag(SR.getBegin(),
+             diag::err_flexible_array_counted_by_attr_field_not_found_in_struct)
+          << ECA->getCountedByField() << SR;
+
+      SR = Result.getAsSingle<NamedDecl>()->getSourceRange();
+      S.Diag(SR.getBegin(), diag::note_var_declared_here)
+          << Result.getAsSingle<NamedDecl>() << SR;
+    } else {
+      SourceRange SR = ECA->getCountedByFieldLoc();
+      S.Diag(SR.getBegin(),
+             diag::err_flexible_array_counted_by_attr_field_not_found)
+          << ECA->getCountedByField() << SR;
+    }
+  } else if (!Decl::isFlexibleArrayMemberLike(Ctx, FD, FD->getType(),
+                                              StrictFlexArraysLevel, true)) {
+    SourceRange SR = FD->getLocation();
+    S.Diag(SR.getBegin(),
+           diag::err_counted_by_attr_not_on_flexible_array_member)
+        << SR;
+  } else if (Field->hasAttr<CountedByAttr>()) {
+    // The "counted_by" field can't point to the flexible array member.
+    SourceRange SR = ECA->getCountedByFieldLoc();
+    S.Diag(SR.getBegin(),
+           diag::err_flexible_array_counted_by_attr_refers_to_self)
+        << ECA->getCountedByField() << SR;
+  } else if (!Field->getType()->isIntegerType() ||
+             Field->getType()->isBooleanType()) {
+    // The "counted_by" field must have an integer type.
+    SourceRange SR = Field->getLocation();
+    S.Diag(SR.getBegin(),
+           diag::err_flexible_array_counted_by_attr_field_not_integer)
+        << Field << SR;
+
+    SR = ECA->getCountedByFieldLoc();
+    S.Diag(SR.getBegin(), diag::note_flexible_array_counted_by_attr_field)
+        << ECA->getCountedByField() << SR;
+  }
+}
+
 void Sema::ActOnTagFinishDefinition(Scope *S, Decl *TagD,
                                     SourceRange BraceRange) {
   AdjustDeclIfTemplate(TagD);
@@ -18048,6 +18125,18 @@
                      [](const FieldDecl *FD) { return FD->isBitField(); }))
       Diag(BraceRange.getBegin(), diag::warn_pragma_align_not_xl_compatible);
   }
+
+  // Check the "counted_by" attribute to ensure that the count field exists in
+  // the struct.
+  if (const auto *RD = dyn_cast<RecordDecl>(Tag)) {
+    auto Pred = [](const Decl *D) {
+      if (const auto *FD = dyn_cast<FieldDecl>(D))
+        return FD->hasAttr<CountedByAttr>();
+      return false;
+    };
+    if (const FieldDecl *FD = FindFieldIf(RD, Pred))
+      CheckCountedByAttr(S, *this, FD);
+  }
 }
 
 void Sema::ActOnObjCContainerFinishDefinition() {
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -8335,6 +8335,20 @@
   D->addAttr(ZeroCallUsedRegsAttr::Create(S.Context, Kind, AL));
 }
 
+static void handleCountedByAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (!AL.isArgIdent(0)) {
+    S.Diag(AL.getLoc(), diag::err_attribute_argument_type)
+        << AL << AANT_ArgumentIdentifier;
+    return;
+  }
+
+  IdentifierLoc *IL = AL.getArgAsIdent(0);
+  CountedByAttr *CBA =
+      ::new (S.Context) CountedByAttr(S.Context, AL, IL->Ident);
+  CBA->setCountedByFieldLoc(IL->Loc);
+  D->addAttr(CBA);
+}
+
 static void handleFunctionReturnThunksAttr(Sema &S, Decl *D,
                                            const ParsedAttr &AL) {
   StringRef KindStr;
@@ -9281,6 +9295,10 @@
     handleAvailableOnlyInDefaultEvalMethod(S, D, AL);
     break;
 
+  case ParsedAttr::AT_CountedBy:
+    handleCountedByAttr(S, D, AL);
+    break;
+
   // Microsoft attributes:
   case ParsedAttr::AT_LayoutVersion:
     handleLayoutVersion(S, D, AL);
diff --git a/clang/test/CodeGen/attr-counted-by.c b/clang/test/CodeGen/attr-counted-by.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/attr-counted-by.c
@@ -0,0 +1,227 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 3
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -DCOUNTED_BY -O2 -Wall -fsanitize=array-bounds,object-size,local-bounds -fstrict-flex-arrays=3 -emit-llvm -o - %s | FileCheck --check-prefix=SANITIZE-WITH-ATTR %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -DCOUNTED_BY -O2 -Wall -fstrict-flex-arrays=3 -emit-llvm -o - %s | FileCheck --check-prefix=NO-SANITIZE-WITH-ATTR %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O2 -Wall -fsanitize=array-bounds,object-size,local-bounds -fstrict-flex-arrays=3 -emit-llvm -o - %s | FileCheck --check-prefix=SANITIZE-WITHOUT-ATTR %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O2 -Wall -fstrict-flex-arrays=3 -emit-llvm -o - %s | FileCheck --check-prefix=NO-SANITIZE-WITHOUT-ATTR %s
+
+#if !__has_attribute(counted_by)
+#error "has attribute broken"
+#endif
+
+#ifdef COUNTED_BY
+#define __counted_by(member)    __attribute__((__counted_by__(member)))
+#else
+#define __counted_by(member)
+#endif
+
+typedef long unsigned int size_t;
+
+struct annotated {
+  unsigned long flags;
+  int count;
+  int array[] __counted_by(count);
+};
+
+// SANITIZE-WITH-ATTR-LABEL: define dso_local void @test1(
+// SANITIZE-WITH-ATTR-SAME: ptr noundef [[P:%.*]], i32 noundef [[INDEX:%.*]], i32 noundef [[VAL:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+// SANITIZE-WITH-ATTR-NEXT:  entry:
+// SANITIZE-WITH-ATTR-NEXT:    [[COUNT:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 1
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = load i32, ptr [[COUNT]], align 8, !tbaa [[TBAA2:![0-9]+]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = sext i32 [[INDEX]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP2:%.*]] = zext i32 [[TMP0]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP3:%.*]] = icmp ult i64 [[TMP1]], [[TMP2]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    br i1 [[TMP3]], label [[CONT7:%.*]], label [[HANDLER_OUT_OF_BOUNDS:%.*]], !prof [[PROF7:![0-9]+]], !nosanitize !6
+// SANITIZE-WITH-ATTR:       handler.out_of_bounds:
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP4:%.*]] = zext i32 [[INDEX]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    tail call void @__ubsan_handle_out_of_bounds_abort(ptr nonnull @[[GLOB2:[0-9]+]], i64 [[TMP4]]) #[[ATTR2:[0-9]+]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    unreachable, !nosanitize !6
+// SANITIZE-WITH-ATTR:       cont7:
+// SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED]], ptr [[P]], i64 0, i32 2, i64 [[TMP1]]
+// SANITIZE-WITH-ATTR-NEXT:    store i32 [[VAL]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITH-ATTR-LABEL: define dso_local void @test1(
+// NO-SANITIZE-WITH-ATTR-SAME: ptr nocapture noundef writeonly [[P:%.*]], i32 noundef [[INDEX:%.*]], i32 noundef [[VAL:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+// NO-SANITIZE-WITH-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[IDXPROM:%.*]] = sext i32 [[INDEX]] to i64
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[IDXPROM]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    store i32 [[VAL]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2:![0-9]+]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test1(
+// SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i32 noundef [[INDEX:%.*]], i32 noundef [[VAL:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+// SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[IDXPROM:%.*]] = sext i32 [[INDEX]] to i64
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[IDXPROM]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    store i32 [[VAL]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2:![0-9]+]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test1(
+// NO-SANITIZE-WITHOUT-ATTR-SAME: ptr nocapture noundef writeonly [[P:%.*]], i32 noundef [[INDEX:%.*]], i32 noundef [[VAL:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[IDXPROM:%.*]] = sext i32 [[INDEX]] to i64
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[IDXPROM]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    store i32 [[VAL]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2:![0-9]+]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+void test1(struct annotated *p, int index, int val) {
+  p->array[index] = val;
+}
+
+// SANITIZE-WITH-ATTR-LABEL: define dso_local void @test2(
+// SANITIZE-WITH-ATTR-SAME: ptr noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR0]] {
+// SANITIZE-WITH-ATTR-NEXT:  entry:
+// SANITIZE-WITH-ATTR-NEXT:    [[COUNT:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 1
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = load i32, ptr [[COUNT]], align 8, !tbaa [[TBAA2]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = zext i32 [[TMP0]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP2:%.*]] = icmp ugt i64 [[TMP1]], [[INDEX]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    br i1 [[TMP2]], label [[CONT12:%.*]], label [[HANDLER_OUT_OF_BOUNDS:%.*]], !prof [[PROF7]], !nosanitize !6
+// SANITIZE-WITH-ATTR:       handler.out_of_bounds:
+// SANITIZE-WITH-ATTR-NEXT:    tail call void @__ubsan_handle_out_of_bounds_abort(ptr nonnull @[[GLOB4:[0-9]+]], i64 [[INDEX]]) #[[ATTR2]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    unreachable, !nosanitize !6
+// SANITIZE-WITH-ATTR:       cont12:
+// SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP3:%.*]] = shl i32 [[TMP0]], 2
+// SANITIZE-WITH-ATTR-NEXT:    store i32 [[TMP3]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITH-ATTR-LABEL: define dso_local void @test2(
+// NO-SANITIZE-WITH-ATTR-SAME: ptr nocapture noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR1:[0-9]+]] {
+// NO-SANITIZE-WITH-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[COUNT:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 1
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = load i32, ptr [[COUNT]], align 8, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = shl i32 [[TMP0]], 2
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    store i32 [[TMP1]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test2(
+// SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR0]] {
+// SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    store i32 -1, ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test2(
+// NO-SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR1:[0-9]+]] {
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    store i32 -1, ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+void test2(struct annotated *p, size_t index) {
+  p->array[index] = __builtin_dynamic_object_size(p->array, 1);
+}
+
+// SANITIZE-WITH-ATTR-LABEL: define dso_local void @test3(
+// SANITIZE-WITH-ATTR-SAME: ptr noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR0]] {
+// SANITIZE-WITH-ATTR-NEXT:  entry:
+// SANITIZE-WITH-ATTR-NEXT:    [[COUNT:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 1
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = load i32, ptr [[COUNT]], align 8, !tbaa [[TBAA2]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = zext i32 [[TMP0]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP2:%.*]] = icmp ugt i64 [[TMP1]], [[INDEX]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    br i1 [[TMP2]], label [[CONT12:%.*]], label [[HANDLER_OUT_OF_BOUNDS:%.*]], !prof [[PROF7]], !nosanitize !6
+// SANITIZE-WITH-ATTR:       handler.out_of_bounds:
+// SANITIZE-WITH-ATTR-NEXT:    tail call void @__ubsan_handle_out_of_bounds_abort(ptr nonnull @[[GLOB5:[0-9]+]], i64 [[INDEX]]) #[[ATTR2]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    unreachable, !nosanitize !6
+// SANITIZE-WITH-ATTR:       cont12:
+// SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP3:%.*]] = shl i32 [[TMP0]], 2
+// SANITIZE-WITH-ATTR-NEXT:    [[CONV:%.*]] = add i32 [[TMP3]], 16
+// SANITIZE-WITH-ATTR-NEXT:    store i32 [[CONV]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITH-ATTR-LABEL: define dso_local void @test3(
+// NO-SANITIZE-WITH-ATTR-SAME: ptr nocapture noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR1]] {
+// NO-SANITIZE-WITH-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[COUNT:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 1
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = load i32, ptr [[COUNT]], align 8, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = shl i32 [[TMP0]], 2
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[CONV:%.*]] = add i32 [[TMP1]], 16
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    store i32 [[CONV]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test3(
+// SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR0]] {
+// SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    store i32 -1, ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test3(
+// NO-SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i64 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR1]] {
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED:%.*]], ptr [[P]], i64 0, i32 2, i64 [[INDEX]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    store i32 -1, ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+void test3(struct annotated *p, size_t index) {
+  // This test differs from 'test2' by checking bdos on the whole array and not
+  // just the FAM.
+  p->array[index] = __builtin_dynamic_object_size(p, 1);
+}
+
+struct annotated_with_anon_struct {
+  unsigned long flags;
+  struct {
+    unsigned char count;
+    int array[] __counted_by(count);
+  };
+};
+
+// SANITIZE-WITH-ATTR-LABEL: define dso_local void @test4(
+// SANITIZE-WITH-ATTR-SAME: ptr noundef [[P:%.*]], i32 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR0]] {
+// SANITIZE-WITH-ATTR-NEXT:  entry:
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED_WITH_ANON_STRUCT:%.*]], ptr [[P]], i64 0, i32 1
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = load i8, ptr [[TMP0]], align 8, !tbaa [[TBAA8:![0-9]+]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP2:%.*]] = sext i32 [[INDEX]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP3:%.*]] = zext i8 [[TMP1]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP4:%.*]] = icmp ult i64 [[TMP2]], [[TMP3]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    br i1 [[TMP4]], label [[CONT18:%.*]], label [[HANDLER_OUT_OF_BOUNDS:%.*]], !prof [[PROF7]], !nosanitize !6
+// SANITIZE-WITH-ATTR:       handler.out_of_bounds:
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP5:%.*]] = zext i32 [[INDEX]] to i64, !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    tail call void @__ubsan_handle_out_of_bounds_abort(ptr nonnull @[[GLOB6:[0-9]+]], i64 [[TMP5]]) #[[ATTR2]], !nosanitize !6
+// SANITIZE-WITH-ATTR-NEXT:    unreachable, !nosanitize !6
+// SANITIZE-WITH-ATTR:       cont18:
+// SANITIZE-WITH-ATTR-NEXT:    [[ARRAY:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 12
+// SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [0 x i32], ptr [[ARRAY]], i64 0, i64 [[TMP2]]
+// SANITIZE-WITH-ATTR-NEXT:    [[TMP6:%.*]] = shl i8 [[TMP1]], 2
+// SANITIZE-WITH-ATTR-NEXT:    [[CONV:%.*]] = zext i8 [[TMP6]] to i32
+// SANITIZE-WITH-ATTR-NEXT:    store i32 [[CONV]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITH-ATTR-LABEL: define dso_local void @test4(
+// NO-SANITIZE-WITH-ATTR-SAME: ptr nocapture noundef [[P:%.*]], i32 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR1]] {
+// NO-SANITIZE-WITH-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP0:%.*]] = getelementptr inbounds [[STRUCT_ANNOTATED_WITH_ANON_STRUCT:%.*]], ptr [[P]], i64 0, i32 1
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP1:%.*]] = load i8, ptr [[TMP0]], align 8, !tbaa [[TBAA6:![0-9]+]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[TMP2:%.*]] = shl i8 [[TMP1]], 2
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[CONV:%.*]] = zext i8 [[TMP2]] to i32
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[ARRAY:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 12
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[IDXPROM:%.*]] = sext i32 [[INDEX]] to i64
+// NO-SANITIZE-WITH-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [0 x i32], ptr [[ARRAY]], i64 0, i64 [[IDXPROM]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    store i32 [[CONV]], ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITH-ATTR-NEXT:    ret void
+//
+// SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test4(
+// SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i32 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR0]] {
+// SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAY:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 12
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[IDXPROM:%.*]] = sext i32 [[INDEX]] to i64
+// SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [0 x i32], ptr [[ARRAY]], i64 0, i64 [[IDXPROM]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    store i32 -1, ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+// NO-SANITIZE-WITHOUT-ATTR-LABEL: define dso_local void @test4(
+// NO-SANITIZE-WITHOUT-ATTR-SAME: ptr noundef [[P:%.*]], i32 noundef [[INDEX:%.*]]) local_unnamed_addr #[[ATTR1]] {
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:  entry:
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAY:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 12
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[IDXPROM:%.*]] = sext i32 [[INDEX]] to i64
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [0 x i32], ptr [[ARRAY]], i64 0, i64 [[IDXPROM]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    store i32 -1, ptr [[ARRAYIDX]], align 4, !tbaa [[TBAA2]]
+// NO-SANITIZE-WITHOUT-ATTR-NEXT:    ret void
+//
+void test4(struct annotated_with_anon_struct *p, int index) {
+  p->array[index] = __builtin_dynamic_object_size(p->array, 1);
+}
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -56,6 +56,7 @@
 // CHECK-NEXT: ConsumableAutoCast (SubjectMatchRule_record)
 // CHECK-NEXT: ConsumableSetOnRead (SubjectMatchRule_record)
 // CHECK-NEXT: Convergent (SubjectMatchRule_function)
+// CHECK-NEXT: CountedBy (SubjectMatchRule_field)
 // CHECK-NEXT: DLLExport (SubjectMatchRule_function, SubjectMatchRule_variable, SubjectMatchRule_record, SubjectMatchRule_objc_interface)
 // CHECK-NEXT: DLLImport (SubjectMatchRule_function, SubjectMatchRule_variable, SubjectMatchRule_record, SubjectMatchRule_objc_interface)
 // CHECK-NEXT: Destructor (SubjectMatchRule_function)
diff --git a/clang/test/Sema/attr-counted-by.c b/clang/test/Sema/attr-counted-by.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/attr-counted-by.c
@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -fstrict-flex-arrays=3 -fsyntax-only -verify %s
+
+#define __counted_by(f)  __attribute__((counted_by(f)))
+
+struct bar;
+
+struct not_found {
+  int bork;
+  struct bar *fam[] __counted_by(blork); // expected-error {{field 'blork' in 'counted_by' not found}}
+};
+
+int global; // expected-note {{variable 'global' is declared here}}
+
+struct found_outside_of_struct {
+  int bork;
+  struct bar *fam[] __counted_by(global); // expected-error {{field 'global' in 'counted_by' is not found in struct}}
+};
+
+struct self_referrential {
+  int bork;
+  struct bar *self[] __counted_by(self); // expected-error {{field 'self' in 'counted_by' cannot refer to the flexible array}}
+};
+
+struct non_int {
+  double non_integer; // expected-error {{field 'non_integer' in 'counted_by' is not a non-boolean integer type}}
+  struct bar *fam[] __counted_by(non_integer); // expected-note {{field 'non_integer' declared here}}
+};
+
+struct array_of_ints {
+  int non_integer[2]; // expected-error {{field 'non_integer' in 'counted_by' is not a non-boolean integer type}}
+  struct bar *fam[] __counted_by(non_integer); // expected-note {{field 'non_integer' declared here}}
+};
+
+struct not_a_fam {
+  double non_integer;
+  struct bar *non_fam __counted_by(non_integer); // expected-error {{'counted_by' only applies to flexible array members}}
+};