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
@@ -4268,6 +4268,19 @@
     return field_begin() == field_end();
   }
 
+  FieldDecl *getLastField() {
+    FieldDecl *FD = nullptr;
+    for (FieldDecl *Field : fields())
+      FD = Field;
+    return FD;
+  }
+  const FieldDecl *getLastField() const {
+    const FieldDecl *FD = nullptr;
+    for (const FieldDecl *Field : fields())
+      FD = Field;
+    return FD;
+  }
+
   /// 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 +478,12 @@
   // Return true if this is a FileContext Decl.
   bool isFileContextDecl() const;
 
+  /// Whether it resembles a flexible array member.
+  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
@@ -4222,3 +4222,18 @@
   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];
+  code AdditionalMembers = [{
+    private:
+      SourceRange countedByFieldLoc;
+    public:
+      SourceRange getCountedByFieldLoc(void) 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
@@ -7189,3 +7189,47 @@
 (if any) of these ``cleanup`` callback functions.
 }];
 }
+
+def CountedByDocs : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+Clang supports the ``counted_by`` attribute for flexible array members. The
+argument for the ``counted_by`` attribute is the name of a field member in
+the same structure holding the count of elements in the flexible array.
+
+For example:
+
+.. code-block:: c
+
+  struct bar;
+
+  struct foo {
+    size_t num_fam_elements;
+    /* ... */
+    struct bar *fam[] __attribute__((counted_by(num_fam_elements)));
+  };
+
+  struct foo *foo_alloc(size_t num_elements) {
+    struct foo *f;
+
+    f = malloc(sizeof(struct foo) + num_elements * sizeof(struct bar *));
+    f->num_fam_elements = num_elements;
+    return f;
+  }
+
+This attribute is used with the ``-fsantize=array-bounds`` flag to trap if a
+flexible array access is outside of the number of elements.
+
+.. code-block:: c
+
+  void mux(struct foo *);
+
+  struct bar *baz(void) {
+    struct foo *f = foo_alloc(128);
+
+    mux(f); /* Fills in fam element. */
+
+    return f->fam[256]; /* Trap. */
+  }
+  }];
+}
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
@@ -6363,6 +6363,10 @@
   "field %0 can overwrite instance variable %1 with variable sized type %2"
   " in superclass %3">, InGroup<ObjCFlexibleArray>;
 
+def err_flexible_array_counted_by_attr_refers_to_self : Error<"">;
+def warn_flexible_array_counted_by_attr_field_not_found : Warning<
+  "counted_by field %0 not found">;
+
 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
@@ -8891,6 +8891,10 @@
 public:
   AttrImporter(ASTImporter &I) : Importer(I), NImporter(I) {}
 
+  // Useful for accessing the imported attribute.
+  template <typename T> T *getAttrAs() { return cast<T>(ToAttr); }
+  template <typename T> const T *getAttrAs() 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.
@@ -9097,6 +9101,15 @@
                   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.getAttrAs<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 +410,80 @@
   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;
+    }
+  }
+
+  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/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -981,7 +981,8 @@
 
     // Register callbacks to schedule sanitizer passes at the appropriate part
     // of the pipeline.
-    if (LangOpts.Sanitize.has(SanitizerKind::LocalBounds))
+    if (LangOpts.Sanitize.has(SanitizerKind::LocalBounds) ||
+        LangOpts.Sanitize.has(SanitizerKind::ArrayBounds))
       PB.registerScalarOptimizerLateEPCallback(
           [](FunctionPassManager &FPM, OptimizationLevel Level) {
             FPM.addPass(BoundsCheckingPass());
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 +853,55 @@
     }
   }
 
+  if (IsDynamic) {
+    const 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 *>(dyn_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 = dyn_cast<ArrayType>(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));
+
+      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 +960,52 @@
   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 (!Decl::isFlexibleArrayMemberLike(getContext(), FD, Base->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 +1015,8 @@
   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
@@ -17988,6 +17988,38 @@
          "Broken injected-class-name");
 }
 
+static const FieldDecl *FindFieldWithCountedByAttr(const RecordDecl *RD) {
+  for (const Decl *D : RD->decls()) {
+    if (const auto *FD = dyn_cast<FieldDecl>(D))
+      if (FD->hasAttr<CountedByAttr>())
+        return FD;
+
+    if (const auto *SubRD = dyn_cast<RecordDecl>(D))
+      return FindFieldWithCountedByAttr(SubRD);
+  }
+
+  return nullptr;
+}
+
+/// CheckCountedByAttr - Return an \p IdentifierInfo if the attribute field
+/// doesn't exist in the enclosing structure. Returns \p nullptr if the
+/// attribute field does exist.
+static const IdentifierInfo *CheckCountedByAttr(const RecordDecl *RD,
+                                                const FieldDecl *FD,
+                                                SourceRange &Loc) {
+  const auto *ECA = FD->getAttr<CountedByAttr>();
+
+  auto It = llvm::find_if(RD->fields(), [&](const FieldDecl *Field) {
+    return Field->getName() == ECA->getCountedByField()->getName();
+  });
+  if (It == RD->field_end()) {
+    Loc = ECA->getCountedByFieldLoc();
+    return ECA->getCountedByField();
+  }
+
+  return nullptr;
+}
+
 void Sema::ActOnTagFinishDefinition(Scope *S, Decl *TagD,
                                     SourceRange BraceRange) {
   AdjustDeclIfTemplate(TagD);
@@ -18045,6 +18077,17 @@
                      [](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))
+    if (const FieldDecl *FD = FindFieldWithCountedByAttr(RD)) {
+      SourceRange SR;
+      if (const IdentifierInfo *Unknown = CheckCountedByAttr(RD, FD, SR))
+        Diag(SR.getBegin(),
+             diag::warn_flexible_array_counted_by_attr_field_not_found)
+            << Unknown << SR;
+    }
 }
 
 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
@@ -8334,6 +8334,21 @@
   D->addAttr(ZeroCallUsedRegsAttr::Create(S.Context, Kind, AL));
 }
 
+static void handleCountedByAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  // TODO: Probably needs more processing here. See Sema::AddAlignValueAttr.
+  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;
@@ -9280,6 +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/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/Misc/warning-flags.c b/clang/test/Misc/warning-flags.c
--- a/clang/test/Misc/warning-flags.c
+++ b/clang/test/Misc/warning-flags.c
@@ -18,7 +18,7 @@
 
 The list of warnings below should NEVER grow.  It should gradually shrink to 0.
 
-CHECK: Warnings without flags (65):
+CHECK: Warnings without flags (66):
 
 CHECK-NEXT:   ext_expected_semi_decl_list
 CHECK-NEXT:   ext_explicit_specialization_storage_class
@@ -52,6 +52,7 @@
 CHECK-NEXT:   warn_fe_cc_log_diagnostics_failure
 CHECK-NEXT:   warn_fe_cc_print_header_failure
 CHECK-NEXT:   warn_fe_macro_contains_embedded_newline
+CHECK-NEXT:   warn_flexible_array_counted_by_attr_field_not_found
 CHECK-NEXT:   warn_ignoring_ftabstop_value
 CHECK-NEXT:   warn_implements_nscopying
 CHECK-NEXT:   warn_incompatible_qualified_id