diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -51,6 +51,11 @@
 
     foo: asm goto ("# %0 %1"::"i"(&&foo)::foo);
 
+- ``__builtin_object_size`` and ``__builtin_dynamic_object_size`` now add the
+  ``sizeof`` the elements specified in designated initializers of flexible
+  array members for structs that contain them. This change is more consistent
+  with the behavior of GCC.
+
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
 - Clang won't search for coroutine_traits in std::experimental namespace any more.
@@ -422,6 +427,10 @@
   (`#61746 <https://github.com/llvm/llvm-project/issues/61746>`_).
 - Clang `TextNodeDumper` enabled through `-ast-dump` flag no longer evaluates the
   initializer of constexpr `VarDecl` if the declaration has a dependent type.
+- Match GCC's behavior for ``__builtin_object_size`` and
+  ``__builtin_dynamic_object_size`` on structs containing flexible array
+  members.
+  (`#62789 <https://github.com/llvm/llvm-project/issues/62789>`_).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11733,7 +11733,14 @@
   auto CheckedHandleSizeof = [&](QualType Ty, CharUnits &Result) {
     if (Ty.isNull() || Ty->isIncompleteType() || Ty->isFunctionType())
       return false;
-    return HandleSizeof(Info, ExprLoc, Ty, Result);
+    bool Ret = HandleSizeof(Info, ExprLoc, Ty, Result);
+    if (Ty->isStructureType() &&
+        Ty->getAsStructureType()->getDecl()->hasFlexibleArrayMember()) {
+      const auto *VD =
+          cast<VarDecl>(LVal.getLValueBase().get<const ValueDecl *>());
+      Result += VD->getFlexibleArrayInitChars(Info.Ctx);
+    }
+    return Ret;
   };
 
   // We want to evaluate the size of the entire object. This is a valid fallback
diff --git a/clang/test/CodeGen/object-size.c b/clang/test/CodeGen/object-size.c
--- a/clang/test/CodeGen/object-size.c
+++ b/clang/test/CodeGen/object-size.c
@@ -525,6 +525,33 @@
   gi = OBJECT_SIZE_BUILTIN(&dsv[9].snd[0], 1);
 }
 
+// CHECK-LABEL: @test32
+static struct DynStructVar D32 = {
+  .fst = {},
+  .snd = { 0, 1, 2, },
+};
+unsigned long test32(void) {
+  // CHECK: ret i64 19
+  return OBJECT_SIZE_BUILTIN(&D32, 1);
+}
+// CHECK-LABEL: @test33
+static struct DynStructVar D33 = {
+  .fst = {},
+  .snd = {},
+};
+unsigned long test33(void) {
+  // CHECK: ret i64 16
+  return OBJECT_SIZE_BUILTIN(&D33, 1);
+}
+// CHECK-LABEL: @test34
+static struct DynStructVar D34 = {
+  .fst = {},
+};
+unsigned long test34(void) {
+  // CHECK: ret i64 16
+  return OBJECT_SIZE_BUILTIN(&D34, 1);
+}
+
 // CHECK-LABEL: @PR30346
 void PR30346(void) {
   struct sa_family_t {};