diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1141,7 +1141,7 @@
   HelpText<"Use Apple's kernel extensions ABI">,
   MarshallingInfoFlag<LangOpts<"AppleKext">>;
 def fstrict_flex_arrays_EQ : Joined<["-"], "fstrict-flex-arrays=">, Group<f_Group>,
-  MetaVarName<"<n>">, Values<"0,1,2">,
+  MetaVarName<"<n>">, Values<"0,1,2,3">,
   LangOpts<"StrictFlexArrays">,
   Flags<[CC1Option]>,
   HelpText<"Enable optimizations based on the strict definition of flexible arrays">,
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
@@ -11618,7 +11618,8 @@
          Designator.Entries.size() == Designator.MostDerivedPathLength &&
          Designator.MostDerivedIsArrayElement &&
          (Designator.isMostDerivedAnUnsizedArray() ||
-          Designator.getMostDerivedArraySize() == 0 ||
+          (Designator.getMostDerivedArraySize() == 0 &&
+           StrictFlexArraysLevel < 3) ||
           (Designator.getMostDerivedArraySize() == 1 &&
            StrictFlexArraysLevel < 2) ||
           StrictFlexArraysLevel == 0) &&
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
@@ -885,17 +885,24 @@
   // the two mechanisms.
   const ArrayType *AT = E->getType()->castAsArrayTypeUnsafe();
   if (const auto *CAT = dyn_cast<ConstantArrayType>(AT)) {
+    // At -fstrict-flex-arrays=3, only IncompleteArrayTypes are considered
+    // FAMs.
+    if (StrictFlexArraysLevel == 3)
+      return false;
+
     // FIXME: Sema doesn't treat [1] as a flexible array member if the bound
     // was produced by macro expansion.
-    if (StrictFlexArraysLevel >= 2 && CAT->getSize().ugt(0))
+    if (StrictFlexArraysLevel == 2 && CAT->getSize().ugt(0))
       return false;
+
     // FIXME: While the default -fstrict-flex-arrays=0 permits Size>1 trailing
     // arrays to be treated as flexible-array-members, we still emit ubsan
     // checks as if they are not.
     if (CAT->getSize().ugt(1))
       return false;
-  } else if (!isa<IncompleteArrayType>(AT))
+  } else if (!isa<IncompleteArrayType>(AT)) {
     return false;
+  }
 
   E = E->IgnoreParens();
 
diff --git a/clang/test/CodeGen/bounds-checking-fam.c b/clang/test/CodeGen/bounds-checking-fam.c
--- a/clang/test/CodeGen/bounds-checking-fam.c
+++ b/clang/test/CodeGen/bounds-checking-fam.c
@@ -1,10 +1,12 @@
 // REQUIRES: x86-registered-target
-// RUN: %clang_cc1 -emit-llvm -triple x86_64 -fstrict-flex-arrays=0 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-0
-// RUN: %clang_cc1 -emit-llvm -triple x86_64 -fstrict-flex-arrays=0 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-0,CXX,CXX-STRICT-0
-// RUN: %clang_cc1 -emit-llvm -triple x86_64 -fstrict-flex-arrays=1 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-1
-// RUN: %clang_cc1 -emit-llvm -triple x86_64 -fstrict-flex-arrays=1 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-1,CXX
-// RUN: %clang_cc1 -emit-llvm -triple x86_64 -fstrict-flex-arrays=2 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-2
-// RUN: %clang_cc1 -emit-llvm -triple x86_64 -fstrict-flex-arrays=2 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-2,CXX
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=0 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-0
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=0 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-0,CXX,CXX-STRICT-0
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=1 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-1
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=1 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-1,CXX
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=2 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-2
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=2 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-2,CXX
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=3 -fsanitize=array-bounds        %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-3
+// RUN: %clang_cc1 -O2 -emit-llvm -triple x86_64 -fstrict-flex-arrays=3 -fsanitize=array-bounds -x c++ %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-STRICT-3,CXX
 // Before flexible array member was added to C99, many projects use a
 // one-element array as the last member of a structure as an alternative.
 // E.g. https://github.com/python/cpython/issues/84301
@@ -24,6 +26,7 @@
   // CHECK-STRICT-0-NOT: @__ubsan
   // CHECK-STRICT-1-NOT: @__ubsan
   // CHECK-STRICT-2:     call void @__ubsan_handle_out_of_bounds_abort(
+  // CHECK-STRICT-3:     call void @__ubsan_handle_out_of_bounds_abort(
   return p->a[i] + (p->a)[i];
 }
 
@@ -32,6 +35,7 @@
   // CHECK-STRICT-0:     call void @__ubsan_handle_out_of_bounds_abort(
   // CHECK-STRICT-1:     call void @__ubsan_handle_out_of_bounds_abort(
   // CHECK-STRICT-2:     call void @__ubsan_handle_out_of_bounds_abort(
+  // CHECK-STRICT-3:     call void @__ubsan_handle_out_of_bounds_abort(
   return p->a[i] + (p->a)[i];
 }
 
@@ -53,6 +57,7 @@
   // CHECK-STRICT-0-NOT: @__ubsan
   // CHECK-STRICT-1-NOT: @__ubsan
   // CHECK-STRICT-2-NOT: @__ubsan
+  // CHECK-STRICT-3: @__ubsan
   return p->a[i] + (p->a)[i];
 }
 
@@ -61,6 +66,7 @@
   // CHECK-STRICT-0-NOT: @__ubsan
   // CHECK-STRICT-1-NOT: @__ubsan
   // CHECK-STRICT-2: @__ubsan
+  // CHECK-STRICT-3: @__ubsan
   return p->a[i] + (p->a)[i];
 }
 
@@ -69,6 +75,7 @@
   // CHECK-STRICT-0: @__ubsan
   // CHECK-STRICT-1: @__ubsan
   // CHECK-STRICT-2: @__ubsan
+  // CHECK-STRICT-3: @__ubsan
   return p->a[i] + (p->a)[i];
 }
 
@@ -77,6 +84,7 @@
   // CHECK-STRICT-0: @__ubsan
   // CHECK-STRICT-1: @__ubsan
   // CHECK-STRICT-2: @__ubsan
+  // CHECK-STRICT-3: @__ubsan
   return p->a[i] + (p->a)[i];
 }
 
@@ -85,6 +93,7 @@
   // CHECK-STRICT-0:     call void @__ubsan_handle_out_of_bounds_abort(
   // CHECK-STRICT-1:     call void @__ubsan_handle_out_of_bounds_abort(
   // CHECK-STRICT-2:     call void @__ubsan_handle_out_of_bounds_abort(
+  // CHECK-STRICT-3:     call void @__ubsan_handle_out_of_bounds_abort(
   return p->a[i] + (p->a)[i];
 }
 
diff --git a/clang/test/CodeGen/object-size-flex-array.c b/clang/test/CodeGen/object-size-flex-array.c
--- a/clang/test/CodeGen/object-size-flex-array.c
+++ b/clang/test/CodeGen/object-size-flex-array.c
@@ -1,6 +1,8 @@
-// RUN: %clang -fstrict-flex-arrays=2 -target x86_64-apple-darwin -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-2 %s
-// RUN: %clang -fstrict-flex-arrays=1 -target x86_64-apple-darwin -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-1 %s
-// RUN: %clang -fstrict-flex-arrays=0 -target x86_64-apple-darwin -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-0 %s
+// RUN: %clang                        -target x86_64 -O2 -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-NO-STRICT %s
+// RUN: %clang -fstrict-flex-arrays=0 -target x86_64 -O2 -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-0 %s
+// RUN: %clang -fstrict-flex-arrays=1 -target x86_64 -O2 -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-1 %s
+// RUN: %clang -fstrict-flex-arrays=2 -target x86_64 -O2 -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-2 %s
+// RUN: %clang -fstrict-flex-arrays=3 -target x86_64 -O2 -S -emit-llvm %s -o - 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-STRICT-3 %s
 
 #define OBJECT_SIZE_BUILTIN __builtin_object_size
 
@@ -26,36 +28,86 @@
 
 // CHECK-LABEL: @bar(
 unsigned bar(foo_t *f) {
-  // CHECK-STRICT-0: ret i32 %
-  // CHECK-STRICT-1: ret i32 %
-  // CHECK-STRICT-2: ret i32 %
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 -1
+  // CHECK-STRICT-2: ret i32 -1
+  // CHECK-STRICT-3: ret i32 -1
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
 // CHECK-LABEL: @bar0(
 unsigned bar0(foo0_t *f) {
-  // CHECK-STRICT-0: ret i32 %
-  // CHECK-STRICT-1: ret i32 %
-  // CHECK-STRICT-2: ret i32 %
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 -1
+  // CHECK-STRICT-2: ret i32 -1
+  // CHECK-STRICT-3: ret i32 0
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
 // CHECK-LABEL: @bar1(
 unsigned bar1(foo1_t *f) {
-  // CHECK-STRICT-0: ret i32 %
-  // CHECK-STRICT-1: ret i32 %
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 -1
   // CHECK-STRICT-2: ret i32 8
+  // CHECK-STRICT-3: ret i32 8
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
 // CHECK-LABEL: @bar2(
 unsigned bar2(foo2_t *f) {
-  // CHECK-STRICT-0: ret i32 %
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
   // CHECK-STRICT-1: ret i32 16
   // CHECK-STRICT-2: ret i32 16
+  // CHECK-STRICT-3: ret i32 16
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
+#define DYNAMIC_OBJECT_SIZE_BUILTIN __builtin_dynamic_object_size
+
+// CHECK-LABEL: @dyn_bar(
+unsigned dyn_bar(foo_t *f) {
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 -1
+  // CHECK-STRICT-2: ret i32 -1
+  // CHECK-STRICT-3: ret i32 -1
+  return DYNAMIC_OBJECT_SIZE_BUILTIN(f->c, 1);
+}
+
+// CHECK-LABEL: @dyn_bar0(
+unsigned dyn_bar0(foo0_t *f) {
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 -1
+  // CHECK-STRICT-2: ret i32 -1
+  // CHECK-STRICT-3: ret i32 0
+  return DYNAMIC_OBJECT_SIZE_BUILTIN(f->c, 1);
+}
+
+// CHECK-LABEL: @dyn_bar1(
+unsigned dyn_bar1(foo1_t *f) {
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 -1
+  // CHECK-STRICT-2: ret i32 8
+  // CHECK-STRICT-3: ret i32 8
+  return DYNAMIC_OBJECT_SIZE_BUILTIN(f->c, 1);
+}
+
+// CHECK-LABEL: @dyn_bar2(
+unsigned dyn_bar2(foo2_t *f) {
+  // CHECK-NO-STRICT: ret i32 -1
+  // CHECK-STRICT-0: ret i32 -1
+  // CHECK-STRICT-1: ret i32 16
+  // CHECK-STRICT-2: ret i32 16
+  // CHECK-STRICT-3: ret i32 16
+  return DYNAMIC_OBJECT_SIZE_BUILTIN(f->c, 1);
+}
+
 // Also checks for non-trailing flex-array like members
 
 typedef struct {
@@ -75,24 +127,30 @@
 
 // CHECK-LABEL: @babar0(
 unsigned babar0(foofoo0_t *f) {
+  // CHECK-NO-STRICT: ret i32 0
   // CHECK-STRICT-0: ret i32 0
   // CHECK-STRICT-1: ret i32 0
   // CHECK-STRICT-2: ret i32 0
+  // CHECK-STRICT-3: ret i32 0
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
 // CHECK-LABEL: @babar1(
 unsigned babar1(foofoo1_t *f) {
+  // CHECK-NO-STRICT: ret i32 8
   // CHECK-STRICT-0: ret i32 8
   // CHECK-STRICT-1: ret i32 8
   // CHECK-STRICT-2: ret i32 8
+  // CHECK-STRICT-3: ret i32 8
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
 // CHECK-LABEL: @babar2(
 unsigned babar2(foofoo2_t *f) {
+  // CHECK-NO-STRICT: ret i32 16
   // CHECK-STRICT-0: ret i32 16
   // CHECK-STRICT-1: ret i32 16
   // CHECK-STRICT-2: ret i32 16
+  // CHECK-STRICT-3: ret i32 16
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
diff --git a/clang/test/Sema/array-bounds-ptr-arith.c b/clang/test/Sema/array-bounds-ptr-arith.c
--- a/clang/test/Sema/array-bounds-ptr-arith.c
+++ b/clang/test/Sema/array-bounds-ptr-arith.c
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -verify=expected -Warray-bounds-pointer-arithmetic %s
 // RUN: %clang_cc1 -verify=expected -Warray-bounds-pointer-arithmetic %s -fstrict-flex-arrays=0
 // RUN: %clang_cc1 -verify=expected,strict -Warray-bounds-pointer-arithmetic %s -fstrict-flex-arrays=2
+// RUN: %clang_cc1 -verify=expected,strict -Warray-bounds-pointer-arithmetic %s -fstrict-flex-arrays=3
 
 // Test case from PR10615
 struct ext2_super_block{
diff --git a/clang/test/SemaCXX/array-bounds-strict-flex-arrays.cpp b/clang/test/SemaCXX/array-bounds-strict-flex-arrays.cpp
--- a/clang/test/SemaCXX/array-bounds-strict-flex-arrays.cpp
+++ b/clang/test/SemaCXX/array-bounds-strict-flex-arrays.cpp
@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -verify=relaxed -fstrict-flex-arrays=1 %s
 // RUN: %clang_cc1 -verify=relaxed,strict -fstrict-flex-arrays=2 %s
+// RUN: %clang_cc1 -verify=relaxed,strict -fstrict-flex-arrays=3 %s
 
 // We cannot know for sure the size of a flexible array.
 struct t {