Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7926,6 +7926,8 @@
 def err_atomic_exclusive_builtin_pointer_size : Error<
   "address argument to load or store exclusive builtin must be a pointer to"
   " 1,2,4 or 8 byte type (%0 invalid)">;
+def err_atomic_builtin_ext_int_size : Error<
+  "Atomic memory operand must have a power-of-two size">;
 def err_atomic_op_needs_atomic : Error<
   "address argument to atomic operation must be a pointer to _Atomic "
   "type (%0 invalid)">;
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -5280,6 +5280,16 @@
   // gracefully.
   TheCall->setType(ResultType);
 
+  // Prohibit use of _ExtInt with atomic builtins.
+  // The arguments would have already been converted to the first argument's
+  // type, so only need to check the first argument.
+  const auto ExtIntValType = ValType->getAs<ExtIntType>();
+  if (ExtIntValType && ExtIntValType->getNumBits() !=
+                           llvm::PowerOf2Ceil(ExtIntValType->getNumBits())) {
+    Diag(FirstArg->getExprLoc(), diag::err_atomic_builtin_ext_int_size);
+    return ExprError();
+  }
+
   return TheCallResult;
 }
 
Index: clang/test/Sema/builtins.c
===================================================================
--- clang/test/Sema/builtins.c
+++ clang/test/Sema/builtins.c
@@ -281,6 +281,42 @@
   __atomic_fetch_add(ptr, 1, 0);  // expected-error {{address argument to atomic operation must be a pointer to non-const type ('const int *' invalid)}}
 }
 
+void test_ei_i42i(_ExtInt(42) *ptr, int value) {
+  __sync_fetch_and_add(ptr, value); // expected-error {{Atomic memory operand must have a power-of-two size}}
+  // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
+  __sync_nand_and_fetch(ptr, value); // expected-error {{Atomic memory operand must have a power-of-two size}}
+}
+
+void test_ei_i64i(_ExtInt(64) *ptr, int value) {
+  __sync_fetch_and_add(ptr, value); // expect success
+  // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
+  __sync_nand_and_fetch(ptr, value); // expect success
+}
+
+void test_ei_ii42(int *ptr, _ExtInt(42) value) {
+  __sync_fetch_and_add(ptr, value); // expect success
+  // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
+  __sync_nand_and_fetch(ptr, value); // expect success
+}
+
+void test_ei_ii64(int *ptr, _ExtInt(64) value) {
+  __sync_fetch_and_add(ptr, value); // expect success
+  // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
+  __sync_nand_and_fetch(ptr, value); // expect success
+}
+
+void test_ei_i42i42(_ExtInt(42) *ptr, _ExtInt(42) value) {
+  __sync_fetch_and_add(ptr, value); // expected-error {{Atomic memory operand must have a power-of-two size}}
+  // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
+  __sync_nand_and_fetch(ptr, value); // expected-error {{Atomic memory operand must have a power-of-two size}}
+}
+
+void test_ei_i64i64(_ExtInt(64) *ptr, _ExtInt(64) value) {
+  __sync_fetch_and_add(ptr, value); // expect success
+  // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
+  __sync_nand_and_fetch(ptr, value); // expect success
+}
+
 void test22(void) {
   (void)__builtin_signbit(); // expected-error{{too few arguments to function call, expected 1, have 0}}
   (void)__builtin_signbit(1.0, 2.0, 3.0); // expected-error{{too many arguments to function call, expected 1, have 3}}