Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6021,9 +6021,8 @@
 def err_atomic_specifier_bad_type
     : Error<"_Atomic cannot be applied to "
             "%select{incomplete |array |function |reference |atomic |qualified "
-            "|sizeless ||integer |integer }0type "
-            "%1 %select{|||||||which is not trivially copyable|with less than "
-            "1 byte of precision|with a non power of 2 precision}0">;
+            "|sizeless ||integer }0type "
+            "%1 %select{|||||||which is not trivially copyable|}0">;
 
 // Expressions.
 def ext_sizeof_alignof_function_type : Extension<
@@ -7941,6 +7940,8 @@
   " 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_builtin_ext_int_prohibit : Error<
+  "argument to atomic builtin of type '_ExtInt' is not supported">;
 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
@@ -4956,6 +4956,11 @@
                 ? 0
                 : 1);
 
+  if (ValType->isExtIntType()) {
+    Diag(Ptr->getExprLoc(), diag::err_atomic_builtin_ext_int_prohibit);
+    return ExprError();
+  }
+
   return AE;
 }
 
Index: clang/lib/Sema/SemaType.cpp
===================================================================
--- clang/lib/Sema/SemaType.cpp
+++ clang/lib/Sema/SemaType.cpp
@@ -8880,11 +8880,8 @@
     else if (!T.isTriviallyCopyableType(Context))
       // Some other non-trivially-copyable type (probably a C++ class)
       DisallowedKind = 7;
-    else if (auto *ExtTy = T->getAs<ExtIntType>()) {
-      if (ExtTy->getNumBits() < 8)
+    else if (T->isExtIntType()) {
         DisallowedKind = 8;
-      else if (!llvm::isPowerOf2_32(ExtTy->getNumBits()))
-        DisallowedKind = 9;
     }
 
     if (DisallowedKind != -1) {
Index: clang/test/Sema/builtins.c
===================================================================
--- clang/test/Sema/builtins.c
+++ clang/test/Sema/builtins.c
@@ -285,12 +285,16 @@
   __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}}
+
+  __atomic_fetch_add(ptr, 1, 0); // expected-error {{argument to atomic builtin of type '_ExtInt' is not supported}}
 }
 
 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
+
+  __atomic_fetch_add(ptr, 1, 0); // expected-error {{argument to atomic builtin of type '_ExtInt' is not supported}}
 }
 
 void test_ei_ii42(int *ptr, _ExtInt(42) value) {
Index: clang/test/SemaCXX/ext-int.cpp
===================================================================
--- clang/test/SemaCXX/ext-int.cpp
+++ clang/test/SemaCXX/ext-int.cpp
@@ -91,10 +91,11 @@
 _Complex _ExtInt(3) Cmplx;
 
 // Reject cases of _Atomic:
-// expected-error@+1{{_Atomic cannot be applied to integer type '_ExtInt(4)' with less than 1 byte of precision}}
+// expected-error@+1{{_Atomic cannot be applied to integer type '_ExtInt(4)'}}
 _Atomic _ExtInt(4) TooSmallAtomic;
-// expected-error@+1{{_Atomic cannot be applied to integer type '_ExtInt(9)' with a non power of 2 precision}}
+// expected-error@+1{{_Atomic cannot be applied to integer type '_ExtInt(9)'}}
 _Atomic _ExtInt(9) NotPow2Atomic;
+// expected-error@+1{{_Atomic cannot be applied to integer type '_ExtInt(128)'}}
 _Atomic _ExtInt(128) JustRightAtomic;
 
 // Test result types of Unary/Bitwise/Binary Operations:
Index: libcxx/test/libcxx/atomics/ext-int.verify.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/atomics/ext-int.verify.cpp
@@ -0,0 +1,11 @@
+// REQUIRES: clang-11
+
+#include <atomic>
+
+int main(int, char**)
+{
+  // expected-error@atomic:*1 {{_Atomic cannot be applied to integer type '_ExtInt(32)'}}
+  std::atomic<_ExtInt(32)> x {42};
+
+  return 0;
+}