diff --git a/compiler-rt/test/builtins/Unit/atomic_test.c b/compiler-rt/test/builtins/Unit/atomic_test.c
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/builtins/Unit/atomic_test.c
@@ -0,0 +1,417 @@
+// RUN: %clang_builtins %s %librt -o %t && %run %t
+// REQUIRES: librt_has_atomic
+//===-- atomic_test.c - Test support functions for atomic operations ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file performs some simple testing of the support functions for the
+// atomic builtins. All tests are single-threaded, so this is only a sanity
+// check.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+// We directly test the library atomic functions, not using the C builtins. This
+// should avoid confounding factors, ensuring that we actually test the
+// functions themselves, regardless of how the builtins are lowered. We need to
+// use asm labels because we can't redeclare the builtins.
+
+void __atomic_load_c(int size, void *src, void *dest,
+                     int model) asm("__atomic_load");
+
+uint8_t __atomic_load_1(uint8_t *src, int model);
+uint16_t __atomic_load_2(uint16_t *src, int model);
+uint32_t __atomic_load_4(uint32_t *src, int model);
+uint64_t __atomic_load_8(uint64_t *src, int model);
+
+void __atomic_store_c(int size, void *dest, void *src,
+                      int model) asm("__atomic_store");
+
+uint8_t __atomic_store_1(uint8_t *dest, uint8_t val, int model);
+uint16_t __atomic_store_2(uint16_t *dest, uint16_t val, int model);
+uint32_t __atomic_store_4(uint32_t *dest, uint32_t val, int model);
+uint64_t __atomic_store_8(uint64_t *dest, uint64_t val, int model);
+
+void __atomic_exchange_c(int size, void *ptr, void *val, void *old, int model)
+                         asm("__atomic_exchange");
+
+uint8_t __atomic_exchange_1(uint8_t *dest, uint8_t val, int model);
+uint16_t __atomic_exchange_2(uint16_t *dest, uint16_t val, int model);
+uint32_t __atomic_exchange_4(uint32_t *dest, uint32_t val, int model);
+uint64_t __atomic_exchange_8(uint64_t *dest, uint64_t val, int model);
+
+int __atomic_compare_exchange_c(int size, void *ptr, void *expected,
+                                void *desired, int success, int failure)
+                                asm("__atomic_compare_exchange");
+
+bool __atomic_compare_exchange_1(uint8_t *ptr, uint8_t *expected,
+                                 uint8_t desired, int success, int failure);
+bool __atomic_compare_exchange_2(uint16_t *ptr, uint16_t *expected,
+                                 uint16_t desired, int success, int failure);
+bool __atomic_compare_exchange_4(uint32_t *ptr, uint32_t *expected,
+                                 uint32_t desired, int success, int failure);
+bool __atomic_compare_exchange_8(uint64_t *ptr, uint64_t *expected,
+                                 uint64_t desired, int success, int failure);
+
+uint8_t __atomic_fetch_add_1(uint8_t *ptr, uint8_t val, int model);
+uint16_t __atomic_fetch_add_2(uint16_t *ptr, uint16_t val, int model);
+uint32_t __atomic_fetch_add_4(uint32_t *ptr, uint32_t val, int model);
+uint64_t __atomic_fetch_add_8(uint64_t *ptr, uint64_t val, int model);
+
+uint8_t __atomic_fetch_sub_1(uint8_t *ptr, uint8_t val, int model);
+uint16_t __atomic_fetch_sub_2(uint16_t *ptr, uint16_t val, int model);
+uint32_t __atomic_fetch_sub_4(uint32_t *ptr, uint32_t val, int model);
+uint64_t __atomic_fetch_sub_8(uint64_t *ptr, uint64_t val, int model);
+
+uint8_t __atomic_fetch_and_1(uint8_t *ptr, uint8_t val, int model);
+uint16_t __atomic_fetch_and_2(uint16_t *ptr, uint16_t val, int model);
+uint32_t __atomic_fetch_and_4(uint32_t *ptr, uint32_t val, int model);
+uint64_t __atomic_fetch_and_8(uint64_t *ptr, uint64_t val, int model);
+
+uint8_t __atomic_fetch_or_1(uint8_t *ptr, uint8_t val, int model);
+uint16_t __atomic_fetch_or_2(uint16_t *ptr, uint16_t val, int model);
+uint32_t __atomic_fetch_or_4(uint32_t *ptr, uint32_t val, int model);
+uint64_t __atomic_fetch_or_8(uint64_t *ptr, uint64_t val, int model);
+
+uint8_t __atomic_fetch_xor_1(uint8_t *ptr, uint8_t val, int model);
+uint16_t __atomic_fetch_xor_2(uint16_t *ptr, uint16_t val, int model);
+uint32_t __atomic_fetch_xor_4(uint32_t *ptr, uint32_t val, int model);
+uint64_t __atomic_fetch_xor_8(uint64_t *ptr, uint64_t val, int model);
+
+// We conditionally test the *_16 atomic function variants based on the same
+// condition that compiler_rt (atomic.c) uses to conditionally generate them.
+// Currently atomic.c tests if __SIZEOF_INT128__ is defined (which can be the
+// case on 32-bit platforms, by using -fforce-enable-int128), instead of using
+// CRT_HAS_128BIT.
+
+#ifdef __SIZEOF_INT128__
+#define TEST_16
+#endif
+
+#ifdef TEST_16
+#define uint128_t __uint128_t
+uint128_t __atomic_load_16(uint128_t *src, int model);
+uint128_t __atomic_store_16(uint128_t *dest, uint128_t val, int model);
+uint128_t __atomic_exchange_16(uint128_t *dest, uint128_t val, int model);
+bool __atomic_compare_exchange_16(uint128_t *ptr, uint128_t *expected,
+                                  uint128_t desired, int success, int failure);
+uint128_t __atomic_fetch_add_16(uint128_t *ptr, uint128_t val, int model);
+uint128_t __atomic_fetch_sub_16(uint128_t *ptr, uint128_t val, int model);
+uint128_t __atomic_fetch_and_16(uint128_t *ptr, uint128_t val, int model);
+uint128_t __atomic_fetch_or_16(uint128_t *ptr, uint128_t val, int model);
+uint128_t __atomic_fetch_xor_16(uint128_t *ptr, uint128_t val, int model);
+#endif
+
+#define U8(data) ((uint8_t)(data))
+#define U16(data) ((uint16_t)(data))
+#define U32(data) ((uint32_t)(data))
+#define U64(data) ((uint64_t)(data))
+
+#define v ((((uint128_t)0x4243444546474849) << 64) | 0x4a4b4c4d4e4f5051)
+#define ones ((((uint128_t)0x0101010101010101) << 64) | 0x0101010101010101)
+
+#define LEN(array) (sizeof(array) / sizeof(array[0]))
+
+int main() {
+  static char data[] = {
+      0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+      0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+      0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
+      0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+  };
+
+  uint8_t a8, b8;
+  uint16_t a16, b16;
+  uint32_t a32, b32;
+  uint64_t a64, b64;
+  uint128_t a128, b128;
+
+  // Loads
+
+  static int atomic_load_models[] = {
+      __ATOMIC_RELAXED,
+      __ATOMIC_CONSUME,
+      __ATOMIC_ACQUIRE,
+      __ATOMIC_SEQ_CST,
+  };
+
+  for (int m = 0; m < LEN(atomic_load_models); m++) {
+    int model = atomic_load_models[m];
+
+    for (int n = 1; n <= LEN(data); n++) {
+      char dst[LEN(data)] = {0};
+      __atomic_load_c(n, data, dst, model);
+      if (memcmp(dst, data, n) != 0)
+        abort();
+    }
+
+    a8 = U8(v + m);
+    if (__atomic_load_1(&a8, model) != U8(v + m))
+      abort();
+    a16 = U16(v + m);
+    if (__atomic_load_2(&a16, model) != U16(v + m))
+      abort();
+    a32 = U32(v + m);
+    if (__atomic_load_4(&a32, model) != U32(v + m))
+      abort();
+    a64 = U64(v + m);
+    if (__atomic_load_8(&a64, model) != U64(v + m))
+      abort();
+    a128 = v + m;
+#ifdef TEST_16
+    if (__atomic_load_16(&a128, model) != v + m)
+      abort();
+#endif
+  }
+
+  // Stores
+
+  static int atomic_store_models[] = {
+      __ATOMIC_RELAXED,
+      __ATOMIC_RELEASE,
+      __ATOMIC_SEQ_CST,
+  };
+
+  for (int m = 0; m < LEN(atomic_store_models); m++) {
+    int model = atomic_store_models[m];
+
+    for (int n = 1; n <= LEN(data); n++) {
+      char dst[LEN(data)];
+      __atomic_store_c(n, dst, data, model);
+      if (memcmp(data, dst, n) != 0)
+        abort();
+    }
+
+    __atomic_store_1(&a8, U8(v + m), model);
+    if (a8 != U8(v + m))
+      abort();
+    __atomic_store_2(&a16, U16(v + m), model);
+    if (a16 != U16(v + m))
+      abort();
+    __atomic_store_4(&a32, U32(v + m), model);
+    if (a32 != U32(v + m))
+      abort();
+    __atomic_store_8(&a64, U64(v + m), model);
+    if (a64 != U64(v + m))
+      abort();
+#ifdef TEST_16
+    __atomic_store_16(&a128, v + m, model);
+    if (a128 != v + m)
+      abort();
+#endif
+  }
+
+  // Exchanges
+
+  static int atomic_exchange_models[] = {
+      __ATOMIC_RELAXED,
+      __ATOMIC_ACQUIRE,
+      __ATOMIC_RELEASE,
+      __ATOMIC_ACQ_REL,
+      __ATOMIC_SEQ_CST,
+  };
+
+  a8 = U8(v);
+  a16 = U16(v);
+  a32 = U32(v);
+  a64 = U64(v);
+  a128 = v;
+
+  for (int m = 0; m < LEN(atomic_exchange_models); m++) {
+    int model = atomic_exchange_models[m];
+
+    for (int n = 1; n <= LEN(data); n++) {
+      char dst[LEN(data)];
+      char old[LEN(data)];
+      for (int i = 0; i < LEN(dst); i++)
+        dst[i] = i + m;
+      __atomic_exchange_c(n, dst, data, old, model);
+      for (int i = 0; i < n; i++) {
+        if (dst[i] != 0x10 + i || old[i] != i + m)
+          abort();
+      }
+    }
+
+    if (__atomic_exchange_1(&a8, U8(v + m + 1), model) != U8(v + m))
+      abort();
+    if (__atomic_exchange_2(&a16, U16(v + m + 1), model) != U16(v + m))
+      abort();
+    if (__atomic_exchange_4(&a32, U32(v + m + 1), model) != U32(v + m))
+      abort();
+    if (__atomic_exchange_8(&a64, U64(v + m + 1), model) != U64(v + m))
+      abort();
+#ifdef TEST_16
+    if (__atomic_exchange_16(&a128, v + m + 1, model) != v + m)
+      abort();
+#endif
+  }
+
+  // Compare and exchanges
+
+  static int atomic_compare_exchange_models[][2] = {
+      {__ATOMIC_RELAXED, __ATOMIC_RELAXED},
+      {__ATOMIC_CONSUME, __ATOMIC_RELAXED},
+      {__ATOMIC_CONSUME, __ATOMIC_CONSUME},
+      {__ATOMIC_ACQUIRE, __ATOMIC_RELAXED},
+      {__ATOMIC_ACQUIRE, __ATOMIC_CONSUME},
+      {__ATOMIC_ACQUIRE, __ATOMIC_ACQUIRE},
+      {__ATOMIC_RELEASE, __ATOMIC_RELAXED},
+      {__ATOMIC_RELEASE, __ATOMIC_CONSUME},
+      {__ATOMIC_RELEASE, __ATOMIC_ACQUIRE},
+      {__ATOMIC_ACQ_REL, __ATOMIC_RELAXED},
+      {__ATOMIC_ACQ_REL, __ATOMIC_CONSUME},
+      {__ATOMIC_ACQ_REL, __ATOMIC_ACQUIRE},
+      {__ATOMIC_SEQ_CST, __ATOMIC_RELAXED},
+      {__ATOMIC_SEQ_CST, __ATOMIC_CONSUME},
+      {__ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE},
+      {__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST},
+  };
+
+  for (int m = 0; m < LEN(atomic_compare_exchange_models); m++) {
+    int m_succ = atomic_compare_exchange_models[m][0];
+    int m_fail = atomic_compare_exchange_models[m][1];
+
+    for (int n = 1; n <= LEN(data); n++) {
+      char dst[LEN(data)] = {0};
+      char exp[LEN(data)] = {0};
+      if (!__atomic_compare_exchange_c(n, dst, exp, data, m_succ, m_fail))
+        abort();
+      if (memcmp(dst, data, n) != 0)
+        abort();
+      if (__atomic_compare_exchange_c(n, dst, exp, data, m_succ, m_fail))
+        abort();
+      if (memcmp(exp, data, n) != 0)
+        abort();
+    }
+  }
+
+  // Fetches with operations
+
+  static int atomic_fetch_models[] = {
+      __ATOMIC_RELAXED,
+      __ATOMIC_CONSUME,
+      __ATOMIC_ACQUIRE,
+      __ATOMIC_RELEASE,
+      __ATOMIC_ACQ_REL,
+      __ATOMIC_SEQ_CST,
+  };
+
+  for (int m = 0; m < LEN(atomic_fetch_models); m++) {
+    int model = atomic_fetch_models[m];
+
+    // Fetch add
+
+    a8 = U8(v + m);
+    a16 = U16(v + m);
+    a32 = U32(v + m);
+    a64 = U64(v + m);
+    a128 = v + m;
+
+    b8 = __atomic_fetch_add_1(&a8, U8(ones), model);
+    if (b8 != U8(v + m) || a8 != U8(v + m + ones))
+      abort();
+    b16 = __atomic_fetch_add_2(&a16, U16(ones), model);
+    if (b16 != U16(v + m) || a16 != U16(v + m + ones))
+      abort();
+    b32 = __atomic_fetch_add_4(&a32, U32(ones), model);
+    if (b32 != U32(v + m) || a32 != U32(v + m + ones))
+      abort();
+    b64 = __atomic_fetch_add_8(&a64, U64(ones), model);
+    if (b64 != U64(v + m) || a64 != U64(v + m + ones))
+      abort();
+#ifdef TEST_16
+    b128 = __atomic_fetch_add_16(&a128, ones, model);
+    if (b128 != v + m || a128 != v + m + ones)
+      abort();
+#endif
+
+    // Fetch sub
+
+    b8 = __atomic_fetch_sub_1(&a8, U8(ones), model);
+    if (b8 != U8(v + m + ones) || a8 != U8(v + m))
+      abort();
+    b16 = __atomic_fetch_sub_2(&a16, U16(ones), model);
+    if (b16 != U16(v + m + ones) || a16 != U16(v + m))
+      abort();
+    b32 = __atomic_fetch_sub_4(&a32, U32(ones), model);
+    if (b32 != U32(v + m + ones) || a32 != U32(v + m))
+      abort();
+    b64 = __atomic_fetch_sub_8(&a64, U64(ones), model);
+    if (b64 != U64(v + m + ones) || a64 != U64(v + m))
+      abort();
+#ifdef TEST_16
+    b128 = __atomic_fetch_sub_16(&a128, ones, model);
+    if (b128 != v + m + ones || a128 != v + m)
+      abort();
+#endif
+
+    // Fetch or
+
+    b8 = __atomic_fetch_or_1(&a8, U8(ones), model);
+    if (b8 != U8(v + m) || a8 != U8((v + m) | ones))
+      abort();
+    b16 = __atomic_fetch_or_2(&a16, U16(ones), model);
+    if (b16 != U16(v + m) || a16 != U16((v + m) | ones))
+      abort();
+    b32 = __atomic_fetch_or_4(&a32, U32(ones), model);
+    if (b32 != U32(v + m) || a32 != U32((v + m) | ones))
+      abort();
+    b64 = __atomic_fetch_or_8(&a64, U64(ones), model);
+    if (b64 != U64(v + m) || a64 != U64((v + m) | ones))
+      abort();
+#ifdef TEST_16
+    b128 = __atomic_fetch_or_16(&a128, ones, model);
+    if (b128 != v + m || a128 != ((v + m) | ones))
+      abort();
+#endif
+
+    // Fetch and
+
+    b8 = __atomic_fetch_and_1(&a8, U8(v + m), model);
+    if (b8 != U8((v + m) | ones) || a8 != U8(v + m))
+      abort();
+    b16 = __atomic_fetch_and_2(&a16, U16(v + m), model);
+    if (b16 != U16((v + m) | ones) || a16 != U16(v + m))
+      abort();
+    b32 = __atomic_fetch_and_4(&a32, U32(v + m), model);
+    if (b32 != U32((v + m) | ones) || a32 != U32(v + m))
+      abort();
+    b64 = __atomic_fetch_and_8(&a64, U64(v + m), model);
+    if (b64 != U64((v + m) | ones) || a64 != U64(v + m))
+      abort();
+#ifdef TEST_16
+    b128 = __atomic_fetch_and_16(&a128, v + m, model);
+    if (b128 != ((v + m) | ones) || a128 != (v + m))
+      abort();
+#endif
+
+    // Fetch xor
+
+    b8 = __atomic_fetch_xor_1(&a8, U8(ones), model);
+    if (b8 != U8(v + m) || a8 != U8((v + m) ^ ones))
+      abort();
+    b16 = __atomic_fetch_xor_2(&a16, U16(ones), model);
+    if (b16 != U16(v + m) || a16 != U16((v + m) ^ ones))
+      abort();
+    b32 = __atomic_fetch_xor_4(&a32, U32(ones), model);
+    if (b32 != U32(v + m) || a32 != U32((v + m) ^ ones))
+      abort();
+    b64 = __atomic_fetch_xor_8(&a64, U64(ones), model);
+    if (b64 != U64(v + m) || a64 != U64((v + m) ^ ones))
+      abort();
+#ifdef TEST_16
+    b128 = __atomic_fetch_xor_16(&a128, ones, model);
+    if (b128 != (v + m) || a128 != ((v + m) ^ ones))
+      abort();
+#endif
+  }
+}